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1 EFFECTS OF INTERCROPPING SWEET POTATO ON THE POPULATION DENSITY OF SWEET POTATO WEEVIL, û1as formicariys (F.) (COLEOPTERA: CURCULIONIDAE) By ALEXANDER YAKU A thes i s submi tted to the Facu l ty of Graduate Studies and Research in partial fulfillment of the requi rement for the degree of Master of Science Department of Entomology l d Campus of McGi 11 University, Montreal Quebec, Canada Il Al exander YakLc June 1992
Transcript of digitool.library.mcgill.cadigitool.library.mcgill.ca/thesisfile56673.pdf · ABSTRACT M.Sc ALEXANDER...
1
EFFECTS OF INTERCROPPING SWEET POTATO ON THE POPULATION DENSITY OF SWEET POTATO WEEVIL, û1as formicariys (F.)
(COLEOPTERA: CURCULIONIDAE)
By
ALEXANDER YAKU
A thes i s submi tted to the Facu l ty of Graduate Studies and Research in partial fulfillment
of the requi rement for the degree of Master of Science
Department of Entomology ~Iacdona l d Campus of McGi 11 University, Montreal Quebec, Canada
Il Al exander YakLc
June 1992
-l
Suggested short title:
Sweet potato weevil control by intercropping
Alexander Yaku
J
ABSTRACT
M.Sc ALEXANDER YAKU ENTOMOLOGY
EfFECTS OF INTERCROPPING SWfET POTATO ON THE POPULATION DENSITY OF SWEET POTATO WEEVIL, Cylas. formicarius (F.)
(COLEOPTERA: CURCULIONIDAE)
Fi el d experi ments were conducted duri ng the 1989 dry season
(Ju 1 y to Oecembe r) a t the Manggoapi Fa rm of the Facu l ty of
Agnculture, Cenderawasih University in Manokwari, Irian Jaya,
Indones i a. The obj ecti ves of the experiments were to determi ne the
effects of four sweet potato cropping systems on the population
densi ty of sweet potato weevils (SPW) and on the diversity of other
i nsec ts wi th in these ag roecosys tems .
Fewer SPW were found in i ntercropped sweet potato + corn
(2 weevi 1 s per kg infected tubers), sweet potato + soybean (21
wccvi 1 s), sweet potato + corn + soybean (8 weevi 1 s) than in
monoc u 1 tu re sweet potato (37 weevi 1 s); percentage of damaged tubers
followed the same trend, ranging from 2.6 % to 14.0 % in
lntercropped sweet potato, to 21.9 % in the sweet potato
monocul ture. However, the hi gher number of SPW and damaged tubers
in the monoculture did not reduce yield below that in the
l ntercropped plot s, wh i ch had lower yi el ds becaus e of reduced sweet
potato density and higher interspecific plant competition.
Consequently, numbers and weight of tubers per plant, as we" as
I11drketable yield, were highest in the monoculture.
i
Insect and spider populations were more diverse in the
i ntercropped sweet pota to sys tems than in monocu lture. Number of
arthropods increased throughout the growl ng season, reélching a peak
at 56 days after planting (DAP). Intercroppi ng may reduce the
popul ation densi ty of other insect pests associated Wl th sweet
potato ( e.g., the spotted tortoise bettle ~doll1orptlg sp. was
less abundant), and may increase the popul ation densi ty of natural
enemies (e.g., the spider Lycosa sp. was more abundant).
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RESUME
M.Se ALEXANDER YAKU ENTOMOLOGIE
EFFETS DE DIVERS TYPES D'ASSOCIATIONS DE CULTURE DE LA PATATE DOUCE SUR LA DENSITE DE POPULATION DU CHARANÇON DE LA PATATE
DOUCE, Çy-l_~~ formicarius (F.) (COLEOPTERE: CURCULIONIDAE)
Des essa i s en champs ont été réa li sés pendant 1 a sa i son sèche
1989 (J U 111 et à Décembre) à l a ferme Manggoapi de la Facu l té
d'Agricultrure de l'Université Cenderawasih en Manokwari, de la
province d'Irian Jaya, en Indonésie. Les obj eet ifs de ces
ex péri en ces vis aient à détermi ner les effects de qua t re sys tèmes de
culture de la patate douce sur la densité de poplilation du
charancon de la patate douce et sur la diversité de l'entomofaune
à l'intérieur de ces divers agroécosystèmes.
Le nombre de cha rançon éta i t bas dans 1 es as soc ; at ions patate
douce - maïs (2 charançons [ch.] Ikg de tubercules i nfestp~ Et. i], patate douce - fève soya (21 ch. /kg de t. i ), patate douce - maïs -
fève soya (8 ch. /kg de t. i .) comparé au nombre de charançons
présent dans la monoculture de patate douce (37 ch./kg de t.i .).
Le pourcentage de tubercules attaqués suit cette même tendance; 2,6
à 14 7., des tubercules étdient attaqués dans les associations de
culture et 21.9 :1., dans la monoculture. Tout de même, le nombre
plus élevé de charançons et de plants infestés retrouvé en
l11onocu l tu re ne corres pond pas à une ba i sse de rendemen t de 1 a
patate douce car il était pl us élevé dans la monocul ture que dans
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T !
les cultures associées; le rendement inférieur dans ces dernières
est attribué à une densité plus faible de plants et li la
compétition interspécifique des plants. Par conséquent, le nombre
et le poids de tubercules frais ainsi que la récolte
commercialisable étaient donc plus élevés dans la monoculture.
L ' entomof aune éta i t plus divers i fi ée cians les sys tèmes de
polyculture que dans la monoculture. Le nombre d'arthropodes a
augmenté au cours de la saison, pour atteindrê un maximum 56 jours
aprè le repiquage. La polyculture a semblp réduire la densité de
population des autres insectes ravageurs de la patate douce (ex. la
casside, Aspidomorpha sp. y était moins abondante) et augmenter la
densité des ennemis naturels (ex. l'araignée Lycosa. sp. y était
plus nombreux).
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ACKNOWLEDMENTS
First of all, my greatest thanks and appreciation go to God,
Who by His infinite grace and faithfulness provided the
unpredictable opportunity to study in Canada. Only through His
power and guidance have 1 been able to undertake this work.
1 want to express my gratitude to the Indonesian Government,
especially to the Ministry of State for Population and Environment
and the Environmental Management Development of Indonesia (EMOI)
ProJect (a Joint proJect of the Indonesian Ministry of State for
Population and Envi!'or.ment, and the School for Resource and
~ Environmental Studies, Dalhousie University for the Environmental
Management Oevelopment of Indonesia) for granting me a scholarship
to study in Canada and to conduct the field research in Indonesia.
In this regard 1 am especiall.\1 indebted for the efforts of the
former EMOI Project Officers, Dr. Shirley A.M. Conover, Dr. Tania
Ll, George Green and the late Ors. Sjafran Sjamsuddin.
1 would also like to thank (1) Dr. Joan M. Campbell, E~1DI
ProJect Officer in Halifax, and her staff, especially Pauline
Lawrence and Lynnc Norrena who have given considerable attention
to the solving of various problems, (2) Barbara C. Duffield, former
EMOI Project Officer in Jakarta and her staff, especially Mrs.
P.M. Abclulkadir, Mrs Wisnu and Mr. Sony for their assistance
during my field research in Indonesia.
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T
l wi sh to acknow1 ecge the he1 p of the Dean of the Facu l ty of
Agricd1ture, Cènderawash University, for providing facilities
during my field research in M~nok~ari.
1 am grateful ta my twa faithful assÎ::,tants, Cornelles Iji and
Paulus Jumame, for their help with field work, and their
willingness ta guard the crops against theft during the nights of
the last six weeks before harvest.
l would like to thank Or. Helene Chiasson, Tanya Searle and
Susan Johnson, fOl"' help in insect identification, writing style and
analysis of data during the preparation of the thesis.
l am extreme1y grateful ta my academic superVlsor, Dr. Stuart
B. Hi 11 for his encouragement and understanding, which gave me the
se l f-confi den ce n€:cessa ry to s ucceed in th i s endeavour. Hi s adVl ce
and patience in readirg and carrecting the manuscript are a1so
appreci ated.
My gra t i tude al so goes to fe 11 ow s tudents and fri ends, Joanny
Zongo and Dr. Graham Thurston for providing time for discussion,
both entomalogy, and on how ta adjust to 11fe in Canada.
Fina11y, 1 would like to thank my wife Poulla and daugther
Catherine for their support and understanding in patiently waiting
for the day of my graduation. 1 wou1d a1so like to thank my
father, mother, l rothers and sisters who have always support~d my
efforts in prayer.
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{:
.,
TABLE OF CONTENTS
ABSTRACT
RESUME
. . . . . . . . . . . . . . . . .. . . . . .
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES
CHAPTER
1 . 1 NTRODUCTION
11. LITERATURE REVIEW
Page
ï i i
v
x
xii
1
12
2. 1. B IOECOLOGY OF THE SWEET POTATO WEEVI L, ÇyJ éiS formicarius (F.) (COLEOPTERA: ~URCULIONIDm 13
2.1.1. 2.1.2. 2.1.3.
2.1.4. 2.1.5. 2.1.6.
Taxonomie status and distribution Host range Life cycle a. Egg .. b. Larva . c. Pupa .. d. Adult .......... . Feedi ng habi ts and crop damage. Reproducti on . • . . . . . . . . . Factors affecting infestation by SPW
2.2. ROLE OF INTERCROPPING IN FOOD PRODUCTION.
2.2.1. 2.2.2. 2.2.3.
Definitions .......... . Status. . . . . . . . . . . . . . Advantages and di sadvantages. . .
2.3. EFFECTS OF INTERCROPPING ON INSECT PES rs. 2.3.1. Rate of eolonization ..•..
a. Visual effeets ...•.. b. 01 factory effects . • .. c. Di vers i on a ry hos t effeets
13 15 16 16 18 19 19 22 23 24
26
26 27 30
34
35 35 36 36
vii
111.
"
1 V •
2.3.2. 2.3.3. 2.3.4.
Oevelopment ......... . Di spersa 1 . . . . . . . . . . . Abundance of natura l enem; es. .
MATERIALS AND METHOOS . .
3.1.
3.2.
3.3.
Site description
Experimental design .
Crop arrangenment and spacing within each treatment ............. .
3.3.1. Sweet potato monocu l ture . . . . 3.3.2. Sweet potato and corn . . . . . 3.3.3. Sweet potato and soybean . . . . 3.3.4. Sweet potato, corn and soybean . 3.3.5. Sweet potato, tomato and cabbage
3.4. Field preparation and management
3.4.1. Land preparation ..... 3.4.2. Preparation of planting materials 3.4.3. Planting and fertilizing ....
3.5. Crop maintenance
3.6. Observations
3.6.1. 3.6.2.
Colonization by SPW ...... . Population density of SPW and percentage
3.6.3. 3.6.4. 3.6.5. 3.6.6.
of damaged tubers . . . . Number and fresh weight of Marketable yield .... . Monetary index ... . Insect diversity .... . a. Sweep net sampling b. Pitfall traps ....
tubers
37 38 38
40
41
43
43
43 43 44 44 44
45
45 45 45
47
47
47
48 49 49 49 50 50 51
3.7. Analysis of data 53
3.7.1. Effects of intercropping on populations of SPW and on sweet potato production. 53
3.7.2. Insect diversity 54
RESULTS . . .. . . . . . . . . . . . 55
4.1. Effects of intercropping on the population of sweet potato weevil (SPW) and sweet potato y; el d . . . . . . . . . . . . . . iii • • • • •• 59
viii
1
V.
4.1.1. Colonization of the sweet potato by SPW in four croppi ng systems . . . . . . .. 59
4.2.
4.3.
4.1.2.
4.1.3.
4.1.4.
Population density of SPW and percentage of dùmaged tubers . . . . . . . . . . . Number and fresh weight of sweet potato tubers. . . . . . . . . . . . Marketable yield and economic value of sweet potato and intercropped plants
Number of insect and spider fa mi lie j, and number of individuals of each family associated with sweet potato agroecosystems ..... .
Aspidomorpha sp. (Coleoptera: Chrysomelidae) and Lycosa sp. (Araneae): two population of arthropod species, within the monoculture and intercropping systems
DISCUSSION
60
61
63
65
70
74
5.1. Effects of intercropping on the population densi ty of the sweet potato weevi 1 (SPW). 75
5.1.1. Effects of intercropping on number of SPW and on percentage of damaged tubers
5.1.2. Level of atte.ck by SPW in relation to tuber formation ........... .
75
77
5.2. Number of insect and spider families associated with sweet potato agroecosystems 80
VI. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH 85
LITERATURE CITED 89
APPENDICES
1.
2.
3.
Field layout of the experiment
Cost of production for monoculture and i ntercropped sweet potato ......... .
Insect and spider families associated with the sweet potato cropping systems at 35, 42, 49 and 56 DAP Il .. .. • .. .. • .. .. • .. .. .. .. .. .. .. .. .. .. ..
106
107
108
111
ix
Number
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
LIST OF TABLES
Page
Insect pests of sweet potato in Iri an Jaya .. 8
Alternative hosts of Cylas formicarius (F.) (Coleoptera: Curcul ionidae) . . . . . . .. 15
Life cycle of the :weet potato weevil, Cv1a) f 0 rm i (. '1 ri us (F.) ( Col e 0 pt a ra: Cu r cul ion i da e , on sweet pota to . . . . . . . . . . . . . .. 17
Effect of temperature on development of Cylas formicarius elegantulus (F.) (Coleoptera: Curculionidae) on sweet potato (cv. 'Jewel') 18
Percentage of cul t i vated land under i nter:rops in selected countries . . . . . . . . . . 29
Total amount of nitrogen, phosphorous and pota3sium (kg/ha) applied to sweet potato, corn, soybean, toma to and cabbage . . . . .. 46
Colonization of sweet potato by the sweet potato weevi l (SPW) at 56 DAP ..... 59
Number of sweet potato weevils (SPW) per kilogram of damaged tubers and percentage of damaged tubers . . . . . . . . . . . . . 60
Effects of intercropping sweet potato on number of tubers per plant and on fresh weight of tubers per 10 plants .......... 63
Marketable yield, economic values, cost of production and monetary index of sweet potato and intercropped plants in intercropped sweet potato wi th corn and soybedn . . . . .. 64
Insect and sRider families (fam.) and individuals (ind.) of each family collected a t 35 DAP . . . . . . . . . . . . . . . . .. 65
Insect and sRider families (fam.) and individuals (ino.) Jf each family collected a t 42 DAP . . . . . . . . . . . . . . . . .. 66
x
13.
14.
15.
T
Insect and spider families (fam.) and individuals (ind.) of each ramily collected a t 49 DAP . . . . . . . .. . .. ..
Insect and spider families (fam.) and individuals (ind.) of each ramily collected at 56 DAP . . . . . . . . . . . . . . . . .
Number of spotted tortoise beetles collected dt 56 DAP and spider collected at 35 DAP, 42 DAP, 49 DAP and 56 DAP, in four sweet potato croppi ng sys tems . ....... .
66
67
71
xi
-_ ...
1 .
Number
1.
2.
3.
4.
5 .
6.
7 .
8.
9.
10.
11.
12.
13.
LIST OF FIGURES
Sweet potato intercropped with corn in Kamu valley of the western highland region of Irian Jaya, Indonesia ............... .
Sweet potato grown intercropped with corn, bean, cabbage in Baliem Valley of the eastern highland region of Iri an Jaya, Indonesi a ....... .
Sweet potato grown intercropped with sugar cane, tometo and cabbage in Baliem valley of the eastern highland region of Irian ~aya, Indonesia
Sweet potato grown with taro in Baliem valley of the eastern highllnd r€gion of Irian Jaya, Indonesia .................. .
Adult sweet potato weevil, Cylas formicarius (F.) (Co 1 eoptera: Curcu l i oni dae) . . . . . . . . . .
Head and antennae of adult Cylas formicarius (F.) (Coleoptera: Curcul ionidae) ....
Monthly rainfall recorded at Manggoapi
Page
4
4
5
5
7
21
experimental farm during 1989, and the average of monthly rainfall in Manokwari recorded during a previ ous 40-yea r peri od (1939 to 1979) . . . . .. 42
Arrangement of pitfall traps in each experimental unit . .. ................ 52
Heavily damaged tomato and cabbage intercropped with sweet potato (treatment E) ..•.••.. 56
Sweet potato monocu l ture (treatment A) . . . . . 57
Sweet potato intercropped with corn (treatment B) 57
Sweet potato intercropped with soybean (treatment C) ........... .
Sweet potato intercropped with corn and soybean (0) . . . . . . . . . . . . . .
58
58
xii
14.
15.
16.
17
,
Relationship between number of sweet potato weev ils and damaged tubers ... . . . . .
Number of insect and spider families associated wi th sweet potato croppi ng systems .. . . . .
Number üf spotted tortoise beetle, Aspidomorp-ha sp. (Coleoptera: Cassididae) collected from sweet potato cropping systems at 35, 42, 49 and 56 DAP.
Number of the spiders, Lycosa sp. (Areneae: Lycosidae)., collected from four sweet potato croppi ng systems at 35, 42, 49 and 56 DAP . . . .
62
69
72
73
xiii
CHAPTER 1
INTRODUCTION
$weet potato, Ipomoea batatas (L.) Lam, a earbohydrate
prod uei ng root erop, ran king seventh i I~ wor l d prod lIet l on a fter
wheat, riee, maize, potato, barleJ' and ea;sava (FAO 19~O), 1S a
staple food in many parts of the tropies (Y~n 1974, Onwueme 1978,
Vill areal 1982, Bouwkamp 1985, FAO 1986, NWJnyi 1987). Worldwide,
it supplies 3.9 % of the ealorie intdke and 1.7 % of the protein
for h uman eonsumpt ion (FAO 1986). 1 n eerta in pa rts of the t ropi es ,
where this crop is the only staple food, sweet potato eontributes
approximately 80 % to 90 % of the calorie intake of the population.
This situation, for example, oeeurs in the central highlands of the
the island of New Guinea, which ineludes Papua New Guinea (Bourke
1985, FAO 1986, Hadfield 1989) and the Irian Jaya province of
Indonesia (Oomen et QI. 1961, Ruin~rd 1969, Oomen 1971, Manwan &
Dimyati 1989, Karafir 1989). It is also the staple food in several
South Pacifie Islands, including the Solomon Islands and Tonga
(Onwueme 1978, Bradbury & Holloway 1988, Horton & Ewell 1991), in
the Visayas region of the Philippines (Villareal 1982, Palomar ~J
li. 1989, MaeKay 1989). in sorne A fri can countri es, i ne l ud i ng Rwanda
(Janssens 1982, Alvarez 1987, Horton & Ewell 1991), the Cameroons
(Pfeiffer 1982), Burundi and parts of Uganda (Alverez 1987), and i~
parts of the Carribean (Horton & Ewell 1991), especially during
drought periods (Bouwkamp 1985).
In addition to human food, sweet potato plays a significant
role as animal feed, and as the raw material of industrial starch
and alcohol production (Edmond & Ammerman 1971, Yen 1974, Hahn
1977, FAO 1986, Jansson & Raman 1991).
:2
,\ .. In Irlan Jaya, the indigenous people have cultivated sweet
potato since the crop was first introduced to the island of New
Gui nea in the 14'" century (Yen 1974). Since then, sweet potato has
become an important s tap le food of the i nd i genous people, and of a t
least one livestock species (the pig) in the central highland
region of the island (Rappaport 1984, Halfield 1989). Thus, based
on its role and importance as human food and animal feed, sweet
potato remains central to the agricultural system in Iri an Jaya
(Karaf, r 1989).
Today it is planted extensively, both in lowland and highland
regions, by various local tribes, each employing its own cropping
sys tem. In the 1 owl ands the crop i s pl anted in a mi xtu re wi th
other crops such as corn, taro, tanni a, cassava, sugar cane,
cucumber, banana, and Hibiscus manihot, whereas in areas of cleared
virgin or secondary forest it is planted under a shifting
cultivation system (Karafir 1989). It is also commonly planted in
ga rdens adj dcent to hou ses and vi 11 ages. In the hi gh 1 ands, sweet
potato is planted in fields in the valleys and on hill sides,
either dS a monoculture, or mixed with corn, taro, beans, cabbage,
tOll1ato and sugar cane (Figs.1-4.)
Although sweet potato has been cultivated for many years, the
yield level is usual1y low: less than three tons per hectare in the
lowlands (Ruinard 1969), to three to six tons per hectare in the
highlands (Pospisil 1963). Recent data (Karafir 1989), however,
indicate that sweet potato production in Irian Jaya has increased
to an average of seven tons per hectare.
3
1
l
Fig. 1. Sweet potato intercropped with corn in Kamu vdlley of the western highland region of Irian Jaya, Indonesia.
Fig. 2. Sweet potato grown intercropped with corn, bean, cabbage in Baliem valley of the eastern highland reglon of Irian Jaya, Indonesia.
--,
4
(.
(
f 1~. 3. Sweet patata grawn intercropped with sugar cane, tomdto and cabbage in Baliem valley of the eastern highland r(>~lion of Irian Jaya, Indanesia.
f Iq. 4. Sweet potata growth with taro in Baliem valley of thr eastern highland regian of Irian Jaya, Indonesia.
5
l
T
This level of production is still only 50 % of average world yield
data, which is 14 t/ha (Horton 1988, 1989). As a consequence,
production of sweet potato in Irian Jaya is sometimes insufficlent
to meet the needs of both humans and livestock. For example, the
consumption of sweet potato in the eastern hlghland reglon of Irian
Jaya in 198~ (359 tons) exceeded th~ production of sweet potato
(230 tons) by 56 %. Su ch production shortfalls (Karafir 1989) have
sometimes been followed by local famine (Oomen ~1 Çil. 1961).
Severa l factors l imi t product i on of sweet potata ln 1 rl an
Jaya. These include low sail fertility, a long dry season, the use
of low yip~ding varieties, poor management of cultivation
techniques, and little or no attempts at pest control.
Among the 16 main insect pests of sweet potato in Irian Jaya
(Table 1; Simon Thomas 1962), the sweet potato weevi l (SPW~, CylÇl~
formicarius (F.) (Figure 5), is the most Economically lmportant
pest (Szent-Ivany 1958, van Driest & Ruinard 1960, Lamb 1974). It
damages the sweet potato vi ne and tuber, and occas i ona lly the
foliage, thereby reducing both the yield and quality of the crop
(Sutherl and 1986a).
6
1
~ lq 5. f\dlllt sweet potato weevil, Cy7as formicarius (F.) (Coleoptera: Cllηlllllonidae). (Dr. G.V.H. Jackson, South Pacifie Commision, Noumea).
7
l
r 1 .
Table 1. Insect pests of sweet potato in Irian Jaya
Classification
Col eoptera Curcul iOllidae
Coccinellidae Chrysomelidae
Lepidoptera: Nympha1idae
Sphlngidae
Insect species
Cylas formicarius (F.) Oribius improvidus Mshl. Epilachna si9natipennis Boisd. Meroleptus Clnctor Mshl. Aspidomor~ adhaeren~ Weber A. austr31asiae Boisd. A. punctul11 F. Cassida diomma Boisd. ~ holmgreni multicolor Blackb. ~ papuana Speath ~ strigula F, Laccoptera impressa Blanch
Appias melania F. Precis orjthya F. P. villida Bod. Herse convolvu1i L.
Pa rt a t tacked 1
S, l , T L L S L L L L L L L L
L L L L
Source: Simon Thomas 1962. 1) S = stem; L = leaf; T = tuber
The fo 11 owi ng cont ro l methods have beeil proposed to reduce
y; el d los s cau s e d b yin sec t s : r es i st a n t cul t i var s ( M li 11 en et i!J '-
1985, AVRDC 1987,1988), cultural techniques such as crop rotation
(Re~nhard 1923, Gonzalez 1925, Cockerham et .9.l... 1954, Sherman &
Tamashiro 1954, Kalshoven 1981, AVRDC 1986, 198ï, 1988), removal of
volunteer plants and crop debris from harvested fields (Reinhard
1923, Gonza les 1925, Cockerham et .9.l... 1954), prompt ha rves t i ng
(Sherman & Tamashiro, 1954, Sutherland 1986"), removal of
alternative wild hosts (Cockerham ~t .9.l... 1954, Talekar 1983, AVRDC
1988), planting away from weevil-infected fields (Sherman & Tamashiro 1954, AVRnC 1988), intercropping (Singh et Ql... 1984,
AVRDC 1987), and maintaining soil to avoid cracking through banking
8
and irrigation (Pardales & Cerna 1987, Talekar 1987). Chemical
control wlth insecticides (Sherman & Tamashiro 1954, Wolfenbarger
& Walker 1974, Muruvanda 1985, Schalk & Jones 1985, AVRDC 1987),
and a synthetic sex pheromone (Proshold et ~ 1986) have also been
proposed. Often these methods are combined in an integrated pest
management strategy (Talekar 1988, 1991). Recent developments
include the use of natural control agents such as predators,
pa ras ito i ds, entomopathogen i c fungi, bacteri a, and nematodes
(Jansson 1991"). Among these agents, two entomopathogenic
nematodes, Heterorhabdit i s bacteri ophora strai n 1 HP88", and
Steinernema carpocapsae strain "All", are promising as biological
control agents of SPW (Jansson 1991b).
Based on the agricultural system in Irian Jaya, the farmers'
< ability, availability of control agents and economic limitations,
control of the SPW by cultural techniques is considered to be the
most appropri ate approach, and the one most 1 i kely to be adopted at
the present time.
Application of these control measures, however, should not be
applied directly without taking into account the traditional
agri cul tural systems employed by the farmers (Jansson & Raman
1991). Modlfication of certain cultural techniques would be useful
and appropriate as such modifications would not dramatical ly change
the farlllers' production system. Moreover, the modification of
production techniques does not usually require additional inputs or
fu rther k nowl edge of cu lt i vat ion techn i ques, al though i t may
9
1
y
requi re a better unders tandi ng of the i nsect and the factors
responsible for its pest status (Jansson & Raman 1991).
1 ntercroppi ng sweet potato with one or more crops i 5 one of
the cultural techniques that is already widely practiced in Irian
Jaya (Karafi r 1989). Genera 11 y, th i s pract i ce i ncreases crop
diversity, which provides both barriers to pest dispersal and more
habitats for natural enemies, thereby reducing both colonization of
the crop by pes ts and the; r subsequent cont ro 1 (l i ts i nge r & Moody
1976, Perrin 1977, Hare 1983, Andow 1983, Al tieri & Liebman 1986,
Altieri 1987, Risch 1987).
The si gn i fi cance of i ntercroppi ng in the contro 1 of SPW,
however, is poorly understood (O'Haïr 1991). Preliminary data that
are available from India (Singh et. li. 1984) and Taiwan (AVRDC
1987) indicate that intercropping a10ne is insufficient for
control l ing SPW, and that the level of control varies \'Iith the
i ntercrop spec i (lS th at i s used. For examp 1 e, i ntercropp i ng swee t
potato wi th proso-mi 11 et and ses ame in 1 ndi a reduced i nfes tat ion by
SPW to 9 % and 6 %, respectively, compared with 28 cy" in sweet
potato monoculture (Singh et li. 1984). Similarly, in Taiwan,
i ntercroppi ng sweet potato wi th ch i ckpea, cori ander, pumpk in,
radish, fennel, blackgram or yardlong bean also significantly
reduced the 1 eve 1 of i nfes tat ion (AVRDC 1987). When sweet pota to
was i ntercropped wi th a number of other crops, such as green gram
(Singh et li. 1984, AVRDC 1987), cabbage, peanut, and corn (AVRDC
1987), reduct ion in i nfes ta t ion by the SPW to 12 % from 20 % was
st i 11 cons i dered to be unacceptab 1 e. The ex tent to wh i cil
10
i ntercroppi ng can reduce i nfes tati on of the sweet potato by SPW and
other i nsect pes ts in 1 ri an Jaya was unknown pri or to the present
resea rch.
ln an attempt to obtain an appropiate method for controlling
SPW among t T'ad; t i ona l sweet potato farmers ; n 1 r; an Jaya, th; s
resea rch program was des i gned to determi ne (1), the effect; veness
of in tercroppi ng of sweet potato wi th corn, soybean, tomato and
cabbage in reduc i ng crop damage, (2), the popu lat; on dens i ty of
SPW a t ha rvest, and (3), the d; vers; ty of i nsects and other
arthropods in sweet potato agroecosystems.
Becaus eth; s resea rch was des; gned to be support ive of sma 11
sca le sweet pota to farmers, the ; ntercropp; ng sys tems tested were
chosen in relation to the farmers' traditional practices.
11
. A
T
CHAPT ER II
LITERATURE REVIEW
2.1.
2.1.1.
BIOECOLOGY OF THE SWEET POTATO WEEVIL r Cylas formicarius (F.) COLEOPTERA: CURCULIONIDAE)
Taxonomie status and distribution
Latreille in 1802 was the first entomologist to use the genus
WA~_ (Neave 1939) for deseribing weevils of the Cyladinae that
have the following characteristies: (a) slender, elongate bOdy with
a eylindrical beak, (b) posterior femora not as a rule exceeding
the tip of the elytra, and (3) the elytra not inflated (Pierce
1940, Subramani an 1957).
The genus Cyl as contai ns 27 spec i es (Scha 1 k & Jones 1985,
Austin et gl. 1991). Of these, h formicarius (Fabricius), h
tureipennis Boheman,_~ brunneus (Fabricius), ~ femoralis Faust,
and ~ puncticollis Boheman, are associated exclusively with sweet
potato (Pierce 1918, 1940, Austin et li. 1991).
A recent taxonomie and distributional study, however,
indicated that there may be as many as nine species of Cylas that
are potential pests of sweet potato. These nine species are
classified into three monophyletic species groups: ~ formicarius,
~ brunneus, Ç..... punctieollis (Wolfe 1988, 1991). Of the above
three main speeies groups, Cylas formiearius and ~ punctieolis are
the most widely distributed. h formicarius oecurs in Africa, the
Americas ~nd Asia, whereas ~ puncticollis occurs only in certain
African countries (Cor.:monwealth Institute of Entomology 1970, <;ingh
1977). ~2 brunneus may have a similar status and distribution
in West Africa as ~ puncticollis (Hahn et li. 1989).
13
Cylas formicarius was first described by Fabricius (1798) as
Brentus formicarius. This was based on specimens from India that
had a piceous-brown body, with a reddish thorax (Pierce 1918). His
description of this species is as follows:
"Habitat Tranquebariae. Parvus in hoc genere. Rostrum cylindricum, atrum antennis rufis, moniliformibus: articulo ultimo longiori, cylindrico, clavato. Thorax rufus, antice globosus. Elytra laevia, atra, nitida. Pedes rufi, femoribus cl a vat i s, a tin e rm i bus: a r. nul 0 n i gr 0" ( Pie r ce 19 18 )
This species seemed not to be the same as the common sweet
potato weevil found in the USA that has shiny blue-black elytra,
red thorax and appendages, and a black head and beak. For that
reason, Summers (1875) named the American species ~ elegantulus to
distinguish it from the Asian species (Pierce 1918).
The name was not, however, applied, partly out of respect to
Fabricius (Pierce 1918). But, based on the color of the elytra
(greenish for the Asian species and bluish for the American
species, Pierce 1918), and karyological differences in the sex
determi ni ng system (Hung 1985), two sub-speci es of Ç-Yl~s.
formicarius have now been recognized:
formicarius (F.), which is widespread throughout the Asian tropics,
and Cylas formicarius elegantulus (Summers), which occurs in
t ropi ca land s ub-trop i ca 1 reg i ons of the Ameri cas (Suther land
1986a , Talekar 1988). In subsequent chapters, unless otherwise
indicated, the use of .c. formicarius refers to .c. formiçariJ!.s.
formicarius, the Asian sub-species.
14
(
2.1.2. Host range
The prefered host for ç. formi cari us i s the sweet potato,
l~oJl!oeQ 12a ta tas (Cockerham 1943, Sherman & Tamas h i ro 1954, Hi 11
1983, Talekar 1989, Austin et M. 1991), although this insect also
a ttacks other Ipomoea and related species (Table 2).
Ta b l e 2. A 1t e r n a t ive ho s t s 0 f Cyl as f 0 rm i car jus ( F . ) (Coleoptera: Curculionidae)
Alternative host
Calonyction aculeata Çlll.Y~:teill sol dane 11 a IRom9.~Q batatas J. ararica T. IjarreTri oi des t. ço.nge~tQ 1. di ssecta r. heder-a-cea t. h'~iif~phyrLQ I. lacunosa f. lateraTT s 1. rëari,-T. rlTtoralis 1. rnacrorliTZa r. muri cata 1. palmata L Qalldura_ta 1. Qres-caprae J. Qurpuria 1. guam~cl i t 1. sa_9ittatQ t . ~~pj_QLiA 1. s.etosil
J. :trj.ç_~oca rpa I. trilobia 1. fTlpTda IJ2orn-oe,i ~. J_ÇlQ.uemontia tamnifolia ThJJllbe"'iLi a ~.
Source: Sutherland 1986d•
Common name
Moon-fl ower Bi ndweed Sweet potato
Blue morning glory A l arno-v; ne Engl i sh Ivy Bush morning 910ry Diminute mornlng glory
Blue dawn-flower
Man-of-the-earth Beach morning-glory Common morning-glory Cypress vi ne
Brazi1ian morningglory
Clock vi ne
Country
Indi a Taiwan Worl dwi de Indi a Indi a Papua New Guinea U.S.A. U.S.A. U.S.A. U.S.A. India. India. U.S.A. U.S.A. U.S.A. Indi a U. S. A Indi a Indi a U. S.A U.S.A. Indi a
U.S.A. U.S.A/lndia Philipp.!India Indi a U. S.A/Taiwan Indi a Indi a
15
r
2. 1. 3 • li f e eye le
Research on the l i fe cyc le of SPW has been rev i ewed by
Sutherland (1986a; Table 3). Apparently it varies from one
location to another, probably mainly in relation to changes in
temperature (Mull en 1981, Sutherl and 1986a). At low temperatures,
fecundity ;s higher and the life cycle is longer (Table 4; Mullen
1981). Thus, these properties vary with the season (Gonzales 1925,
Rajamma 1983).
The optimum temperature for development is 27°C to 30 llC, wh en
the life cycle is completed in ca. 33 d. At 27°C and 60 % RH the
adul t weevi l lives for 94 d (Mu 11 en 1981).
a. Egg
! The adult female lays cream colored eggs (0.75 x 0.40 mm),
T
singly in a cavity in vines or tubers (Reinhard 1923, Sutherland
1986a) • After l ayi ng each egg, she sea l s the ca v ity wi th a grey
fecal plug. This conserves moisture, protects the eggs from
predacious mites, and "hides" the location of the oviposition site
( S h e rm a n & T am a 5 h i r 0 1954).
Recorded oviposition rates differ from one geographic area to
another in re lat ion to the temperatu re of the regi on. For examp le,
in India, Rajamma (1983) found that females lay 1 to 9 eggs per
day, wi th an average of 3 to 5 eggs. In the USA, however, r~ei nhard
( 1923) repo rted an ovi pos i t i on rate of up to 2 eggs per day.
16
Table 3. Life cycle of the sweet potato weevil, Cylas formicarius (F.) Coleoptera: Curculionidae), on sweet potato
Stage
Temperature (OC)
Egg (d)
Larva (d)
No. of instars
Pre-pupa (d)
Pupa (d)
Pre-oviposition (d)
Oviposition (d)
No. of eggs
Longev. female (d)
Longev. male (d)
Egg to egg (d)
Reinhard Sherman & Kemner
(1923) USN
28
5-11
20
1-3
15
6-9
104
56
53
Tamashiro (1954 )
Hawai il
27
8
15
3
4
8
32
(l924 ) Indone-si aZ
5-9
25-26
6-7
7-9
43-51
Gonzales
(1925) Philippines2
6-9
4-6
63-120
256
26-52
Trehan & Subramanian Bagal (1957) (1959) Jndi a Indi a2
6
17
7
83
83
31
6
24
5
1-2
10
7
80
148
94
110
47
Source: Sutherland 1986". l) Cylas formicarius eleçantulus (Summers) Z) Cyl as formi cari us formi cari us (F.) - = data unavailable
Jayaranaiah Rajamma
(1975) (1983) India2 India2
9
28
5
7
90
166
46
6
16
4
8
83
30-36
17
l
T À
Stage
Egg
Larva
Pupa
Tabl e 4. Effect of temperature on development of Cylas formjcorius e7egafitulus (Summers) (Coleoptera: Curculianidae) on sweet potato (cv.'Jewel')
Average durat i on (d) of deve l opment stages of the SPW at di Herent temperatures
7.9 5.7 4.8 4.0
58.2 23.7 16.3 16.2
10.7 5.0 5.5 8.6
Pre-ovipos i ti on 7.7 6.5 6.3 4.5
Egg ta egg 84.5 40.9 32.9 33.3
Source: Mullen 1981.
The egg stage 1 asts 4 to 8 d (Sutherl and 1986"). Over the 60 to
120-day ovi pos; t i on peri od, the total number of eggs 1 ai d ranges
from 50 to over 250.
b. larva
The newl y hatched 1 a rva has a de 1 i ca te appea rance and i s
initially white in color (Reinhard 1923, Gonzales 1925, Cockerhan
et QI. 1954, Trehan & Bagal 1957). As it matures its body darkens
(Reinhard 1923, Cockerhan et QI. 1954) and becomes slightly curved
(Trehan & Baga l 1957, Sutherl and 1986a).
Its body si ze i ncreases through the four 1 arva 1 i ns tars from
< 1 mm a t h a t chi n 9 t 0 u p t 0 8 . 5 mm dur i n g the fin a 1 i n s t a r
(Re; nha rd 1923, Cockerhan et QI. 1954, Trehan & Baga 1 1957).
18
c. Pupa
The flnal larval stage excavates a pupal cavity , measuring
two or three times the size of its body, inside the tuber (Reinhard
1923, Cockerhan et .li. 1954, Sherman & Tamashiro 1954, Sutherland
1986") . 1 t then 5 tops feed i ng and becomes qu i escen t for one or
more days before pupating (Cockerhan et QI. 1954).
The first external indication of pupation is the splitting of
the head capsul e of the prepupa between the rudimentary antennae
and the skin of the dorsal thoracic region (Reinhard 1923,
Cockerhan et QI. 1954).
The pupa usually remains motionless, but if disturbed it makes
a circular twisting movement of the abdomen, and sometimes turns
over (Reinhard 1923, Cockerhan et QI. 1954).
At first the pupa is white (Reinhard 1923, Trehan & Bagal
1957), and later it becomes yellowish. It gradually darkens, pr;or
to transformation to the adult (Reinhard 1923).
lhe pupa is ca. 5 mm long by 1.5 mm wide (Reinhard 1923,
Sutherland 1986d). The length of the pupal period varies with
temperature, from 5 to Il d (Table 4; Mullen 1981).
d. Adult
At the end of the pupal stage, the pupal skin splits down the
back begi nni ng near the head. The new adul t pull s i ts head and
th en its legs out of the old skin. As soon as the legs become hard
they are used to push the skin off the rear of the body.
19
t j ~
The partially exposed hind wings are wrinkled at first, but,
after a short period of time, body fluids flow into them, expanding
them to their full length beyond the elytra, in which position they
rema in unt i l ha rdened. The wi ngs are then fo 1 ded in the norma 1
position under the elytra.
The newl y t rans formed adu 1 t i s al mos t wh He and ra ther
helpless. A min'imum of 4 d is required before it is able to eut a
passageway to the surface of the potato and emerge.
The adult weevil (Asian subspecies) is black ln color with ~
reddi sh brown prothorax (Gonzales 1925, Trehan & Bagal 1957,
Rajamma 1983, Sutherland 1986a). The elytra and head are black and
the 1 egs are redd i sh-brown and b lac k (Ka l s hoven 1981, Sutherland
1986a) •
T Body size and antennal structure differ in males and females. l
,. 1 1
The adult female is usually smaller than the male ( 5.8 x 1.5 mm
compared with 6.1 x 1.6 mm, for the male; Rajamma 1983). Body
size, however, is not a reliable criterion for sex determination,
since it varies significantly in nature (Gonzales 1925, Trehan &
Bagal 1957).
The sexes can be separated reliably on the basis of
di fferences in the si ze and shape of the lOlh anten n a 1 segment
(Fig. 6).
20
1
(a) ( b.)
Fi gu re 6. Head and an tenna of 'âdu 1 t ~las formi ca ri us (F.) (Coleoptera: Curculionidae). (a) femaTeand (b) male. Sou rce: Su therl and 1986".
f
21
, •
.,
2.1.4. Feeding habits and crop damage
Adult weevils feed on the exposed parts of the sweet potato
plant, including the foliage, vine, stem, and tuber (Reinhard 1923,
Gonzales 1925, Cockerham et Ql. 1954, Trehan & Bagal 1957,
Kalshoven 1981, Rajamma 1983, Sutherland 1986"), although the tuber
i s the preferred food source (Rei n hard 1923, Gonza les 1925,
Cockerham et QI. 1954).
The adult weevil feeds on the tuber surface, parti~ularly if
it is shaded (Reinhard 1923). On tubers, the damage appears as
patches of shallow feeding punctures (Reinhard 1923). Females also
deposit eggs, which are usually covered with frass, on the tubers.
On the vine, the adult weevil feeds by gnawing rather than by
making distinct punctures; on the stems, petioles and leaf veins
the feeding scars often run together or overlap (Cockerham ~1 SLl.
1954).
The larvae feed inside the tubers and underground inside the
lower portions of the stems by tunnelling into them (Trehan & Bagal
1957, Cockerham et. gl. 1954, Rajamma 1983). The tunnels inside the
tubers follow a zig-zag pattern (Reinhard 1923, Rajamma 1983).
The tunnels are usually closed with excreta or remains of food
materials (Rajamma 1983); feeding rarely occurs in open tunnels.
The level of damage that results depends on the parts of the
plant that are attacked. Damage to above ground stems, vines and
foliage is usually not significant, in contrast to underground
damage, especially to the tuber, which can be devastating. The
greenish black color and bitter taste of infested tubers containing
22
terpenoids (Akazawa et ~ 1960, Uritani et g~ 1975, Sato et ~
1977) make them unfit for human and animal consumption (Rajamma
1983, Raman 1989).
In addition to attacking tubers in the field, losses of sweet
potatoes in storage to the SPW are also significant (Rajamma 1983,
Raman 1989).
The level of damage both in the field and in storage ranges
from 5 % to 90 % (Sutherland 1986a, Raman 1989). Loss of production
was found to be 10 % to 20 % in Hawaii (Sherman & Tamashiro 1954),
60 % in Papua New Guinea (Szent-Ivany 1958), 60 % to 70 % in
Malaysia (Ho 1970), 16% to 80 % in India (Rajamma 1983), 12 % to
90 % in Africa (Alverez 1987) and 5 % to 20 % in China (Lu et gl.
1989). Therefore, 10s5 of yield caused by the SPW can be one of
the ma in l imi ts on product i on (Raman 1989, Horton & Ewell 1991).
2.1.5. Reproduction
Sexual attraction in SPW has been claimed to be poorly
deve l oped, si nce in the l aboratory ma 1 es were found to show no
response to the presence of females (Reinhard 1923). Recent
s tud i es, hO\'Iever, have shown tha t the ma l es are a t t racted to sex
pheromones released by the females (Nottingham et gl. 1986). These
pheromones are released only when the females have found and fed
upon an appropriate hosto
Copulation takes place after both sexes have fed on a tuber;
it occurs several times (Reinhard 1923). During copulation, the
weevils remain relatively motionless. If disturbed, however, the
23
female immediately begins to crawl, either carrying the male with
her or separating (Reinhard 1923).
At first the eggs are laid inside the vines, and then, when
the tubers develop, inside them (Reinhard 1923). The female can
l ay eggs in a 11 parts of the tuber, in especi a 11 y prepared
cavities. The egg cavities are usually wider, but shallower than
the feeding punctures, and are oriented obliquely.
After digging out the cavity, the female turns around and
inserts the tip of the abdomen, which moves from side to side.
Eventually the ovipositor is protruded into the cavity and an egg
is laid (Reinhard 1923). Adults mate within 3 to 5 d of emerging
(Subramanian 1959) and after feeding (Reinhard 1923).
2.1.6. Factors affecting infestation by SPW
Infestation of sweet potato crops by SPW is affected direct1y
and i ndi rect l y by the age of plants, type of soi l , season,
especialy rain fall, elevation, source of the weevi 1s, and the
cultural techniques being used.
The age of plants influences the level of infestation by SPW
(Q'Hair 1991). This reaches a peak at the same time as storage
root formation and development, which starts as early as 28 DAP and
reachs its peak between 56 and 84 DAP (Wilson & Lowe 1973, Wilson,
1982).
Soils with a higher clay content tend to shrink when dry and
form cracks through which weevils can enter and reach underground
tubers (Hahn & Leuschner 1982, Eusebio 1983, Q'Hare 1991). When
24
thlS occurs, high infestations are common, as has been documented
in Papua New Guinee (Bourke 1985). Soil pH also affects the level
of SPW infestation, high weevil infestation being associated with
high soil acidity (pH 4.6 to 5.5), and low infestation with low
soil acidity (pH 8.6 to 9.5; Abella 1982). Thus, applying lime to
adJust soil pH to approximately neutral is often suggested as part
of a weevil control program.
Infestation by SPW is claimed to be low at high altitudes,
since the lower temperatures slow the developmental rate of SPW
(Eusebio 1983); however, the situation may be reversed if there is
a long dry season. Consequent 1 y, h; gh damage to tubers us ua 11 y
occurs in highland regions during longer th an usual dry seasons.
For examp le, the dry season of 1954 in Bena-Bena, Goroka, and
Chimbu subdistricts of the eastern highland region of Papua New
Guinea, coincided with devastation of the sweet potato crop, and
resulted in food shortages (Zsent-Ivany 1958, Anas 1960).
Infestation may also be a symptom of poor farm practices, such
as a 11 owi ng the s pread of the SPW from a prey i ous crop, from
adjacent alternative hosts, or from infected planting materials.
It is also affected by poor land preparation, failure to hill-up
the plants, and by late harvesting (Franssen 1934, Kalshoven 1981,
O'Hare 1991).
25
1
1
2.2. ROLE OF INTERCROPPING IN FOOD PRODUCTION
2.2.1. Definitions
Intercropping is a form of multiple cropping in which two or
more crops are planted simu1taneous1y in the same field (Andrews & Kassam 1976). Thus, cropping is intensified in both time and
space, as is inter-specific competition (Andrew & Kassam 1976, Roy
& Braun 1983, Gomez & Gomez 1983, Francis 1986). Consequently,
ach i ev i ng success requ ires a hi gher l eve l of management ski 11 s than
in a monocropping system.
Intercropping may be classified into mixed intercropping (no
distinct rows), row intercropping (single rows), strip
intercropping (several rows) and relay intercropping (overlapping
in time) (Andrews & Kassam 1976, Roy & Braun 1983, Gomez & Gomez
1983, Francis 1986).
Intercropping is commonly practiced by subsistence farmers in
tropical developing countries, at all levels of agricultural
technology (Andrews & Kassam 1976, Gomez & Gomez 1983)
Traditionally, two or more crops are grown on the same field,
primarily to achieve optimal use of space, diversify the range of
products, and reduce risk in the face of a crop failure (Gomez &
Gomez 1983).
Today, with the increasing 1055 of agricu1tural land and soi1
degradation, intercropping is being recognized as a way to enhance
efficiency and conserve soi1 (Gomez & Gomez 1983).
26
~~--J
2.2.2. Status
1 ntercropp i ng has long been recogn i zed as a way in whi ch
fa rmers in the t ropi cs and s ubt ropi cs, wi th li mi ted land resources,
can more effciently and economically produce food and cash crops
(Beets 1982, Kass 1978, Willey 1979, Roy & Braun 1982). It now
plays a significant role in the production of staple crops in
Africa (Okigbo & Greenland 1976, Steiner 1982), Latin America, and
Asia (Harwood & Price 1976, Gomez & Gomez 1983).
The significance of intercropping in developing countries is
appa rent from the large a rea of land that i s devoted to thi s
cropping system. In Africa, for example, almost 80 % of cultivated
land is intercropped (Steiner 1984), and this is probably also the
case for much of Latin America and Asia, where most staple crops
ft are produced in intercropping systems.
The type of i ntercroppi ng and s peci es used vari es wi th
geography and culture. For example, 98 % of cowpeas in Africa, and
more than 60 % of maize and beans in Latin America, are grown in
crop mixtures (Francis et M. 1976); whereas in Asia, especially
India, 5 to 6 % of rice, and 70 to 80 % of other crops, are grown
as mi x tures (Kas s 1978) . The commones t i ntercropped plants in
Asia include upland rice, sorghum, millet, maize, rainfed wheat,
and soybean.
Even though intercropping is prevalent in tropical areas where
farm5 are 5mall and farmers lack capital (Roy & Braun 1982, Liebman
1987), sorne farmers in temperate regions who have large farms and
.1df'quate capital are starting to practice intercropping, for a
27
, ,
1
l
range of reasons. Such farmers may be using intercropping to solve
problems of soil depletion and contamination (Poincelot 1986),
which are often associated with long periods of monoculture
agriculture.
Monocul ture agri cul ture i s characteri zed by a heavy dependence
on petrochemi ca l energy for opera t i ng fa rm mach i nery, and on
synthetic fertilizers and pesticides (Hill & Ramsay 1977).
Extens ive use of energy has i ncreased food product ion (Horwi th
1985, Power & Follett 1987), but it has also resulted in changes in
farrning rnethods (Horwith 1985, Power & Follett 1987, Altieri 1987,
Poincelot 1986), and a decrease in energy efficiency (Hill &
Rams ay, 1977).
Monocult~re tends to degrade the environment by accelerating
soil erosion, increasing the potential for depleting or degrading
ground water resources, reducing the quality of surface ~ater, and
using up fossil energy resources (Power & Follett 1987). It also
causes hea 1 th hazards and pest prob 1 ems as a resul t of the wi de use
of pesticides and fertilizers (Poincelot 1986), and supplementary
fertilization may be unable to compensate for the drop in yield
(Power & Follett 1987).
As 3. resu 1 t of energy rel ated prob lems, i ne l ud i ng i ncreased
costs, environrnental damage and health hazards, sorne farrners in
developed countries have become rnotivated to practice
i ntercroppi ng.
Estimation of the percentage of crops that are planted ~~
intercrops in various countries are given in Table 5.
28
Table 5. Percent age of cultivated land under intercrops in selected countries
Country
Dominican Rep. El Salvador Jamalca Mexico Braz il
Paraguay
Venezue la
Col umbi a Guatema 1 a Bhutan 1 ndones i a
Pakistan
Cenl.African Rep.
Senegal
Nigeria
Uganda
Malawi
Main crop
Maize Maize Maize Maize Rice Maize Bean Beans Sweet potato Maize Rice Maize Bean Cassava Cotton Bean Bean Potato Maize Rice Wheat Barley Cotton Cotton Coffee Cassava Groudnuts Mi 11 et Cowpeas Groundnut Melon Mi llet Cocoyam Cotton Ma ize Maize Bean Pi geon peas Coffee Cowpeas Goundnut Ma l awi
% Intercropped
> 40%
50 % 20 % 6 %
11% 80 % 33 % 10 % 10 % 16 % 33 % 20 % 20 % 50 % 90 % 73 % 40 %
25 % 33 % 20 % 25 % 25 % 99 % 95 % 93 % 90 % 86 % 80 % 76 % 84 % 81 % 76 % 63 % 62 % 56 % 90 %
Intercropl
b,s, * b, s, ')t'
* * * * * * * * * * * * * * *
m,b * * * * * -t
* * * * * * * * * * * * * * * * m
21) b = bean; s= sorghum; m= maize; * other crops
) 1 = FAO, 1973; 2. Francis, 1986; 3. Okigbo & Greenland, 1976; 4. Francis et al., 1976; 5. Os;ru, 1982; 6. Edje, 1982; 7. Roder et al. 1992; 8. Coaker 1990.
References2
1 1 1 1 1 1 4 1 1 1 1 1 1 1 1
4,8 4 7 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 3
3,5 3 3 3 3
6,8
29
2.2.3. Advantanges and disadvantanges
The advantanges of intercropping that are likely to appedl to
subsistence farmers include: the possibility of obtaining a higher
yield, reduced risk of total crop failure, enhanced resource use,
reduced fertilizer and pesticide requirement, absorption of excess
farm labor, and improved nutrition (Gliessman 1985, Vandermeer
1990).
The posibility of achieving a greater yield from the limited
area available is often the main reason why more and more farmers
are choosing to ir1tercrop (Kass 1978, Liebman 1987, Gliessman
1985). Total yield per area is usually greater in intercropped
systems, even though the individual species invariably yield more
in monocultures (Trenbath 1974, Hardwood & Price 1976, Willey 1979,
f Gliessman 1985). The greater total yield in intercropping OCCt!rs
if the relative yield total (RYT), which is the sum of the
intercropped yield divided by yields of monoculture crops, is
greater than 1.0 (Gliessman, 1985).
T 1
Not a 11 the i ntercroppi ng sys tems, however, ach i eve RYT' s
greater than 1.0. Trenbath (1986), for example found that among
572 comparisons of crop mixtures, only 66 % had RYT's close to 1.0,
i n d i ca tin 9 no dis tin c t a d van t age t 0 the sem i x t ure sin te rm s 0 f
yield. On the other hand, 20 % of the mixtures had RYT's greater
th an 1.0, ranging up to 1.7, indicating a distinct advantage over
the monocultures. Only 14 % had RYT's less than 1.0, indicating
distinct disadvantages.
30
1 n add i t ion to yi el d advantages, i ntercroppi ng may reduce
economic loss because of the failure of a particular crop within
the system. Even wh en the RYT is close to 1 (e.g., shows no
i ne reased yi el d ove r the monocu l ture), the other benefi ts of
intercropplng, such as lower energy costs and less pest problems,
will serve to make intercropping competitive with the monoculture
system (Harwood & Price 1976, Willey 1979, Horwith 1985, Liebman
1987, Gliessman 1985).
Ava i l ab le li ght, wa ter and nut ri ent resou rces maya l so be used
more efficiently in intercropping systems (Liebman 1987). Thus,
because total densities are usually higher in such systems, more
light can be intercepted early in the growing season. This has
been demons t ra ted fo r mi xtu res of ma i ze and mungbean, peanut or
sweet potato (Liebman 1987). Moreover, because intercropped plants
have non-synchronous patterns of canopy development and different
matu rat ion t imes, the l eaf a rea produced over the growi ng season i s
greater and therefore abl e to i ntercept more li ght than are
monocultures (Liebman 1987).
One outcome of the i ncreased canopy cover i s that a greater
proportion of available soil water is channelled through the crops
as transpiration, rather than being lost as evaporation from the
soi 1 surf ace. Futhermore, i ncreased canopy coverage can al so
increase penetration of rainfall into the soil, and decrease soil
erosion by lessening the impact of rain and wind on the soil
surface. This has been documented, for example, with maize
intercropped with cassava (Lal 1986, 1989).
31
T ,
Intercropping of plants with different rooting patterns
permi ts greater exp loi tat i on of a l arger vo l ume of soi land
impro"es access to relatively immobile nutrients. As a result,
i ntercropped pl ants tend to absorb more nutrients th an those in
monocultures (Horwith 1985, Liebman 1987).
In addition, ir.tercropping with legumes may enhance nutrient
avai labi l ity for the non-legume crop, e.g., maize with soybean,
cowpea or mungbean. The l egumes may prov i de bath add i t i ona l
nitrogen through theirmutualistic association with nitragen-fixing
bacteri a such as Rh i zobi um (Ni co l 1935, Gomez & lands t ra 1977,
Horwi th 1985) , and phasphorous thraugh thei r mutua 1 i st i c
as soci at ion wi th ves i cu la r arbuscu 1 a r mycorrhi za l (VAM) fung i
(Hetrick 1984). The mutualistic associations, especial1y those
involving VAM, may also occur with non-legume intercrops
(Ch i ari e 11 0 et QI, 1982). Such rel at i onsh i p can reduce the need
for imported nutri ents, whether as manures or synthet i c
fertilizers.
Intercropping may increase or reduce pest populations.
Reduction mechanisms include the provision of physical barriers to
the pest's ability to find suitable hosts (Litsinger & Maody 1976,
Perrin 1977); the production of chemicals that disrupt the
searching behaviour of the pest and pravide associational
resistance (Perrin 1977); and the provision of shelter and
alternative food sources for predators and parasites (Litsinger & Moody 1976, Perrin 1977, Horwith 1985, Liebman 1987). By taking up
excess available nutrients, especially nitrogen, intercropping may
32
prevent the main crop from becoming more attractive to pests, such
as through the accumulation of free amine acids and sugars in the
plant tissue.
Intercropping can affect the deve10prnent of diseases,
nematodes and weeds (Litsinger & Moody 1976, Liebman 1987). Thus,
a susceptible crop planted between a resistant crop may be
protected from a di sease by the i ntercept i on of the i nocu l um
(Liebrnan 1987). Also, the microclimate provided by the intercrop
may be less or more favorable for disease development (Litsinger & Moody 1976, Li ebman 1987). Sorne crops are known to excrete
substances that are toxi c to nematodes, thereby 1 oweri ng the
incidence of infection in susceptible hosts (Liebman 1987).
The more complete canopy and plant cover associ ated wi th
intercropping is a1so an effective way to control weeds (Litsinger
& Moody 1976, Li ebman 1987). 1 fi these ways, i ntercroppi ng can
reduce the need for pesticides in crop production, and so reduce
production costs.
Because intercropping requires more labor and management than
monoculture (Andow 1983, Gomez & Gomez 1983, Liebman 1987), it can
dbsorb exces s fa rm l abor (Gomez & Gomez 1983). Consequent 1 y,
intercropping is like1y to be most profitable in labor intensive
production systems (Andow 1983, Hare 1986; Liebrnan 1987).
There are, however, sorne di sadvantanges. These i nc 1 ude yi el d
reduct i on of the Ina in crop, loss of product i vi ty duri ng drought
periods, and high labor inputs in regions where labour is scarce
and expensive (G1iessman 1985).
33
T
j
l
T
It is we11 documented that in most cases the main crop in an
i ntercroppi ng system wi 11 not reach as hi gh a yi el d as in il
monoeu1 ture, beeause there i s compet i t ion among i ntercropped
plants for light, soil nutrients and water (Willey 1979, Fordham
1983, Gliessman 1985). This yield reduction may be economieally
significant if the main crop has a higher market priee than the
other intereropped plants.
Another disadvantange that is like1y to oeeur is the higher
cost of maintainance, in partieular, weeding, which may have to be
done by hand. Th i sis not a seri ous prob lem in count ri es where
excess farm labor ;s cheap, but for eountr;es laeking such a labour
force, intereropping will result in inereased costs. Furthermore,
harvesting of one crop may cause daw.age to the others (Gliessman
1985). Finally, the increased canopy cover may result in a
microelimate with a higher relative humidity conducive to disease
outbreaks, especially of fungal pathogens (Gliessman 1985).
2.3. EFFECTS OF INTERCROPPING ON INSECT PESTS
Intereropping can result in a signifieant reduction of insect
pest problems within agroecosystems (Altieri & Letourneau 1982,
1984, Cromatie 1983, Perrin 1977, Altieri & Liebman 1986). This
occurs because intercropping may disturb the insect's activities
and deve l opment, make the hos t 1 es s av a il ab le, and en hance the
development of the pest's natural enemies.
34
The activities that are affected include the rate of
colonization, movement, and development (Perrin 1977, Altieri &
liebman 1986). The main ways in which intercropping has
significant effects on insect populations are explained below.
2.3.1. Rate of colonization
Intercropping affects the rate of insect colonization by
disturbing the visual and olfactory responses that are employed by
many insects in searching for suitable host plants (Cromatie 1983,
Ferro 1987). As a result, they do not easily recognize and locate
suitable hosts that are dispersed amongst other vegetation
(Cromatie 1983, Kareiva 1983).
Colonization of large, closely spaced fields of the same crop
is likely to be more efficient than if the fields are small and
widely spaced. Thus, colonization by insects may be less intense
when the agroecosystem contains a relatively low proportion of food
useable by the particular insect pest (Cromatie 1983).
a. Visual effects
In an intercropping system, host plants are usually scattered
among other crop plants, 50 that the plant is camouflaged by the
non -hos t crop (Perri n 1977, Croma t i e 1983). Conseq uent l y, for a
pest that is flying over the field, intercropping makes host
recognition more difficult (Perrin 1977).
35
... b. Olfactory effects
Intercroppi ng of host and non-host plants may produce a
mixture of odors that fill the air and 50 mask the smell of host
plants and disorient insect pests as they attempt to locate their
hosts (Perrin 1977, Cromatie 1983). As a result, intercropping
makes it more difficult for insects ta find their ho~~ plant, and
50 results in less plant damage. For example, when cabbages are
in tercropped wi th tomato, they are somet i mes protected aga lrlS t fl ea
beetle infestation (Burroughs 1982). Also, certain intercropped
species release chemicals that reppl and antagonize insect pests of
other crops. For example, the diamond-back math, Plutella
xylostella, causes less damage on cabbage intercropped with tomato,
as the tomato odors repel the math and sa reduce colonization on
1 the cabbage crop (Buranday & Raros 1975).
T l
Sorne insect species, however, may be attracted ta the mixed
odor produced by part i cu 1 a r combi na tians of i ntercropped plants
(Kayumbo 1976). Therefore, to avoid the undesired result of
in creas i ng i nsect pes ts as ares u 1 t of i ntercroppi ng, ca refu l
consideration must be given in selecting the species to be
intercropped (B~rroughs 1982).
c. Diversionary host effects
In intercropping, the combination of crop plants may shift
insect feeding to the more tolerant or less valuable crop, or the
pest may colonize one particular crop in a mixture, thereby
36
protectl ng and reduci ng the feeding damage to the more economically
valuablc crops, which may be more susceptible (Perrin 1977,
Cromatie 1983). In this way, economic losses can be decreased.
The intercropped plants may function as divers;onary hosts
for a particular pest (Cromatie 1983). This may involve careful
timing of planting so that the particular growth stage of the
intercrop that is most attractive to the pest is present at the
time when the main crop is most susceptible (Perrin 1977).
2.3.2. Deve10pment
The degree of shadi ng and the nature of cul tura l practi ces
often differ between multiple and monoculture systems, and this
us Uà 11 y a ffects the c rop mi cro-c l i mate, whi ch may become l ess or
more favourab le for a part i cu l ar i nsect pest (Suryatna & Harwood
1976). Also, the confusing olfactory and visual stimuli received
from hos t sand non-hos ts may dis rupt norma l feed i ng and mati ng
behaviours (Tahvanainen & Root 1972). In addition, the pests that
dre associated with a particular crop combination m;ght disperse
el sewhere because of the low qua l i ty of food obta i ned from the
intercropped plants This may interfere with the insects' growth
and development (Kare;va 1983), and, as a result, the population
of i nsect s, as we 11 as the crop damaged, will be low .
37
, , 1
2.3.3. Dispersal
Intercropp i ng affects the movement of both adu l t and 1 arv a l
stages of i nsect pes ts because i t may provi de a phys i ca l barri er
that prevents their dispersal (Perrin 1977, Cromatie 1983). For
example, tall i ntercrops grown as rows between shorter crops may,
by reduc i ng air fl ow, cause more i nsect pests to set t 1 e thù (J if the
air flow were uni nterupted (Lits i nger & Moody 1976). Moreover, as
a physical barrier, intercropping may be valuable in reducing
colonization, thus preventing high infestation and crop damage
(Cromati e 1983). For exampl e, the cabbage rootfly, Del; a DJtkMm,
can be i mpeded from 1 ayi ng i ts eggs in fi e"1 ds that have a cover of
clover (Burroughs 1982).
2.3.4. Abundance of natural enemies
By providing a more diverse environment, intercropping may
createmore favorable conditions fornatural enemies, both in terms
of numbers and diversity (Perrin 1977, Cromatie 1983). This may
occur through the provision of essenti al resources for predators
and parasites, and 50 enable them ta obtain all of the; r
requirements near to the pest population, rather than having to
seek it farther away (Cromatie 1983). Important resources include
food, cover and alternative prey (Way 1977).
Intercropping provides more pollen and nectar sources, which
may attract natura 1 enemi es and i ncrease the; r reproduct; ve
38
potential (Kareiva 1983, Altieri 1987). Moreover, it may ;ncrease
9 round cover, wh i ch favors certa in predators such as ca rabi d and
s taphy 1 in id beet 1 es, and i ncrease the divers i ty of herb; vorous
i nsects that can serve as al ternati ve food sources for natural
enemies. Therefore, creating an environment suitable for a
diversity of insect species will help prevent the 10s5 of
benefi ci al i n sects (preda tors and pa ras; toi ds) when the; r ma in
hosts are in low number5 ( Altier; 1987). In these various ways,
i nterc r0pDi ng can i ndi rect lys upport the benef; ci al i nsects that
prey on the pests (Burroughs 1982, Fl; nt 1990).
39
r 1
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,
CHAPTER III
MATERIALS AND METHODS
40
CHAPTER III
MATERIAlS AND METHODS
,. \
3.1. Site description
A fi el d experiment was conducted duri ng the dry season in
1989 (July to December) at the Manggoapi Experimental Station of
the Faculty of Agriculture, Cenderawasih University in Manokwari
(134°05'E, 0°50'5). The station is 110 m above sea level, with
plate topography and a red-yellow Mediterranean soil type, with a
pH of 5 to 7 (Karyoto et gl. 1987).
The average rainfall and rainy days per month during the field
experiment were 187 mm and 16 days, respectively. 8ased on average
monthly rainfall recorded during 40 years , the location is
classified as having 5 to 6 months wet period and < 2 months of dry
period, with an annual rainfall of approximately 2390 mm (Oldeman
et gl. 1980). Low rainfall « 200 mm) occurs during May to
October, and high rainfall (> 200 mm) occurs during November to
April each year (Fig. 7); these periods are considered as dry and
wet season, respectively.
Accord i ng to 1 and use hi s tory, the experi menta l l oca t ion was
previously planted with mungbean, Phaseolus radiatlJs, corn, lJ~a
mQ.ll, and sweet potato, Ipomoea batatas., after which it was
abandoned for fou r yea rs, and had become weed in fes ted. Vegetation
analysis carried out prior to land clearing showeo that elephant
grass, Pennisetum purpureum, as well as Sida rhomlLifoliQ., and
Callopogon;um mucuniodes, were the dominant weeds.
41
1
nRla~ln~f~.I~I~(m~m~) ________________________________ ~ 360.-
300
FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEe Month
- 1989 ralnfaJl ~ 1939-1979 ralnfall
Fi g. 7. Nonth 1 y ra in fa 11 recorded at Manggoapi experi menta 1 farm during 1989 and the average of monthly rainfall in Nanok\Yari recorded during a prev10us 40-year period (1939 to 1979).
42
1
l
3.2. Experi menta l des i gn
The fol 1 ow; ng fi ve t reatments were tes ted us i ng a randomi zed
complete block design with three repl;cations of 150 ml plots
(Appendi x 1).
A = sweet pota to monocul ture
B = sweet pota to + COl~n
C = sweet pota to + soybean
0 = sweet potato + corn + soybean
E = sweet pota to + tomato + cabbage
3.3. Crop arrangement and s,paci ng wi thin each treatment
3.3.l. Sweet potato monoculture
A sweet potato monocul ture was pl anted with a spaci ng of 100
cm between ridges and 40 cm within rows, giving a density of 370
plants per 150 m2 plot (24.668 sweet potato plants per ha).
3.3.2. Sweet potato and corn
Two ri dges of sweet potato were planted between two double
rows of corn plants. The corn s pac; ng between rows was 50 cm, 50
that the populations obtained were 240 corn plants and 222 sweet
potato plants per plot (16.000 corn plants and 14.800 sweet potato
pl ants per ha).
43
1
1
J
3.3.3. Sweet potato and soybean
Three rows of soybean were pl anted, fo 11 owed by two rows of
sweet potato. The soybean spaci ng was 25 x 25 cm. The sweet
patata spacing was the same as in the monoculture treatment. The
popu 1 at ion s of soybean and sweet potato in th; s t reatment were 720
and 222 plants per plot, respectively (48.002 soybean plants and
14.800 sweet potato Dl ants per ha).
3.3.4. Sweet potato, corn and soybean
One row of corn was pl anted between two rows of soybean, to be
fo 11 owed by two rows of sweet potato. The same arrangement was
repeated for the rest of the plot, 50 that the populations of
soybean, corn and sweet potato were 720, 90 and 148 plants per
'f plot, respectively (48.002 soybean plants, 6.000 corn plants, and
9.867 sweet potato plants per ha).
3.3.5. Sweet potato, tomato and cabbage
Two rows of cabbage were pl anted between two rows of tomato,
fo11owed by two rows of sweet potato. Th; s arrangement was
repeated for the rest of the plot. The popul ations of tomato,
cabba ge and sweet pota to were 180, 222 and 148 plants per plot,
respect ive 1 y (12.000 tomato plants, 14.800 cabbage plants, and
9.867 sweet potato plants per ha).
44
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3.4. Fiel d preparation and management
3.4.1. Land preparation
The experimental site was cleared of grasses and shrubs, and
then plowed. The land was then divided into 15 plots of 150 ml'
each, and ri dges for sweet potato were formed in each plot.
3.4.2. Preparation of planting materials
Termi na 1 cut t i ngs of sweet pota to vi nes, about 25 to 35 cm in
length with a few leaves at the tip, were prepared three days
before planting. The cuttings were tied in groups of 10; wet jute
sack was wrapped around the cut end and the bundl es were stored in
a col d and shady place to acce l erate root growth.
Loca 1 vari et i es of corn, soybean, tomato and cabbage were
~ sel ected. The seeds of cabbage (cv. K-K Cross) and a l oca l va ri et y
l
of tomato were sown in the nursery four weeks pri or to plant i ng
out. After one week the seedl i ngs were transfered ta Pandanus-leaf
corta i ners .
3.4.3. Planting and fertilizing
All crops were planted simultaneously; the sweet potato
cutt i ngs, one per ho le, on the ri dge tops, and corn and soybean, 3
or 4 seeds per hole, according ta planting design.
Transp l anti ng of tomato and cabbage was carri ed out by openi ng
the containers and planting the seedlings in holes that had been
filled with 250 gram chicken manure. as a basic fertilizer, three
days before planting.
45
~--
A 11 c rops recei ved one or two app li cat i ons of i norgani c
sources of nit rogen t phosphorous and potass i um. Total amounts
applied are given in Table 6.
Crop
Table 6. Total amount of nitrogen, phosphorous and potassium (kg/ha) applied to sweet potato, corn, soybean, tomato and cabbage.
Fertilizer (kg/ha)
Nitrogen Phosphorous Potass i um (N) (P2Os) (KP)
----------------------------------------------------------------Sweet potato 90 JO 240
Corn 120 45 25
Soybean 45 90 50
Tomato 100 150 50
Cabbage 90 60 0
The first application took place immediately after planting,
and t for mos t cr0p, the second app 1 i cat ion occu red 30 days a fter
planting (DAP).
For sweet pota to, ha 1 f of the fert il i zer was app 1 i ed as a spot
application at planting time, and the other half at 30 DAP. For
corn t two-third of the nitrogen was applied at planting, the
remaining one third at 30 DAP. The complete dose of phosphorous
and potassium was applied at the time of planting.
Soybean fertilizer was applied once at planting time as a
cont in uous band 7 to 10 cm to one si de of the row. Tomato and
cabbage were fert il i zed once by means of spot and ri ng
applications, respectively, at 21 DAP.
46
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l
3.5. Crop maintenance
Crops maintenance included watering, replanting, weeding and
hi11ing up of the soil, staking of tomato plants, lifting of vines,
and pruning of the old leaves.
Wa teri ng was provi ded, es peci a 11 y for cabbage and toma to,
du ri ng the fi rs t two weeks before the plants were we 11 es tab 1 i shed.
Sweet potato was al so watered ea r l yin the growi ng season, when the
weather was dry. All dead and missing plants were replaced ~ithin
7 to 10 DAP.
Hand weeding, especially of nutsedge, Cyperus roton~, which
domi nated the experi menta 1 a rea, was underta ken twi ce. Weed i ng was
done 14 and 28 DAP. Hi 11 i ng up of the soi 1 was done i mmedi a te 1 y
after weeding to support the crops.
Tomato plants were staked with 75 cm long sticks, 21 DAP.
Lifting of sweet potato vines to prevent the growth of adventious
roots took place twice, at 28 and 56 DAP. Pruning of old sweet
potato leaves was carried out as necessary.
3.6. Observations
3.6.1. Colonization by SPW
Observation of colonization of the sweet potato crop by SPW
was carried out at 56 DAP on 10 randomly selected plants from each
plot. A plant was considered to have been co1onized if a SPW was
present or if damage by SPW was evident. Percentage colonlzation
data was calculated as follows:
47
III
"
where:
N c = -------------- x 100 % T
C = percentage colonizat-jon of the sweet potato plant in a plot
N = number of samples colonized T = total number of samples per plot
Percentage of co 1 oni zati on data were tabul ated and transformed
w i t han arc sin t ra n s f 0 rm a t ion p rio r t 0 a n a lys i S 0 f var i an ce, as
recommended by Gomez & Gomez (1976, 1984 ), when data are in
proport ions.
3.6.2. Population density of SPW and percentage of damaged tuber
Ten samp l i ng un i ts (p 1 ants) were taken random 1 y from each
experimental unit to determine the size of the SPW population.
The tubers and vines (15 cm above the crown) of each plant were
dis sected direct 1 y a t ha rves t; and the number of weevi 1 1 arvae,
pupae and adul ts were total ed for each sampl; ng un; ts (pl ant) per
experimental unit.
The percentdge of infested (damaged) tubers WdS calcul ated as
fo 11 ows.
a l = ---------- x 100 %
a + b
Ivhere: l = percentage of i nfected tubers a = i nfected tubers b = hea l thy tubers
48
, ,
Data for thl~ number of i nsects, and the percentage of i nfected
tubers, were tabulated and transformed with a square root + 0.5,
and arcsin transformation, respectively, prior to analysis. The
square root + 0.5 and arcsin transformation are used when data
record rare events and relate to proportions, respectively (Gomez
& Gomez 1976, 1984).
3.6.3. Number and fresh weight of tubers
Number of tubers from ten plants from each plot were counted
and their fresh weight was recorded.
3.6.4. Marketable yield
Ma rketab le yi el d of sweet pot a to, corn and soybean were
wei ghed from each plot pri or to bel ng ma rketed. Prof; t obta; ned
from marketing the yield of a particular cropping system, without
ta king in to account the cos t of product ion, was recorded as i ts
economi c value. The value of each of the crops was based on the
market priees in the Manokwari market in December 1989.
3.6.5. Monetary index
Monetary index (Gomez & Gomez 1983) was used to evaluate the
yield advantage of each treatment. This was obtained by
substracting the total cost of production from total economic value
of the oroduce from each cropping system.
Total cost of production included operational cost and
materi al s. Operational cost consisted of land preparation,
49
planting, fertilizing, maintaning, harvesting, washing of the
tubers, drying and seeding of corn and soybean; while materials
included cost of sweet potato cuttings, corn and soybean seeds, and
fert il i zers (Urea, TPS, and Kel).
3.6.6. Insect diversity
Insects from the monoculture and the intercropped sweet potato
were sampled using a sweep net and pitfall traps as described
below.
a. Sweep net sampling
Sweep net sampling was done as described by Risch (1979) in
Costa Rica. One sampling unit (30 sweeps) was replicated four
tlmes in each experimental unit (plot). Thus, 120 sweeps were
taken from each experimental unit and a total of 360 sweeps were
taken from each tredtment. Sweeping w~s done in straight lines and
the vegeta t ion was never swept tw; ce duri ng a samp 1 i ng date.
Sweeping was carried out on four sampling dates, 35 DAP, 42 DAP, 49
DAP and 56 DAP, always between 08.00 a.m and Il.00 a.m.
1 nsec t s co 11 ected from each s amp 1 i ng un i t (30 sweeps) were
placed in a sma" plastic bag, killed with chloroform, separated
fram the vegetation, and stored in vials in 75 % methyl alcohol.
The insects were then separated into families under a stereoscopie
mi c roscope, accord i ng to the cl ass; fi cati on scheme ; n Borror et
ILL. (1989).
50
1 b. Pitfall traps
Five pitfall traps per plot were used to capture ground
inhabiting arthropods. The traps (12 cm diam. x 15 cm high
plastie containers), were p1aced diagonally at five points in each
plot 35 DAP (Fi g. 8). They were buri ed in the soil 50 tha t the
edge of the container was flush with the soil surface.
To prevent escape, the t raps were ha l f fi 11 ed wi th a near
saturated solution of water, salt and detergent.
51
10 !IL
o o
o
o o
Figure 8. Arrangement of pitfall traps in each cxperimental unit
15 zn.
52
, 1
, ,
3.7. Analysis of data
3.7.1. Effects of intercropping on populations of SPW and on sweet potato production
For testing the significance of the effects of intercropping
on the populations of SPW (number of SPW and percentage of damage)
and on sweet potato production (number of tubers and fresh weight
of tubers), these data were subjected to analysis of variance.
Fisher's Least Significant Difference (LSD) test was used to locate
differences among means.
The relationship between numbers of SPW, and percentage of
damaged tubers were analyzed by means of regression analysis.
Marketable yield was tabulated to compare yield advantage.
Comparison of yield advantage among treatments using Land
Equivalent Ratio (LER) could not be carried out, because in the
present study corn and soybean were not planted in monoculture. As
a result, the yield advantage is expressed as a Monetary Index
(MI). This index can be obtained by using the following formula:
where
n MI = E (a
1X
1 - b) (Gomez & Gomez 1983).
;=1
MI = monetary index a = species of crops bl = economic value of crops in a cropping system X: = yield of a species of crop
53
3.7.2. Insect diversity
AnalYS1S of the diversity of insects and spiders within the
four different sweet potato agroecosystems was determined by using
the Kruskal-Wallis non-parametric analysis of variance (Daniel
1990). This analysis is based on Kruskal-Wallis test statistic
(H) .
12 R ? 1
H = -------- E -----N(N + 1) n
1
3(N + 1) (Daniel 1990)
where: N = the total number of insect and spider families w;thin the four sweet potato cropping systems.
Rl = the sum of the ranks assigned to the number of insect and spider families within the four sweet potato cropping systems.
ThlS analysis was used to test the fo1lowing hypotheses:
1. Nu 11 hypot hes i s (Ho): The number of i nsect and s pi der famil i es
within each of the four sweet potato cropping systems are
sim; lare
2. Alterna t ive hypothes i s (HA): The number of ; nsect and spi der
fami 1 i es w; thi n each of the four sweet potato croppi ng systems
are no t sim i l a r .
The null hypothesis is rejected if the value of H in Kruskal-
Wa Il is non-parametric ana1ysis of variance is greater th an X23
. 0 05.
54
1
CHAPT ER IV
RESULTS
,.
Dald VIere collected from the four intercropped treatments:
rJw:(; t pota to mon ocu ltu re, sweet pota to + corn, sweet pota to +
soybean, and sweet potato + corn + soybean (Figs. 10,11,12 & 13
respectlvely) . A fi fth treatment (sweet potato + tomato +
cahbage), was abandoned because the cabbage and tomato crops were
hr~vlly damaged by the diamond-back moth, Plutella xylostel1a, and
fWJdriulll wl1t, L!"·t~.~trlum oxysporum, respectively (Fig. 9).
F l g. 9. Heav i l Y damaged tomato and cabbage i nter-cropped with sweet potato (treatment E).
56
1
1
1
Fig.IO. Sweet potata monoculture (treatment A).
Fig. Il. Sweet potato intercropped with corn (treatment B).
57 ,
1
_J
Fig. 12. Sweet patata intercropped with soybean (treatment C).
Flg. 13. Sweet patata intercropped with corn and soybean (treatment 0).
58
l 4.1. Effects of intercropping on the population of sweet potato
weevil (SPW) and on sweet potato yield.
4.1.1. Colonization of the sweet potato by SPW in four cropping systems
Colonization by the SPW at 56 DAP was significantly lower for
intercropped sweet potato compared with the sweet potato
monoculture (Table 7). ~here was, however, no significant
difference in colonization among the three intercropped treatments,
even though the level of colonization in the sweet potato + corn
(11 %) was only half of that in the sweet potato + soybean (23 X).
Table 7. Colonization of sweet potata by the sweet potato weevil (SPW) at 56 DAP.
Colonization by SPW (%) Sweet potato agroecosystem
Original Transformed Ratio data datai
------------------------------------------------------------------Sweet potato monoculture (A) 70 70.0 a?
Sweet potato + soybean (C) 23 23.2 b
Sweet potato + corn + soybean (D) 20 20.0 b
Sweet potato + corn (B) 10 10.8 b
Coefficient of variance (CV) 31.4
1) Arc sine transformation was used for transforming original percentage data.
2) Any two means in one column followed by different letters differ significantly at P < 0.05 using Fisher's Least Significant Different (LSD) test.
6.5
2.2
1.9
1.0
59
4.1.2. Population density of SPW and percentage of damaged tubers
Population density of SPW and percentage of damaged tubers
were significantly lower on the intercropped sweet potato
(treatments B, C, and D) compared with the sweet potato monoculture
(treatment A; Table 8). These rneasures were also significantly
lower in mixtures containing corn (treatment B & C) than in those
wlthout this crop (treatment C; Table 8). Thus, among the
interc~ops tested, corn was most able to reduce the population of
SPW and of damaged tubers.
Table 8. Number of sweet potato weevils (SPW) per kllogram of damaged tubers, and percentage of damaged tubers.
Swcet potato agroecosystem SPW Damaged tubers
No.jkg damaged Ratio tubers
% Ratio
------------------ -------------------------------------------------------Sweet potato monoculture (A) 37.0 al 16.0 21.9
Sweet potato + soybean (C) 21.0 b 9.1 14.7
SW(let potato + corn + soybean (0) 7.7 c 3.3
Swcet potato + corn (B) 2.3 c 1.0
Coffecient of Variance (CV) 20.9
1) Any two mean~ in one column followed by different letters differ significantly at P < 0.05 using Fisher's Least Significant Different (LSD) test.
6.6
2.6
al 8.4
b 5.7
c 2.5
c 1.0
60
1 Al though the i ncl us i on of soybean in the i ntercroppi ng mi xture
appeared to reduce this effect of the corn, the differences between
the mixture with and without soybean were not statistically
significant.
A high population of SPW is more likely to result ln a high
percentage of damaged tubers (Table 8). ThlS relationship is
depicted by the regression line of Y = 2.00 + 0.55 X (Fig. 14),
which indicates that there is a significant positive correlation
between population density of SPW and number of damaged tubers
(rca1 = 0,98 > ro 05 = 0.87).
4.1.3. Number and fresh weight of sweet potato tubers
Even though sweet potato intercropped with corn (treatments B
and D) significantly reduced the population density of SPW and
number of damaged tubers, the 1 eve l of product ion wi th corn,
measured as number of tubers per plant and as weight of fresh
tubers per 10 plants, was significantly lower th an in the cropping
systems without corn (Table 9).
Interestingly, the weight of fresh tubers in the sweet potata
+ corn + soybean mixture (treatment D) was half that in the sweet
potato + corn mixture (treatment B), although this difference was
not statistically significant. Furthermore, it appears that
whereas the i ncl us i on of soybean may have reduced sweet potato
yield, it did not affect sweet patata number, which was identical
in the two corn mixtures.
61
.' '\
.30 r y = 2.05 + 0.55 X r = 0.98
25 -A
,.r-....
n} ~'J --'-
IJI .-'IJ il ::J
.. ..J 1!J ", 'V U' r_, ~: L
r.J lU L-l
~I
o 5 10 15 20 ':0 .35 40
Fly. 14. Relationship betweEn number of sweet patata w~evl1s (SPW) and damaged tubers. A = ~weet potato monoculture, B = sweet potato + corn, C = sweet potato + soybean, 0 = sweet patalo + corn + soybean.
62
, A
-
Table 9. Effects of intercropping sweet potato on number of tubers per plant and on fresh weight of tubers per 10 plants.
Sweet patata agroecosystem Number of Ratio tubers per
Weight of Ratio fresh tubers
Sweet potato monoculture (A)
Sweet potato + soybean (C)
Sweet potato + corn (B)
per plant l
2.3 a
1.1 b
Sweet potato + corn + soybean (0) 1.1 b
Coefficient of variance (CV)
2.5
2.1
1.0
1.0
pet' 10 pl ants!
488.1 al
440.2 a
143.3 b
75.2 b
28.8
1) Any two means in one column followed by different letters differ significantly at P < 0.05 using Fisher's Least Significant Different (LSD) test.
6.6
5.9
1.9
1.0
4.1.4. Marketable yield and economic value of sweet potato and intercropped plants
Marketable yield and economic value of sweet potato, corn and
soybean from the four di fferent sweet potato agroecosystems are
presented in Table 10. The economic value of the marketable yield
was hi ghest for the sweet potato monocul ture and lowest for the
sweet potato + corn + soybean mi xture. Yi el d advantages in the
present study was calculated by using the monetary index (MI; Table
10) .
63
L
Table 10. Marketable yield, economic values, cast of production and monetary index of sweet patata and intercrapped plants in intercrapped sweet patata with carn and soybean.
Cropping Yield Economie value Total Cast of system (t/ha) (Can. $/ha/season)* economic production
------------------------ ------------------------ value Ratio (Can. $/ha)** sweet corn soy- sweet corn soy-patata bean patata bean ($)
Monetary index
----------------------------------------------------------------------------------------------------------------Al 7.0 2808 2808
B 0.9 1.6 344 822 1226
C 2.9 0.5 1157 400 1558
0 0.8 0.6 0.3 30Q 324 256 890
*) Marketable price of sweet patato, corn, and soybean were 0.40, 0.54 and 0.77 dollars per kllogram, respectively.
**) Cast of production is presented in Appendix 3. 1) A= sweet potato monoculture
B= sweet potato + corn C~ sweet patata T soybean 0= sweet patata + corn + soybean
1.0 1539 1269
0.4 1268 -42
0.6 1484 74
0.3 1158 -256
64
4.2. Number of insect and spider families, and number of individuals of each family associated with sweet potato agroecosys tems
The number of i nsect and spi der fanlll i es, and number of
individuals of each family (Tables Il,12, 13, 14), were obtained
from sweep net samples; the pitfal1 traps did not generate data as
they proved to be i nope rat ; ve in the cond it i ons of the present
study. Detailed information on individuals collected from each
family i5 reported in Appendix 4.
fable 11. Insect and spider families (fam.) and individuals (ind.) of each family ca 11 ected at 35 DAP.
Order of B C i nsects & spi ders
fam. ind. fam. ind. fam. ind.
Coleoptera 1 8 3 31 5 27 Hymenoptera 3 3 3 3 7 18 Homoptera 2 9 4 9 Hemiptera 1 2 Orthoptera 3 25 3 16 4 15 l epi doptera 3 17 3 7 3 15 Di ptera 2 4 4 12 13 96 Araneae (spiders) l 1 1 10 1 56 ------------- ----------------------------------------------------Total 13 58
Ratio (fam.) 1.0
Ratio (ind.) 1.0
1) A = Sweet potata monocu lture B = Sweet potata + corn C = Sweet potata + soybean o = Sweet potata + corn + soybean
19 88 38 238
1.46 2.92
1. 52 4.10
D
fam. ind.
4 40 6 9 4 21 1 2 4 44 4 10
14 183 1 51
-----------38 360
2.92
6.21
65
rdblr~ 12. In'J(!ct and spider families (fam.) and individuals (ind.) of (!ach family collected at 42 DAP.
Order of insecls & spiders
B c D
fam. ind. fam. ind. fam. i nd. fam. i nd.
Co l coptera 2 29 4 72 4 64 4 Hymenoptera 1 2 4 12 6 20 5 lIomoptera 1 2 2 17 2 23 3 Il('m i ptera 2 15 2 344 5 200 6 Orlhoplera 4 28 4 26 4 26 4 1 Pp Idopter'a 2 5 3 7 4 [)Iptpra 3 14 8 102 12 325 14 r~dr" odea 1 4 1 BlattdrJd 1 2 Ardneae (spIders) 1 38 1 48 1 137 1
lot al 17 135 25 621 38 806 42
Ratio (fam.) 1.0 1. 47 2.24 2.47
I<alio (lnd.) 1.0 4.60 5.97
1) seeTabJe]l.
lable ]3. Insect and spider families (fam.) and individuals (ind.) of each family collected at 49 DAP.
215 16 26
543 75 12
277 3
247
1414
10.47
-------- ------------------------------------------------------ ---------------Order of AI B C 0
ln~('cts & spiders ----- ------------------------------------------------------fam. ind. fam. ind. fam. inde fam. inde
----------- -----------------------------------------------------------------Lo J coptera 3 29 4 56 3 122 3 89 IIYlllenopt era 3 23 4 23 7 40 7 35 lIol11optera 2 16 4 29 5 70 6 77 Ilellliptera 2 19 4 69 6 287 5 327 Ort hoptera 3 67 4 142 4 259 4 124 Lepidoptera 3 13 1 1 3 7 3 9 Di ptera 3 54 10 247 10 247 14 283 Mantodea 1 4 1 6 At'anea~ (spider) 1 72 l 79 1 246 1 343 ------ ___________________________________ a ____________ ________________________
Total 20 293 32 646 40 1282 44 1293
Rat io (fam.) 1.0 1. 60 2.0 2.20
Ratio (ind.) 1.0 2.20 4.38 4.41
1) see Table Il.
66
Table 14. Insect and spider families (fam.) and individuals (ind.) of each fami l y co 11 ected a t 56 DAP.
Order of insects & spiders
B C D
fam. ind. fam. ind. fam. ind. fam. ind.
Coleoptera Hymenoptera Homoptera Hemirtera OrUloptera Lepldoptera Neuroptera Diptera Mantodea Bl attari a Areneae (spiders)
Total
Ratio (fam.)
Ra t ion (i nd . )
1) see Table 11.
3 2 1 4 3 1
7 1
1
23
1.0
76 16 15
172 72 8
83 2
57
501
1.0
3 5 3 5 4
14 1
1
36
l. 57
47 Il 23
118 29
58 1
41
328
0.65
6 6 S 5 4 3 1
14
1 l
46
2.0
372 47 33
2418 342
5 1
120
4 171
3513
7.01
6 8 5 5 4 3 1
15 1 l 1
50
2.17
414 34 36
2320 238
2 2
215 1 3
161
3426
6.84
The i ntercropped systems supported more fami lies of i nsects
and spiders than did the monoculture (Tables 11,12, 13 and 14).
Wi th one except ion (sweet potato + corn at 56 DAP, t rea tmen t B,
Table 14) the intercropped systems also supported higher numbers of
arthropods, even up to 10 times the population density.
Compared with the monoculture, number of insect and spider
fami 1 ies in treatments B, C, and D are 46 %, 192 % and 192 % higher
at 35 DAP, 47 %, 124 ~~ and 147 % high:r at 42 DAP, 60 %,100 %, and
120 % higher at 49 DAP, and 57 %,100 % and 117 % higher at 56 DAP.
The i ncreases in number of i nsect and spi der fami lies over 100 %
occurred in the treatments that inclLded soybean: C (sweet potato
+ soybean) and B (sweet potato + corn + soybean). Cumulative data
for the entire sampling periods is shown in Figure 15.
67
The hi gh number of i nsect and spi der fami lies in the
i ntercroppi ng systems suggest that these systems provided a greater
diversity of habitats. A reason of the diversity of insect and
splder families within the different sweet potato cropping systems
was obtained by subjecting the data to Kruskal-Wallis non
paramctrlc analysis of varlance. The analysis tested whether the
null hypothesis (the number of lnsect and spider families within
each of the four sweet potato croppi ng systems are simi l ar) or
alternative hypotheses (the number of insect and spider families
within at least one of the four sweet potato cropping systems is
not simllar) would be acceptable based on the H value of the
Kruskal-Wallis test.
The H val ue, 12.88, exceeded X23 . 0 05 (7.815), therefore the null
l hypothes 1 sis rej ected: hence, the numbers of i nsect and spi der
families for the four sweet potato cropping systems are diverse.
The average rank in diversity was higher in sweet potato
intercropped with corn and soybean (53.0), followed by sweet potato
intercropped with soybean (47.0), sweet potato intercropped with
corn (26.0), and sweet potato monoculture (10.0).
68
.-
No. of famille. ~~-----------------------------
60~---------------------------------------
~O~---------------
301----1
20~--f
10
o 36 4f2 ~9 66
DI~' aller planllng
- T,..IMenI Il œ 1'r..IIMftI a t;;;;ll'rNJa.nl C _ T""-.nl D
Figure 15. Number of insect and spider families associated with sweet potato cropping systems
59
4.3. AsQj domorpha sp. (Col eoptera: Chrysome li dae) and lycosa sp. lAraneae): two populations of arthropod species within the monoculture and intercropping systems.
The popu 1 a t i on of the spot ted tortoi se beet le, Asp i domorpha
sp., a 1 eaf-feedi ng i nsect assoc; ated wi th the sweet potato pl ant
and other J~omoea species (Simon Thomas 1961, 1964, Kalshoven
1983), wa s lower ln the i ntercropped sweet pota ta th a n in the sweet
potato monoculture (Fig. 16). The effect of the intercropped sweet
potata on the populatlons of this beetle was more evident and
statistically significant at 56 DAP, although not for all intercrop
trcatmcnts (Table 15). At 56 DAP, lntercropped sweet potato + corn
(trcatment B) had the lowest number of beetles (1.0), followed by
bot h i nte rcropped sweet pota to + soyhean (t rea tment C; 2.0), sweet
potato + corn + soybean (treatment 0; 2.0) with the sweet potato
monoculture having the highest number of beetles (treatment A;
4.0) .
The appos i te was obs erved wi th the popu 1 a t i on of the
predaceous spider, ~cosÀ sp. (Table 15, Fig. 17). The spider
populations were signicantly higher in intercropped sweet potato
whcre soybean was used as an intercropped plant (treatments C and
D; Table 15) and this was so throughout the cropping season, except
at 56 DAP, I"hen sweet potato + corn (treatrnent B) had less spiders
than the monoculture, though not significantly so.
70
•
Table 15. Number of spotted tortoise beetles colleeted at 56 DAP and spiders eolleeted at 35 DAP, 42 DAP, 49 DAP and 56 DAP. in four sweet potato cropping systems.
Croppi ng system·
A
B
C
D
Number of spotted tortoise beetles1
56 DAP
3.7 a
1.0 c
2.0 b
2.0 b
Number of the spiders l
35 DAP
0.3 b
3.3 b
18.7 a
17.0 a
42 DAP 49 DAP 56 DAP
12.7 b 24.0 b 19.0 be
16.0 b 26.3 b 13.7 b
45.7 a 82.0 a 57.0 a
82.3 a 114.3 a 53. 7 ab
1) Any two means in one column followed by different letters are differ signifieantly at P < 0.05 using fisher's Least Signi fi cant Di fferent (LSD) test.
*) A:.. sweet potato monoculture B = sweet potato + corn C = sweet potato , soybean D = sweet potato + corn + soybean
71
No. of bootloe 12~~-----------------------------------------'
101------------------------
o 1---------------
6f--------
4.--------
2
oLJli~~---<42 <49 01)'1 Iller plantlng
66
mrI TnJatmont A ~ Tr.unenl B D Tre&tlhlnl C • Tr •• llnOnl 0
Fi qure 16. Number 0 f spotted torto i se beetles t Aspidomorpha sp. {Co 1 eopterae: Cassididae} collected from four sweet potato cropping systems at 35, 42, 49 and 56 DAP. .
72
• "
1
No. of epldere 400
360
300
250
200
160
100
60
0 36 042 ~9 &6
Day. aUer plantlng
III Tre.lment A • Trwatlntnt B D Treatnent C _ Tr .. lrnenl 0
Figure 17. Number of spiders, Lycosa sp. (Areneae: Lycosidae)" coll ected from four sweet patata croppJ ng SYS tems a t 35, 42, 49 and 56 DAP.
73
l
CHAPTER V
DISCUSSION
5.1. Effects of intercrop~ing on the population density of the sweet potato weevil (SPW)
5. 1. 1. Effects of i ntercroppi ng on number of SPW and on percentage of damaged tubers
The low percentage of S PW co 1 oni z i ng sweet pota ta i ntercropped
with corn and/or soybean (Table 7) suggests that intercropping may
have affected its host searching behaviour. For example,
percentage colonization by SPW, number of SPW per plant, and
percentage of damaged tubers in the sweet pota to monocu ltu re was 7,
16, and 8 times that in the sweet potato + corn mixture,
respectively (Table 7 & 8). Similar high levels of damage in sweet
pota to monocu 1 tu res were al so reported in Papua New Gu i nea
(Sutherl and 1986b). Howpver, number of tubers per pl ant, weight of
fresh tubers per 10 plants, and total value of the marketable yield
in the monoculture were 3, 7 and 3 times that in the sweet potato
+ corn + soybean mixture and 3, 3 and 2 times that in the sweet
potato + corn mixture, respectively (Table 9 & 10). For farmers
who grow sweet potato primarily for home consumption, the
monoculture would be attractive, because of the higher produ·:tion
of tubers which may give them a marketable surplus. The monei..ary
index (MI) used to determi ne yi el d advantage of the di fferen t
treatments (Table 10), i11 ustrates that the greatest profit was
obtained by growing sweet potato as a monocul ture (treatment A),
and then vJhen i ntercropped wi th soybean (t reatment C), res u 1 t l ng in
a net gain of Cano $ 1269/ha and Cano $ 74/ha, respectively.
75
1
Thus, although the mixtures, particularly those including
corn, were effect ive in reduci ng numbers of SPW, they were
uneconomlC, especially when they included both corn and soybean.
The low percentage of colonization and low number of SPW in
1 nterc ropped sweet pota to i s probab l y a resu 1 t of phys i ca 1 and
biological effects of the intercropped plants on the weevils'
activi ty, growth and development (Perrin 1977, Altieri 1987). The
taller corn and soybean plants may act as physical barriers against
SPW invasion of the sweet potato crop. Presumably, the SPW either
moved away from the intercropped sweet potato to a more sui table
l oca t ion 0 r had to spend ext ra t i me and energy to fi nd the crop
(Kareiva 1983). In a corn + bean mixture, Parfait & Jarry (1987)
found that corn made the mi crocl imate unfavorable for the bean
weevil and modified the bean's phenology. Similar effects may be
partly responsible for the reduced of SPW population in the corn +
sweet potato mixture in the present study.
Furthermore, it is suspected that during the extended period
of hos t sea rch i ng in the i ntercropped sys tems, S PW wou l d have been
exposed to various environmental pressures, including higher
popu lat ion dens i ty of n atura 1 enem i es (Ka rei va 1983, van Emden
1990). Al though, in the present study the natura l enemi es were not
stud i ed quantitat i vely, they were recorded in both the sweet potato
agroecosystems and the surrounding area. For example, preying
Illant id s, wh i ch are genera l feeders (Ho 11 i ngswort h & l doi ne 1992),
\'Jcrr often seen searchi ng for prey on the sweet potato fol i age,
although their effects on SPWs were not evaluated. Also, chickens
76
.,
...
from a nearby village were commonly seen searching for insects in
the area surrounding the plots. Thus, the SPW would have been
vulnerable to attack by these and other natural enemies while
searching for a suitable hast.
Preda tors and pa ras i tes are usua 11 y more abundant with in
mixtures than in monocultures (Perrin 1977, Altieri 1987). This
may accou n t for much of the reduct i on of the popu lat ions 0 f SPW
within the mixtures.
SPW orients to its hosts by means of chemical eues produced
and re l eased by the sweet potato l ea ves a nd tuber sk in (Not th i ngham
et il. 1988). In intercropped sweet patata fields, however, such
chem; cal s may be masked by other chem; cal s from the i ntercrop
plants, thereby making it more difficult for the SPW to locate and
recogn i ze ; ts hos t.
5.1.2. Level of attack by SPW in relation to tuber formation.
l nterc rop plants maya ffect the growth and deve l opment of the
sweet pota to crop, thereby maki ng ; t 1 es s attractive ta SPW.
The sweet potato that attracts SPW is released chemical
dur; ng the forma tian of new tubers, wh; ch accu rs between 28 and 56
DAP (Wi 1 son & Lowe 1973, Wi 1 son 1982). Because i ntercropped sweet
potato probab 1 y recei ves l ess li ght than in monocul ture, the
resultant delayed tuber formation would also delay release of the
att ractant, and therefore effect the movement of SPW towa rds i ts
hos t .
77
l
The low number of tubers per pl ant, and the low wei ght of
fres Il tubers pe r 10 plants in sweet pota to i ntercropped wi th corn
were most 1ikely caused by the lack of light that resulted from
shading by the corn plants. Hahn (1977) found that a lack of light
caused a decrease in the net assimilation rate and dry matter
production, esp(lcially in the form of tubers. Moreno (1982), found
that intercropping sweet potato with corn in Guatemala reduced
sweet pota to yi el d by 63 percent; whereas Roberts et iÜ. ( 1983) ,
found that in Trinidad the reduction ranged from 10 % to 44 ~,.
Variations in yield were related to rainfall, the sweet potato
cultivar, planting date and crop spacing (Roberts et al. 1983).
In the present study, yield was lower when sweet potato was
i ntercropped wi th corn than wi th soybean (Table 9), pres umab 1 y
1 a rge 1 y because of the greater shad i ng effect 0 f corn. Soybean may
also have provided additional nitrogen to the sweet potato. This
possible benefit, however, was not evident when sweet potato was
intercropped with corn and soybean (treatment D; Table 9), probably
because of competi ti on from the corn for l imi ted resources,
particulélrly light, nutrients, water, and space; and under these
cond i t i ons the sweet potato plants produced on l y a few and mos t l Y
sma 11 tubers. Therefore , al though i ntercroppi ng reduced the number
of both pest i nsects and damaged tubers, it al so resul ted in a
greater reduction in marketable yield.
Because of the lower numbers of SPW in the corn + sweet potato
mixture, intercropping with corn seems to be the most promising
strategy for control of SPW, and should be investigated further,
78
,
~---~-~~--_ ..
but the 60 % reduction in economic value of marketable yield
1'1"0'1 i des a cons i derab le cha 11 enge to be overcome.
Unti 1 i ntercroppi ng systems are found that both reduce pest
damage, and produce higher or comparable yields and levels of
prof i t to monocu l ture, the 1 a tter i s li ke l y to be preferred by
farmers in Irian Jaya.
Among the in tercroppi ng systems exami ned in the present s tudy,
the sweet potato + soybean mi xture (t reatment C) i s the best choi ce
for i ncreas i ng in come and meet i ng n ut rit i ona l requi rements of the
indigenous people in both the lowland and highland regions of Irian
Jaya.
The in t roduct i on of soybean in to an i ndi genous sweet potato
cropping system in Irian Jaya presents several advantages. (1)
Because soybean roots are i nfected wi th the nitrogen fi xing
bacteri a, Rh i zobi um, thei r presence can i ncrease the amount of
nitrogen in the soil available to the sweet potato. (2) Protein
rich soybeans would also enhance the nutritional quality of the
i nd i genous di et. The nu t rit; ona l va l ue of soybean has not yet been
recog ni zed, however, by l ri an Jaya' sind i genous sweet potato
farmers (La Ahmady 1988), who still primari ly regard ~oybean as a
cash crop. (3) The soybean crop by provi di ng the farmers wi th a
second marketable commodity would help them to diversify their
production base (Kass 1978). (4) The failure of one crOD within an
; ntercroppi ng sys tem as a resu l t of adverse env i ronmenta l
cond i t ions, s uch as drought, pests, or di sease, can be compensated
for the other trops (K(!ss 1978, Beets 198?). Th us, i ntercroppi ng
79
mdy provide yield stability, because an alternative crop may be a
cri ti ca l asset when the ma in crop i s compromi sed by poor weather or
other environmental stre5ses. (5). Soybean and sweet potato
i ntercropped together appear to harbour fewer inseet pests,
diseases and weeds than when bath crops are grown in pure stands.
5.2. Number of insect and spider families associated with sweet potato agroecosystems
The number of insect and spider families in the intercropped
sweet potato systems were generally higher tnan in the sweet potato
monocul ture. Results indicate that the insect and spider
popu lat ion i s more diverse in i ntercropped sweet potato than in a
sweet potato monocu 1 ture and tha t th i 5 di vers i ty changes wi th e rop
phenology.
This inerease in arthropod diversity in intercropping sweet
potato plots may s imp l y represent the addit i on of the faunas of the
eomponent crops An individllal crop and its associated
phytoph agous fauna (usually speeialized insects) in an
intercropping system may directly or indirectly provide chemical
eues attract i ng natura l enemi es (Pri ce 1986). Predators and
parasitoids, for example, may search and attack their Ilost in the
i ntercroppi ng systems, based on the a -t: tract i ve body odor rel ea sed
by phytophagous i nsects. A l sa, they may fi nd the phytophagous
i nsects i nd i rect l y th roug h chemi ca 15 re l eased by the hos t plan t on
which their potential prey or host is feeding
Therefore, an i ndi vi dual crop and its assoei ated
(Priee 1986).
80
fauna in sweet potata crappi ng systems may resul t ; n a hi gher
n umber of in sect s pee; es in in tercropped sys tems (Ri s ch et al.
1983).
There are numeraus parasitoids and predators associated with
i nsect pests of sweet patato and the other i ntercrops. For
examp1e, trichagrammids are known to parasitize the sweet potato
horn worm, Herse conva1vu1 i, L., and the Asian corn borer, Ostrinia
furnacalis_ Guinee (Nafus & Schreiner 1986). The latter ;s also
attacked by braconids, ichneumonids, chôlcidids, eu1ophids,
tachinids and by predators such as Orius spp.(Hemiptera:
Anthocori dae), Che l i soches spp. (Dermaptera: Che 1 i soch i dae), and
other i nsects and spi ders (Nafus & Schrei ner 1991). Many
parasitoids ènd predators associated with each species of insect
pest, wi 11 have contributed ta the higher insect diversity found in
, the i ntercropped sweet potato systems.
Even though the divers i ty of the a rthropod fauna in an
intercropped system is generally higher, the population density of
each species of herbivore, however, is generally lower (Risch et
al. 1983, Letourneau 1990). This is i1lustrated in the present
study, where a smaller population of the spotted tortoise bettle,
A~Qidoll1o~ha sp., a leaf-feeding insect exclusively associated with
sweet potato, tended to be lower in the intercropped systems than
in the monoculture (Fig. 17). Even though reasons for the lower
popu1 at i on of the beet lei Il the i ntercropped sweet potato were not
determi ned, prev; ous research has shown that i ntercroppi ng
frequently causes dramatic decrease in a pest population (Pimentel
8l
1961, Root 1973, Dempster & Coaker 1974, Perrin 1977, Karel ~ ~l.
1982, Altieri & LetournealJ 1982, 1984, Cromatie 1983, Altieri &
Liebman 1986, Tingey & Lamont 1988, van Emden 1990). For example
the popu lat ion dens i t i es of EmRoasca f abae (Ha rri s) and Mh~ fj!Jli\~
Scopol~, were significantly less in plot~ intercropped with winter
wheat th an in those grown in monocu l t rure (Ti ngey & Lamont 1988).
Simi1ary the reduction of the popu1dtion of the diamond-back moth,
Plu tell a x y los te 11 â ( L), w a s sig TI i fic an t l Y les 5 i n plo t s wh e re
cabbages were intercropped with tomatoes (Buranday & Raros 1975).
These authors a1so noted that int~rcrop plants acted as effective
physical and biological bal~riers to insect pest infestation.
However, the mechani sms in wh i ch i ntercropped plants prevent
infestations are not always similar for all species of insects.
In the case of the spotted tortoise beet1e, two hypotheses as
suggested by Root (1973), may be used two exp 1 ai n the beet le' s
lower popu 1 at ion dens i ty in i ntercropped sweet pota to: the resou rce
concentration hypothesis and the natural enemies hypothesis. The
first hypothesis predicts that specialized insect pests will be
1 ess abundant in i ntercropped systems when the mi xtures are
composed of both host and non-host crops (Sheehan 1986, Altieri
1987) . Therefore, in the present s tudy, the s potted torto i se
beetle, a specialist pest of the sweet potato crop, may have had a
difficult time locating, remaining on, and reproducing on the sweet
potato crop; the corn and soybean plants may have acted as physica1
barriers limiting movement of the beetle into or within the sweet
potato plants. The beetle a1so may not have recognized the sweet
82
potato plants because of chemicals produced by the other
intercropped plants m-lsking those of the sweet potato (Altieri
1987) .
The second hypothes i 5 states that hi gh veceta t ion divers i ty
improves conditions for natural enemies by providing a variety of
habitats, and abundar,t food and shelter, resulting in increasing
their numbers and efficiency in intercropping plots. For example,
i ntercroppi ng systems can provi de more pollen and nectar sources
that attract natural enemies and increase their reproductive
potent i al (Sheeh an 1986, Al t i eri 1987). Interc roppi ng sys tems can
also increase ground coyer which fa vors certain predators such as
carabid and staphylinid beetles, centipede and various arachanids,
and can increase diversity of herbivorous insects, which can serve
as alternative foud sources for natural enemies, making them less
likely to leave when the main pest species are rare (Altier; 1987).
1 n t he present s tudy, the s potted torto i se beet le may have
been regu l ated by an i ncreased number of natural enemi es in the
i ntercropped systems as compared wi th the monocul ture. For
cxample, chalcid wasps and encyrti:1îd hymenopterans that
parasitize the oothecae and larva, respectively (Simon Thomas 1964,
Ka l s hoven 1981), may be more abundant in the i ntercropped sweet
potato, which provides a more enriched microenvironment for their
development, than in the monculture.
The spi der, Lycos a sp., was more abu ndant in the i ntercropped
sys tems th an in the monocu l ture, and numbers i ncred sed over t ime ,
reaching a peak at 56 DAP. Spiders are considered as pioneer
83
-
arthropod species readily colonizing new habitélts (Bishop &
Riechert 1990). ïheir role in the present sweet potato cropping
sys tems was not cl ea rl y defi ned, but 5 pi ders are cornillon ly known as
effective natural enemies (Bishop & Riehert 1990).
The l arger spi der popul at ion in the i ntercropped t reatments
may be, in pa rt, accounted for by a more abündant and di verse
popul at i on of i nsect prey found in these ag roecosys tems, and th i s
would be favorable to the generalist spiders (Riehert & Lockley
1984) .
84
CHAPTER VI
CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH
T
In the present s tudy, i ntercroppi ng sweet pota to wi th either
corn and/or soybean was found to reduce the SPW popul ation, rate of
colonization by SPW, and the associated level of damage to the
sweet potato tubers. Number of tubers and yield were higher,
however, in the sweet potato monoculture.
1 ntercropp i ng w loh corn was mos t eff ect ive from a pes t
man agement poi nt of vi ew, probab l y beeause i t prov i ded a more
significant physical barrier to the movement of the SPW than did
the soybean pla n ts. Moreover f by de 1 ayi ng tuber formé't ion. it may
have made the sweet potato plants less attractlve to the SPW.
Un for t u n a tel y, h 0 w ev e r, th i s r e t a rd a t l 0 n 0 f tub e r f 0 rm a t l 0 n
a1so signi ficantly reduced sweet potato yield, making it unlikely
to be accepted as an acceptable agronomie prnctice by sweet potato
farmers in IriRn Jaya.
Sweet pota to in tercropped wi th soybean yi el ded the second
hi 9 hes t i ncome a fter the sweet potato monocu lture, even thoug h
colonization and number of SPW's were higher than in the corn
mixtures. Thus, further work with soybean may be warranted to find
an appropiate crop ratio and spacing that is both effective in
controlling the SPW and economical1y acceptab1e to the sweet potato
farmers. Such development should take into account the indigenous
knowledge and traditional methods of the sweet potato farmers.
More insect and spider families, and therefore possibly a
grea ter divers i ty of s peci es, occu rred in i nterc ropped sweet pota to
than in sweet potato monocul ture. Popul ations of the phytophagous
spotted tortoise beet1e , Aspidomorpha sp., were lower in the
86
1 ntercropped systems than in the monoculture, but the oppùsi te was
found \'lÎth the predacious spider, Lycosa sp. The lower population
of ASPJ.Qo_mgTJ2hg sp. may be associated w-ith a higher number of
parasites and predators found in the inteï"cropped systems. If
Aspidomorph~ sp. is a threat to sweet patato, it may be controlled
by planting sweet potato with another crop that is not
taxonomically related, and is a non-host plant for this insect.
The larger population of the generalist spider may be related to
the larger number of prey species found in intercropped systems.
The present study is a first attempt to investigate
lntercropping as a potential cultural control strategy for SPW, or
for any other insect pest in Irian Jaya. Although the results are
prellminary, they indicate important directions for further
research. Four topics, in particular, are suggested for further
investigation.
1. Evaluation of thè contribution of the different mechanisms by
wh i ch corn red uces damage by the SPW, e. g., phys i ca 1 barri er,
chemical repellent, retardation of tuber formation, and
provision of a suitable habitat for natural control organisms.
ThlS might lead to further intercroppi ng experiments with
shorter cultivars of corn, shade tolerant sweet potato
cultivars, and different spacing between rows and times of
planting.
? ,- . Effects of the sweet potato crop on the productivity of the
intercrop(s). Again, experimentation with different cultivars
is called for to identify compatibility, and to avoid
87
competitlon and antagonism.
3. Such studies should also take into account both the
nutritional and economic implications of intercropping systems
for local farmers.
4. To increase the chances of adoption by the indigenous people,
it will also be necessary to find cropping patterns that build
on their indigenous knowledge and customs, and that are
compat i b le wi th the; r trad; t i ona l methods of product; on.
88
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, Â
APPENDICES
Appendix 1.
Fiel d layout of the experiment
10 Cil.
A c B
c D D
D A
B. c
E A E
~·------------------------_I l III
107
r , j,
1
,
A.
I.
II.
Appendi x 2.
Cost of production for monoculture and intercropped sweet potato
SWEET POTATD MONOCULTURE
OPERATIONAL COST
1. Land preparati on 1.1. Clearing and cutting of grasses and
bushes of 10.000 W @ Rp. 35 ......... 1.2. Ploughing, 10.000 W, Rp ...•.....•... 1.3. Plotting and leveling ................ 1.4. Mounding . . . . . . . . . . . . . . . . . . . .........
2. Planting and fertilizing 2.1. Planting ............................ 2.2. Ferti1izing .........................
3. Maintaining 3.1. Rep1anting . . . . . . . . . . . . . . . . . ......... 3.2. Weeding ............................. 3.3. Hi 11 i ng u~ the soi l •................ 3.4. Lifting t e vines ...................
4. Harvesting ........... Il •••••••••••••••••• 5. C1eaning and washing of tubers ...........
MATERIALS
Rp. 350.000 Rp. 200.000 Rp. 100.000 Rp. 167.500
Rp. 75.000 Rp. 75.000
Rp. 30.000 Rp. 80.400 Rp. 40.200 Rp. 20.000 Rp. 100.000 Rp. 40.000
1. Sweet potato cutting ..................... Rp. 619.750 2. Fertilizer .......••.. ...•• ............... Rp. 101.919
---------------------------TOTA!_ Rp. 2.000.569
B. SWEET POTATD + CDRN
1. OPERATIONAL COST
1. Land prepa rat ion 1. 1. Clearing and cutting of grasses and
bushes, 10.000 M2, @ Rp.35 .......... Rp. 350.000
1. 2. Ploughing, 10.000 W, Rp. 20 ....... Rp. 200.000 1.3. Plotting and leveling . . . . . . ........ Rp. 100.000
2. Planting and fertilizing 2.1. Planting . . . . . . . . . . . . . . . . . . . ........ Rp. 75.000 2.2. Fertilizing . . . . . . . . . . . . . . . . . . . . . . . . Rp. 75.000
108
3. Maintai ning 3. 1. Rep l ant i ng .... . . . . . . . . . . . . . . . . . . . . . Rp. 3.2. Weeding ............................ Rp. 3.3. Hilling up the soil ........... ..... Rp. 3.4. Lifting the vine ........ ........... Rp.
4. Harvesting .............................. Rp. 5. Cleaning and washing of the tubers ...... Rp. 6. Dryi ng and seedi ng of corn .............. Rp.
II. MATERIALS
1. Sweet potato 2. Corn seeds 3. Fertilizer
cuttings .................. . Rp. Rp. Rp.
30. 000 80.400 40.200 20.100
100.500 40.200 75.000
371.850 5.000
85.246
TOTAL Rp. 1.648.496
C. SWEET POTATO + SOYBEAN
1. OPERATIONAL COST
1. Land preparation
2. 3. 4. 5. 6.
Planting and fertilazing .............. . Maintaining ............•............... Harvesting ............................ . Cleaning and washing of the tubers .... . Drying and seeding of the soybean ..... .
II. MATERIALS
1. Sweet potato cuttings .................. . 2. Soybean seeds ...........•....•......... 3. Fertilizer ............................ .
TOTAL
D. SWEET POTATO + CORN + SOYBEAN
1. OPERATIONAL COST
1. Land preparat ion ..................... . 2. Planting and fertilizing ............. . 3. Maintaining .......................•... 4. Harvesting ........................... . 3. Cleaning and washing of the tubers ... . 4. Drying and seeding of soybean and corn.
Rp.
Rp. Rp. Rp. Rp. Rp.
Rp. Rp. Rp.
650. 000
150. 000 170.700 100.500 40.000 75. 000
619.750 5.000
118.125
Rp. 1.924.275
RP. Rp. Rp. Rp. Rp. Rp.
650. 000 150. 000 170.700 100.500 40.200 75. 000
109
II. MATERIALS
1. Sweet potato cuttings ................. Rp. 247.900 2. Corn seeds ............................ Rp. 2.500 3. Soybean seeds ... ........ ...... ........ Rp. 2.500 4. Fertilizer ............................ Rp. 66.088
------------------------TOTAL Rp. 1.505.3ll6
SUMMARY OF THE SWEET POTATO INTERCROPPING PRODUCTION COSTS
Type of sweet potato intercropping system
Production cast
Sweet pûtato monoculture
Sweet potato + Corn
Sweet potato + Soybean
Indonesi an Rupi ah (Rp)
2.000.569
1. 648.496
1.924.275
Canadian dollar (Can. $)'
1,538.89
1,268.07
1,480.21
~ Sweet potata + Carn + Soybean 1.505.388 1,157.99 ---------~---------------------------------------------------------
* In 1989 Cano $ 1 was equal ta Rp.1300,-
110
t
Appendi x 3 . •
Insect and spider families associated with sweet potato croppi fig systems a t 35. 42. 49 and 56 DAP.
A. Insect and spider families associated with the sweet potato cropping systems sampled at 35 DAP.
-----------------------------------------------------------------Order & Familiy of Sweet potato agroecosystem insects & spiders -------------------------------------------
A B C D -----------------------------------------------------------------COLEOPTERA F-:-Chrysome li dae 5 27 18 34 F. Cocci ne 11 i dae 1 4 3 F. Curculionidae 2 1 F. Cassidae 3 1 F. Nitidulidae 2 1 F. Carabidae 2 2 F. Buprestidae HYMENOPTERA r. Argidae 1 2 F. F ormi cac i dae l. 1 8 1
l F. Braconidae 3 4 F. Chalcididae 1 1 3 2 F. Eurytomi dae 1 1 F. Au 1 aei dae 1 1 F. Andrenidae 1 -F. Scelionidae 2 2 HOMOPTERA F. Cicadelidae 8 2 13 F. Delhpacidae 1 1 2 F. Cercopi dae 1 3 F. A~ididae 5 3 HEM! TERA r~-Mi ri dae 2 2 ORTHOPTERA F~---~yrgomorph i dae 2 3 4 F. Acrididae - Acridinae 8 3 3 2 - Cyrtacanthaerinae 13 10 4 7 F. Tetrogi ni idae - Pseudophyll i dae 2 8 - Phaneropterinae 4 14 F. Gryll i dae - Nemobunae 2 1 1 9
111
LEPIDOPTERA F. Pyra 1 i dae 12 2 10 4 F. Sphi ngi dae 2 2 2 2 F. Nymph a li dae 3 3 3 1 F. Noctu i dae 6 DI PTERA F. Phori dae 1 2 F. Lonchopteri dae 4 2 1 F. Asilidae 1 7 F. Bombyl i dae 1 2 F. Empididae 2 2 F. Cono~i dae 2 1 3 2 F. Syrp i dae 4 3 2 1 F. Agromyzi dae 4 73 159 F. Musci dae 2 3 F. Anthomyi i dae 3 1 F. Otti dae 2 3 F. 00 l i ch 0 p 0 d i d a e 2 4 F. Strat i orny; i dae 2 2 F. Tepth ri dae 2 ARANEAE F. Lycos i dae 1 10 56 51
-----------------------------------------------------------------
1 B. Insect and spi der associ ated wi th sweet potato 1. cropping systems at 42 DAP.
.,
Order & Family of i nsects & spi ders
A
Sweet potato agroecosystem
B c o ------------------------------------------------------------------COLEOPTERA F. Chrysome li dae 22 59 37 180 F. Cocci nel idae 1 3 15 25 F. Curcu li oni dae 2 1 2 F. Cassididae 6 3 2 5 F. Carabi dae 5 9 3 HYMENOPT ERA F. Formi caci dae 2 3 2 3 F. Bracon i dae 3 3 F. Chalcididae 7 Il 7 F. Scelionidae 1 2 1 F. Cephi dae 1 2 2 HOMOPTERA F. Ci dade 1; dae 14 19 20 F. De l ph a cid a e 2 3 3 2 F . Acana 1 oni i dae 4 4
112
------ -- ~
HErH PTERA ~ .... r~-PenTatomi dae 13 271
F. Thyreoco r i dae 119 123 F. Lygaei dae 1 2 2 1 F. Mi ri dae 14 342 63 144 F. Coreidae 2 1 F. Reduvi dae 3 3 ORTHOPTERA r-~ Pyr~omorph i dae 5 7 1 1 F. Acndidae - Acrididae 9 3 3 36 - Cyrtacan t hacri da 1 6 8 F. Tetriginidae - Pseudophyll i dae 7 3 7 10 - Phaneropteri nae 1 12 F. Gryll i dae - Oecanthi nae 1 3 - Nemobunae 5 7 6 13 LEPIDOPTERA r.-P-yra 1 i dae 3 1 5 F. Spingidae 2 3 1 F. Nymphal i dae 3 3 F. Noctui dae 3 DIPTERA F. As i 1 i dae 8 3 37 18 F. Bombyl i dae 5 3
l F. Stratiomydae 4 2 F. Rhagi on; dae 2 F. Phori dae 3 5 F. Lonchopteri dae 12 4 F. Therevi dae 4 4 F. Empi di dae 1 8 F. Conohi dae 39 3 9 F. Syrp idae 2 2 1 4 F. Dolichopodidae - 3 1 F. Tephriti dae 6 2 6 F. Agromyzi dae 32 265 211 F.Ottidae F. Muscidae 4 1 4 F. Anthomyi i dae 2 2 MANTODEA 4 3 BLATTARIA 2 ARANEAE r-:-Lycos i dae 38 48 137 247 ----------------------------------------------------------------
113
C. Insect and spider associated with sweet potato cropping .. systems collected at 49 DAP .
-----------------------------------------------------------------Order & Fami ly of Sweet potato agroecosystem insects & spiders --------------------------------------------
A B C D ------------------------------------------------------------------COLEOPTERA F. Chrysomel idae 8 47 86 58 F. Cocc;nel idae 4 2 27 24 F. Curcul i oni dae 6 1 5 F. Cassididae 11 5 4 2 F. Bupresti dae 1 F. Carabi dae 5 HYMENOPTERA F. Formi cac i dae 15 9 21 Il F. Braconi dae 4 2 5 F. Chalcididae 3 8 16 4 F. Scelionidae 2 2 3 F. Ichneumon i dae 4 F. Megachi 1 i dae 5 4 6 F. Spheci dae 1 6 HOMOPTERA F. Ci dadell i dae 12 21 45 48 F. Del phaci dae 4 6 5 14
1 F. Acanaloni idae 14 6 F. Ci cadi dae 1 3 5 F. Cercopod i dae 1 - 3 F. Aphididae HEMI PTERA
3 1
F. Pentatomi dae 2 19 133 F. Thyreocori dae 5 - 125 92 F. Lygaei dae 2 4 4 F. Mi ri dae 14 62 131 93 F. Corei dae 6 9 F. Tinfidae 3 2 ORTHOP ERA F. Pyr~omorphi dae 2 28 15 F. Acrl di dae - Acri di dae 39 67 56 44 - Cyrtacanthacri da 6 6 80 23 F. Tetriginiidae
Pseudophyll i dae 4 12 Il - Phaneropteri nae 14 55 27 5 F. Gryll i dae - Oecanth i dae 4 - Nemobunae 4 12 52 26 LEPIDOPTERA F. Plutellidae 5 1 3 3 F. Sphi ngi dae 3 1 1 F. Nympha 1 i dae 5 3 5
~ ,. 114
NEUROPTERA 1 DIP-rERA -r~ As lTdae 7 15 19 22 F. Tipulidae F. Mydi dae F. Stratipmydae 3 1 F. Phori dae 31 9 F. Locho~teridae 2 8 F. Bomby i dae 8 13 7 F. fmpididae 30 16 13 F. Conohi dae 3 F. Syrp i dae 8 26 17 8 F. Pi)unculidae -F. 00 ichopodidae 46 17 7 F. Tephritidae 9 7 2 F. Agromyzidae 52 112 191 F. Se)t i dae 2 F. He epmyz i dae F. Musc; dae 22 28 43 9 F. Anthom~i idae 2 1 1 F. Sa rcop agae
6 MANTODEA 4 ARANEAE f~--Lycos i dae 72 79 246 343 -----------------------------------------------------------------
:1 D. Insect and spider associated with sweet potat croppi ng sys tems at 56 DAP.
------------------------------------------------------------------Order & Family of Sweet potato agroecosys tem insects & spiders --------------------------------------------
A B C D ------------------------------------------------------------------COLEOPTERA F. Chrysomelidae 45 31 313 339 F. Coccinelidae 12 Il 44 52 F. Curculionidae 7 2 2 12 F. Cassididae Il 3 6 6 F. Bupresti dae 1 1 2 F. Carabi dae 2 2 F. Nitidulidae 4 3 HYMENOPTERA F~--Formi caci dae 3 19 14 F. Braconi dae 3 4 8 5 F. Cha 1 ci di dae 2 2 15 5 F. Au 1 aci dae 1 1 1 F. Scelionidae 1 2 5 F. Ichneumonidae 2 1 F. Cyn i pi dae 1 F . Andreni dae 2 . -
115
HOMOPTERA F. Ci cadell i dae 6 17 14 29 • F. De l phaci dae 2 3 7 F. Acana10ni idae 4 14 2 F. Cicadidae 1 3 F. Cocci dae 1 4 HEMI PTERA F. Pentatomi dae 48 24 1693 1276 F. Thryreocoridae 27 12 468 624 F. lygaei dae 3 4 2 2 F. Mi ri dae 86 75 209 396 F. Corei dae 8 3 46 22 ORTHOPTERA F. PyrQomorphidae 3 23 10 F. Acr,didae - Acri di dae 4 9 30 65 - Cyrtacanthacaridae 6 2 125 46 F. Tetriginiidae
Pseudoph; 11 i dae 10 1 14 19 - Phaneropteri dae 19 9 56 45 F. Gry11 i dae - Oecanthi nae 5 32 30 - Nemobunae 27 5 62 23 lEPIDOPTERA F. Sphingidae 3 1 F. Nymphal·: dae 3
1 F. Noctui dae 2 NEUROPTERA 1 DIPTERA F. As il i dae 6 3 4 1 F. Bomby1 i dae 6 5 12 24 F. St rat i omy; da e 4 6 4 F. Rhagionidae 1 F. Phori dae 10 5 27 12 F. lochopteri dae 12 7 23 1 F. Therevi dae 1 1 F. Empididae 7 5 12 4 F. Cono~i dae - - 1 F. Syrp i dae 3 5 5 6 F. Pi ~uncul i dae 1 1 F. Do ;chopodidae 1 4 6 F. Tephritidae 1 4 5 F. Agromyzi dae 4 4 F. Sepsidae 2 F. Muscidae 20 14 15 146 F. Anthomyi i dae 2 1 1 F. Api oceri dae 2 MANTODFA 2 1 BLATTARIA 4 ARANEAE F. lycos i dae 57 12 371 161 -----------------------------------------------------------,.
116
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