J. Asia-Pacific Entomol. 9(4): 361-367 (2006)www.entomology.or.kr

ECOLOGY AND BEHAVIOR

Relationship between Feeding Damage by Beet armyworm,Spodopfera exigua (Lepidoptera: Noctuidae) and Leaf TrichomeDensity of Potato

Min Kwon*, Hyeon-Mook Cho and Young-Joon Abol

Crop Protection Lab., National Institute of Highland Agriculture (NIHA), Rural Development Administration (RDA), 232-955Pyeongchang, KorealEn~omo~ogy Division, School of Agricultural Biotechnology, College of Agriculture and Life Science, Seoul NationalUniversity, 151-818 Seoul, Korea

Abstract The responses (infestation level and feedingpreference) of 50 potato cultivars, including sevenKorean recommended cultivars to Spodoptera exigualarvae were examined in the field and the laboratory.Based on the number of potato leaves damaged byS. exigua larvae in the field, seven cultivars (Anco,Denali, Bintje, Jidose, Sandra, Shimabara, and Spunta)were found tolerant to the insect attack, whereasNicola and Dejima cultivars were highly susceptible.In feeding preference tests using leaflets of 10 selectedcultivars (Anco, Denali, Bintje, and seven recom­mended cultivars), no significant differences wereseen among the cultivars, indicating no involvementof chemical resistance. In trichome density per mm'leaf area, underside leaf surface contained higher tri­chome densities than the upper side, and the numberof long trichome were distributed four times morethan short trichome on average. Total leaf trichomedensity was the highest on cv. Dejima of 32.1, fol­lowed by cvs. Bintje and Denali, but the lowest oncv. Jopung of 6.9. From correlation analysis, infesta­tion level in the field by S. exigua was correlatedwith the density of shorter trichome on upper sideof leaf surface (r = 0.737* in 1996 and 0.772** in1997). However, it was not correlated with the densityof the longer trichome on underside.

Key wonts Spodoptera exigua, Solanum tuberosum,resistance, leaf trichome

*Corresponding author.Email: mkwon@rda.go.krTel: +82-33-330-7985; Fax: +82-33-330-7715

(Received October 13, 2006; Accepted December II, 2006)

Introduction

Since the last 80's the beet armyworm, Spodopteraexigua (Hubner), has been found an economicallyimportant insect pest of S. tuberosum in Korea, amongits 40 arthropod pests (Ahn et al., 1989). It is a poly­phagous insect pest infesting a wide variety of cropsincluding potato, and known as one of the three mostimportant pest species in the genus Spodoptera. Ifnot managed properly, its larvae would cause seriouslosses due to excessive feeding on potato leaves. Con­trol of S. exigua populations in the world is primarilydependent on repeated applications of conventionalinsecticides such as organophosphates, carbamates,and pyrethroids. Although effective, their repeated usehas disrupted natural biological control systems, some­times resulting in the development of resistance(Brewer et al., 1990; Aldosari et al., 1996). Theseproblems have highlighted the need for the develop­ment of new types of S. exigua control alternativesor alternative control methods without or with thereduced use of synthetic insecticides.

Potato cultivars resistant to S. exigua could be analternative tool that would improve control becausein an integrated pest management (IPM) program evenpartial resistance could be valued significantly (Beckand Maxwell, 1976). Host plant characteristics in­cluding morphological, physical, or structural qualitiesinterfere with insect behavior such as mating, ovi­position, feeding, and food ingestion and mediateantibiosis to insects (Smith 1989). It has been wellnoted that the plant foliages are protected by trichomessuch as hairs, scales, or pelate hairs (panda and Khush,1995). Most studies on potato resistance to insectshave concentrated on Phthorimaeea operculella (Zeller)(Malakar and Tingey, 1999), Leptinotarsa decem­lineata (Say) (Sinden et al. 1986; Tingey and Yencho,1994), Myzus persicae (Sultzer) (Gibson, 1971), and

362 1. Asia-Pacific Entomol. Vol. 9 (2006)

Empoasca fabae (Harris) (Maxwell et al., 1972; DeMedeiros et al., 2004). In the cultivars now beingplanted in Korea, little information has been knownon the mechanism and level of resistance to S. exigua.

This paper describes the laboratory and field studiesdone to evaluate 50 potato cultivars for resistance toS. exigua larvae, and investigated correlation betweenleaf trichome density of potatoes and feeding damagecaused by the larvae.

Materials and Methods

Insects

A colony of S. exigua was obtained from the NationalInstitute of Agricultural Science & Technology, RuralDevelopment Administration, Suwon, Gyeonggi Pro­vince, Korea. They were reared on artificial diet (Shinet al., 1997) at 22±I°C, 70±5% relative humidity (RR),and a photoperiod of 16:8 (LD) h without exposure

Table 1. Source of potato cultivars tested

to any insecticide. The second instar larvae were usedin feeding preference test.

Potato cultivars and agronomicpractices

Fifty potato cultivars used in this study were II early-­maturing, 11 mid-maturing, and 28 late-maturing cul­tivars (Table 1). Of these, Atlantic, Dejima, Irish Cob­bler, Jopung, Namsuh, Shepody, and Superior wereKorean recommended cultivars. They were stored ina cold room at 4°C, and were used whenever required.Seed pieces were pre-sprouted before planting. Wholeseed tubers showing sprouts of 1 to 2 em were cutinto two to four pieces with sterilized knives. Aftersuberization indoor for three days or longer, 30 cutseed potato tubers of each cultivar were hand-plantedin an area of 18.25 m2 (2.5 by 7.5 m) at 10 seedsper row. Just before planting, farmyard manure mixedwith horse feces and chemical fertilizers (N-·P-K) wereapplied at 15 MT ha

o

' and 150-180-120 kg ha', res­pectively. Dimethomorph (Forum 25 WP, 100 g ha")

AK-Red 2 Round Pale redAlamo 2 Oval White

Allegan/ Oval WhiteAnco I Oval Pale yellow

Atlantic" Oval WhiteBintje' Oval YellowCardinae Oval WhiteCarlton I Oval Yellow

Corine3 Long oval Yellow

Dejima" Oval WhiteDenali2 Oval Pale yellow

Desiree l Oval Pale redEigenheimer' Oval Pale redEpicure Oval Pale redHatsuhubuki3 Oval Pale yellowIrish Cobblera l Oval WhiteIsola 3 Oval Pale yellowJidose3 Oval WhiteJopung'" Oval WhiteKumsisuh3 Oval Pale redMarijke'' Long oval Pale yellowMaritta 1 Oval Pale yellow

Namsuha' Oval YellowNicola2 Oval YellowNorgold Russet2 Oval Dark yellow

Characteristics of tuberCultivar tested

Characteristics of tuber

Tuber shape Flesh colorCultivar tested

Norin#2 3

Norland2

Ojiro''

Penobscot2

Pentland Envoy"

ReceneRed Lasoda3

Red Pontiac3

Renemahue3

Rosa 3

Russet Norkotah2

Sandra!

Sebago 3

Shepody"Shimabara3

Shinyuseo/

Sieglinde'Snowchip'

Some Miore 3

Spunta'Superior"Toyoshiro"

Warba 'Wheeler3

Whitu 3

Tuber shapeOvalOvalOval

OvalOval

RoundOval

OvalOval

OvalOval

OvalOval

Very long oval

OvalOvalOval

RoundOval

Long ovalOvalOvalOvalOval

Oval

Flesh color

YellowPale redPale yellow

WhitePale yellowPale yellowRed

Pale redWhite

YellowPurple

Pale yellowWhiteWhite

YellowPale redPale yellowRedYellowWhiteYellowWhite

Pale yellowWhiteWhite

a Potato cultivars recommended by Korea GovernmentI early maturing cultivar; 2 mid maturing cultivar; 3 late maturing cultivar.

was applied at 7 to 10 d intervals with a hydraulicsprayer to control late blight, but no insecticides wereapplied.

Field experiment

The field studies were conducted at Gangneung,Gwangwon Province, Korea, where there was naturaloccurrence of S. exigua for 50 potato cultivars on3 May 1996 and for selected 10 cultivars on 6 May1997. The experimental design used the randomizedcomplete block with three replicates. The number ofleaves injured by S. exigua larvae for all plants perplot was counted three times at seven day intervals,around the most flourishing time (ca. 80 days afterplanting) of upper part of potato plant.

Laboratory experiment

To provide leaflets for laboratory bioassays, totally32 to 80 seed potatoes from each of 10 cultivars wereplanted in the experimental field in the Daegwall­yeong area located 800 meters above sea level. Threepotato leaves were collected from each of four plantsper cultivar 80 days after planting. Three second instarlarvae reared on artificial diet were placed on a potatoleaflet on a moistened kitchen towel (9 em in dia­meter) in a petri dish (11 em in diameter). Leaf areaconsumed by the larvae was measured with LI-3l00Area meter (LI-COR, Inc., Lincoln, Nebraska, USA)24 hours after inoculation.

Leaf trichome density

Trichomes on both the abaxial and adaxial surfacesof leaflets from 10 selected potato cultivars werecounted by the method of Dimock and Tingey (1987)with a slight modification. Twelve fully expandedupper-plant leaves (4th to 6th leaf from terminal nod~)

from four plants of each cultivar were collected mthe field and immediately transported to the labo­ratory. A binocular stereomicroscope (SOx) with anocular grid (mm') was used to count the nu~ber oftrichomes. To scrutinize the shape of leaf tnchome,potato leaflets were examined by a Hitachi S-2460Nscanning electron microscope (450 x ) .

Statistical analysis

The number of leaves damaged, the leaf area con­sumed, and the number of trichomes were determinedfor analysis of variance (ANOVA). Means±SD were

Beet armyworm and Potato leaf trichome 363

compared and separated by Tukey's Studentized ~ngeTest at P = 0.05 (SAS Institute, 1996). A possiblerelationship between damage level caused by S. exigualarvae and the number of leaf trichomes was estimatedby correlation analysis (PROC CaRR, SAS Institute,2004).

Results

In the 1996 field experiment, the number of leavesdamaged by S. exigua varied according to cultivars,ranging from 0.2 to 23.6 (Table 2). Of 50 tested cu~­

tivars, seven (Sandra, Spunta, Jidose, Anco, Denah,Shimabara and Bintje) were less infested, below 0.5damaged leaves at the lowest estimate. However, cvs.Nicola, Jopung, and Warba were severely damaged,as damaged leaves were more than 13.3, while thedamaged leaf number of Russet Norkotah, Eigen­heimer, Red Lasoda, Dejima, Sieglinde, and Wheelercultivars ranged from 9.7 to 12.3. The other 34 cul­tivars were moderately infested with damaged leafnumbers ranging from 0.5 to 8.8. In the 1997 fieldexperiment, the number of damaged leaves of 10selected cultivars (Anco, Denali, Bintje, and sevenKorean recommended cultivars) to S. exigua larvaewas ranged from 2.7 to 130.0 (Table 3). The mostseverely damaged cultivar by the infestation of S.exigua was Jopung and followed by Dejima andSuperior, but cvs. Anco and Denali showed lowdamage estimate under 8.3. In leaf feeding test, therewere no significant differences by potato cultivars inleaf area consumed by S. exigua larvae (Table 4).

The trichome density of potato leaf was dependenton cultivar. Regardless of potato cultivars, undersideleaf surface contained more trichomes than the upperside and the number of long trichomes was as fourtimes more as shorter ones. The highest density ofleaf trichome was on cv. Dejima of 32.1 mm", butthe lowest on cv. Jopung of 6.9 mm". Short trichomedensity on upper side of leaf showed positive cor­relation coefficients with infestation levels by 0.737*in 1996 and 0.772** in 1997.

Discussion

Both in 1996 and 1997 experiments, one of the mostseverely damaged cultivar by the infestation of S.exigua was Jopung, which was bred in Korea by usingthe Dutch cultivar Resy as the seed parent and theAmerican cultivar Superior as a pollen parent (Kimet al., 1990). This cultivar has been distributed nation-

364 J. Asia-Pacific Entomol. Vol. 9 (2006)

Table 2. The number of damaged leaves (Mean±SD) of potato cvs. tested to S. exigua larvae in the field in 1996

Cultivar No. of damaged leaf(Mean±SD)

Cultivar No. of damaged leaf(Mean±SD)

same letter are not significantly different at P = 0.05 using Tukey's Studentized Range Test.

SandraSpuntaJidoseAncoDenaliShimabaraBintjeRecentKumsisuhAlamoNorlandAlleganySnowchipNorgold RussetHatsuhubukiMarijkeSebagoShinyuseoIsolaDesireeCardinalPentland EnvoyNorin#2Irish CobblerToyoshiro

, Means followed by the

0.2 ± 0.32t' Atlantic0.3 ± 0.12t Red Pontiac0.3 ± 0.21t Some Miore0.3±0.15t Maritta0.4 ± 0.15st Penobscot0.4 ± 0.21st Shepody0.4 ± 0.06st Epicure0.5 ± 0.21rst Renemahue0.5 ± 0.50qrst Rosa0.5 ± 0.50qrst Ojiro0.5 ± 0.23qrst Namsuh0.6 ± 0.60qrst Corine0.6 ± 0.15qrst Carlton0.7 ± 0.35pqrst Superior0.9 ± 0.42pqrst AK-Red1.0 ± 0.210pqrst Whitu1.3 ± 0.21nopqrst Wheeler1.5 ± O.1Omnopqrst Sieglinde1.6 ± 0.35lmnopqrst Dejima1.7 ± 0.25lmnopqrst Red Lasoda2.0 ± 0.26lmnopqrst Eigenheimer2.3 ± 0.21klmnopqrst Russet Norkotah2.6 ± 0.15jklmnopqrst Warba3.0 ± 0.45ijklmnopqrst Jopung3.1 ± 0.21hijklmnopqr Nicola

3.1 ± 0.40hijklmnopq3.4 ± O.l7hijklmnop3.5 ± 0.44hijklmno3.6 ± 0.53hijklmno3.6 ± 0.36hijklmno3.7 ± 0.80hijklmn4.0 ± 0.25hijklm4.0 ± 0.45hijklm4.2 ± 0.61hijkl4.6 ± 0.57hijk5.0 ± 0.82ghij5.4 ± 0.12ghi5.4 ±: 0.36ghi5.6 ± 0.60gh7.6 ±: 1.15fg8.8 ±: 1.1lef9.7 ±: l.72def

10.0 ±: 1.15def10.2 ±: 1.20def10.2 ±: 1.3Ide11.8 ±: 1.81cd12.3 ±: 1.46cd13.3 :1: 1.90c16.7 :1: l.75b23.6 :1: 1.5Ia

Table 4. Leaflet feeding rates (M<:an±SD) of 10 potatocultivars by S. exigua larvae in feeding tests

Table 3. The number of damaged leaves of potatoes(Mean±SD) by S. exigua larvae in the field for 10 cultivarsin 1997

Cultivar No. of damaged leaf (Mean±SD) Cultivar Damaged leaf area (crn'') of potatoleaflet" after 24 hrs

Anco 2.7 ± 1.53g1

Denali 8.3 ± 3.06fgBintje 18.0 ± 3.00efgIrish Cobbler 20.7 ± 4.04efAtlantic 30.0 ± 6.56deShepody 32.7 ± 5.5IdeNamsuh 45.3 ± 7.09cdSuperior 53.3 ± 7.37cDejima 77.0 ± 5.57bJopung 130.0 ± Il.79a

Means within a column followed by the same letter arenot significantly different at P = 0.05 using Tukey's Stu­dentized Range Test.

wide since 1988. The cultivation in the area graduallyincreased because of its strong resistance to potatoleaf roll virus and potato late blight, Phytophthorainfestans (Mont.) de Bary (Hahm et al., 1996), andwireworms (Kwon et al., 1999). Contrary to Jopung,

Anco 0.47 ± 0.12Atlantic 0.43 ± 0.12Bintje 0.43 ± 0.15Dejima 0.47±0.15

Denali 0.37 ± 0.12Irish Cobbler 0.50 ± 0.17Jopung 0.53 ± 0.12Namsuh 0.37 ± 0.12Shepody 0.40 ± 0.17Superior 0.47 ± 0.06

Leaf area (ern') fed by S. exigua was measured with areameter (Ll-3100) 12 hr and 24 hr after infestation.a Potato leaflets cultivated in the field were used by leaf

feeding method.

cvs. Anco, Denali and Bintje were stronger to theinfestation of S. exigua larvae than the others. Thesecultivars have yellow to pale yellow flesh color, whilecvs. Jopung and Dejima have white flesh color. It

Beet annywonn and Potato leaf trichome 365

Table 5. Number (Mean±SD) of long and short trichornes at interveinal area (mrrr') of the under or the upper side of leavesfor 10 potato cultivars

CultivarUnder side Upper side

Long Short Sum Long Short SumTotal

Anco 7.5±0.97c 1 l.3±0.48def 8.9±1.32e 0.8±0.42d 0.1±0.32ab 0.9±0.57de 9.8±1.25eAtlantic 6.9±1.20cd 2.4±0.52c 9.3±1.34e 0.4±0.52d 0.1±0.32ab 0.5±0.7le 9.8±1.62eBintje 11.9±1.10b 3.5±0.85d l5.4±1.65b 2.2±0.42b 0.2±0.42ab 2.4±0.52b 17.8±1.62bDejima 21.9±1.66a 5.8±0.92a 27.7±1.89a 4.0±0.82a 0.4±0.52a 4.4±1.17a 32.l±2.64aDenali 11.6±1.35b 1.0±0.67f l2.7±1.58c 1.6±0.52c Ob 1.6±0.52c l4.3±1.55cIrish Cobbler 7.4±0.97c 1.8±0.63de 9.2±1.14e 0.7±0.42d Ob 0.8±0.42de 9.9±1.25eJopung 4.7±0.67e 1.2±0.42ef 5.9±0.88f 0.6±0.52d 0.4±0.52a 1.0±0.47cde 6.9±1.20fNamsuh 7.0±0.82cd 1.4±0.52def 8.4±0.97e 0.5±0.53d Ob 0.5±0.53e 8.9±1.37eShepody 7.5±1.08c 3.4±0.52b 10.8±1.20d 0.6±0.52d 0.1±0.32ab 0.7±0.67de 11.5±1.l7dSuperior 6.2±0.79d 1.9±0.57cd 8.l±0.74e 0.9±0.32d 0.4±0.52a l.3±0.67cd 9.4±1.07e

1 Means within a column followed by the same letter are not significantly different at P = 0.05 using Tukey's StudentizedRange Test.

Table 6. Correlation coefficients (r) between infestation level by S. exigua in the field and leaf trichome density of 10 potatocultivars

Infestation levelShort trichome density

Under side Upper sideLong trichome density

Under side Upper side1996 0.136 0.737*1997 0.135 0.772**

In 1996, n= 10, F=9.53, P=0.015; in 1997, n= 10, F= 11.8, P=0.009.

0.026 0.1190.005 0.112

was not certain whether a feeding preference of S.exigua was affected by potato flesh color or not.

The leaf trichome densities were dependent on cul­tivars. Regardless of potato cultivars, underside leafsurface contained more trichomes than the upper side,and total trichome density was mostly influenced bylonger tones on underside leaf. This result was coin­cident with trichome survey of wild type potato aswell as some commercial potato cultivars (Tingey andLaubengayer, 1986). Total leaf trichome density waslowest on cv. Jopung which showed the highest da­mage degree to the S. exigua in field tests. The cor­relation coefficients between infestation level and leaftrichome density by 10 potato cultivars representedthat short trichome density on upper side of leaf waspositively correlated with damage degree by S. exigua.With regard to plant pubescence as being involvedin susceptibility or resistance to some insects, Lee(1983) reported that the trichomes of soybean deterredoviposition and feeding of E. fabae by preventing theinsect's ovipositor or proboscis from reaching theplant epidermis. In contrast, the cotton ballworm,Helicoverpa zea, preferred pubescent strains of cottonfor oviposition and attachment, because the ovipo­siting females use plant hairs as footholds, which aidattachment to plant (Meredith and Schuster, 1979).

It has been well known that wild potato has con­tained defense mechanism against some insect pests,

because trichomes of wild potato have insecticidalactivity by trapping or fixing insects (Dimock andTingey, 1987; Gibson, 1971; Tingey and Sinden, 1982).In the present study, Dejima and Jopung cultivarscontained the highest and the lowest trichome den­sities, respectively, and were susceptible to S. exigualarvae. These results suggest that total number of thetrichomes do not affect the infestation by S. exiguaon the basis of correlation analysis. Leaf area ratioconsumed by S. exigua larvae was correlated withonly the density of shorter trichome on upper sideof leaf surface. In general, leaf trichomes of potatocould effectively interrupt the feeding by small body­sized insects such as aphids (Gibson, 1974), spidermites (Gibson and Valencia, 1978), leafhoppers andflea beetles (Tingey and Sinden, 1982), while the bigbody-sized insects like Colorado potato beetle showeddifferent tendencies (Dimock and Tingey, 1987). Todate, there has been little information about potatotrichomes resistant to the S. exigua, which is relativelybig-sized than other insects.

Lee et al. (1999) demonstrated that two types (Aand B) of glandular trichomes were present on leafsurface of S. tuberosum cv. Dejima, and density oftype-A trichome was 8.37±1.58 mm", This figure wasdifferent from our result, for the trichome density perunit leaf area will be sparse as potato plant grows.To identify the trichome type, the enlarged shape of

366 1. Asia-Pacific EntomoL Vol. 9 (2006)

shorter trichomes was examined through the scanningelectron microscope (S-2460N SEM, HITACHI).They had typical four-lobe structure. So far, manytoxic substances have been identified in plant tri­chomes. Gibson (1971) reported that some wild Sola­num species (s. polyadenium Greenm., S. berthaultiiHawkes, and S. tariyense Hawkes) discharged water­soluble exudates from the four-lobed head of theglandular trichomes, which were mediated resistanceto M persicae or Macrosiphum euphorbiae (Thomas).Type A trichomes contained some phenolic exudatesin S. berthaultii leaf, which functioned as defensemechanism to aphids (Harborne, 1988). Most of cul­tivated potatoes, however, have non-glandular tri­chomes of both A and B types morphologically similarto those in wild potatoes. Some breeding efforts havebeen focused on producing hybrid potatoes with genesfrom wild cultivars conferring resistance to insectpests, based primarily on glandular trichomes charac­teristics (Mehlenbacher et al., 1984). Kowalski et al.(1993) demonstrated that leaves of commercial potatocultivars contained very low level of resistant-relatedsubstances compared with wild potatoes, S. berthaultii.This study indicates that cvs. Sandra, Spunta, Anco,Jidose, Shimabara, Bintje, and Denali are promisingcultivars resistant to S. exigua. However, cvs. Jopungand Dejima, two important cultivars in Korea, wouldrequire intensive control against S. exigua to assurestable production in the field. In terms of resistancemechanism to potato leafhopper, De Medeiros et al.(2004) stated that leaf trichome density and dropletsize and total content of foliar glycoalkaloids arenecessary to fully elucidate this question. Althoughmore work would be required to further elucidate theexact mechanisms of potato resistance to beet army­worm, our data would encourage breeding efforts toincorporate the resistance present in some cultivarsinto Korean potato cultivars.

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