J. Asia-Pacific Entomol. 9 (1): 55~ 59 (2006)www.entomology.or.kr

BIOLOGICAL CONTROL

An Evidence to Use Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae)as a Field Control Agent against Diamondback Moth, Plutella xylostella L.(Lepidoptera: Plutellidae)

I d M . Yi Li 2Min Kwon*, Seung-Hwan Lee, an el- 109 n

Environment Management Division, National Institute of Highland Agriculture (NIHA), Rural Development Administration(RDA), Pyeongchang 232-955, Korea .. .. .[Laboratory of Insect Biosystematics, School of Agricultural BIOtechnology, Seoul National University, Seoul,151-742, Korea2Entomology Unit, Asian Vegetable Research & Development Center (AVRDC), Shanhua, Taman 741, Taiwan, ROC

Abstract The parasitic effectiveness of Cotesia plu­tellae Kurdjumov for the control of diamondbackmoth (DBM), Plutella xylostella L., was investigatedwithout spraying pesticides in the greenhouse, an~

the damage degrees of Brassica rapa cv. Pakchoigrown in parasitoid-applied plot and control plotwere evaluated. Among the 961 fourth instar DBMlarvae collected in the common cabbage fields atKwanshan region in Taiwan, 336 cocoons of C.plutellae were harvested from DBM larvae, whichshowed a parasitism rate of 35.0%. A total of 650DBM adults and 600 C. plutellae adults were re­leased according to the releasing scheme in theexperiment plot. As a result, 56.7% of the DBMlarvae were parasitized by C. plutellae adults 12days before harvest, and the parasitism reached80.8% at six days before harvest. At the harvesttime, a population density of C. plutellae cocoonsin the experiment plot was 2.2/plant, and that ofnormal DBM larvae was 0.5/plant. The density ofDBM in the control plot was much higher than inthe parasitoid-released plot by 7.7/plant. In the firs~

survey, the density of DBM larvae on a Pakchoiplant was quite low, 0.3 ~0.8 larvae. However, theDBM population increased dramatically after sixdays later, 5.7-10.1 larvae/plant. The p~pulation

density of DBM larvae showed clear differencebetween the parasitoid-released plot and control plot.The parasitism increased to nearly two fold withina week after the release of DBM adults. As a furtherstudy, a combining use of two or more species para­sitoids could be considered to improve a parasitismagainst DBM in the fields.

Key words Cotesia plutellae, Plutella xylostella,parasitism, Brassica rapa cv. Pakchoi, greenhouse

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

(Received December 27, 2005; Accepted March 4, 2006)

Introduction

The Diamondback moth (DBM), Plutella xylostellaL. is the most destructive pest of cruciferous crops,particularly of Brassica species, which include sucheconomically important vegetables as common cab­bage, cauliflower, broccoli, Chinese cabbage andradish (Salinas, 1972; Talekar and Shelton, 1993).Many researchers demonstrated that DBM couldreproduce round the year and are capable to com­plete 13-14 generations in a year, because t?eactivity of DBM could be observed between 10 Cand 50'C, and pupae and adults survived for severalweeks at O'C (Hardy 1938; Jayarathnam, 1977).DBM caused 52% loss in market yield in cabbage(Krishnakumar et al., 1986). It is estimated that thecost to manage DBM was at US$ 1billion per year(Talekar et al., 1992). In Korea, the insect has be­came an important pest of Chinese cabbages, com­mon cabbage and broccoli from the late 1980s (Kim,1990), and it is found in most parts of the KoreanPeninsular, from the southern island, Jeju-do, to thenortheast highland area, Daegwallyeong (NIAST,2000).

In the most countries, control of DBM by con­ventional chemical insecticides has so far been theonly practice. This over-reliance on chemicals haveled to several problems; increasing incidence ofpesticide poisoning of the farmers, development ofinsecticide resistance, presence of insecticidal resi­dues on marketed brassicas, hazards to other orga­nisms in the ecosystem, and spiraling costs of cropproduction (Talekar and Shelton, 1993). In particular,repeated and continuous spraying of chemical in­secticides for the control of this pest has resulted inDBM developing resistance to all commonly usedinsecticides (Shelton and Wyman, 1992; Shelton etal., 1993), including cartap, spinosad, chitin inhi­bitors and even Bt products (Hama, 1986; Zhao etal., 2002). Despite the development of resistance

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

to insecticides, worldwide vegetable growers con­tinue to rely on pesticide application for control ofDBM in the field.

A solitary larval-endoparasitoid wasps of DBM,Cotesia plutellae, was formerly known as Apantelesplutellae and is specifically adapted to wanner en­vironments, where temperatures can go above 30°C(Lim, 1982). They are found in tropical and subtro­pical countries wherein cabbage and other cruci­ferous crops are being grown. Field experimentsusing cabbage cultivars, grown under pesticide-freeconditions, indicated that in winter and early spring,parasitism by C. plutellae was less than 30% in thefield (Waladde et al., 2001). Encouraged by thishigh natural parasitism of C. plutellae in the field,researchers have studied on mass-rearing methods toutilize as biological control agent. Although the rea­ring technique was successfully carried out, thefield-release effect of parasitoids produced in thelaboratory was not as good as they had expected(Waladde et al., 2001). Recently, some studies toestablish an integrated control approach by usingthis parasitoid together with low-toxic pesticideshave been tried. This study was carried out to in­vestigate the parasitic effectiveness of C. plutellaefor the control of DBM without spraying pesticidesin the greenhouse, and to compare the damage de­grees between parasitoid-applied plot and controlplot.

Materials and methods

Test insects and plants

The larvae of Diamondback moth (DBM), Plutellaxylostella, and Cotesia plutellae cocoons were col­lected on 29-June in 200S, from the common ca­bbage fields in Kwanshan region located about 180km southeast from the Asia Vegetable Research andDevelopment Center (AVRDC) headquarters, Taiwan.DBM larvae were reared successively on commoncabbage at 23±2°C, 7S% RH, and a photoperiod of16:8 (L:D) h without any exposure to insecticides.The host plants were maintained in the greenhouseuntil they had 10- 12 leaf stage, at about 4 - 6weeks after transplanting into clay pots. This stagewas so ideal for rearing DBM larvae that weretransferred to the rearing room fitted with air­conditioner to maintain the temperature. Newly pu­pated DBM and parasitoid cocoons collected werestored at 10°C in a refrigerator until use.

The test plant was Brassica rapa cv. Pakchoiwhich is a preferable host of DBM larvae and needs

a short period for cultivating. Three thousands oftest plant seeds were sowed on July 1St, 200S. Thehealthy seedlings were chosen and transplanted intoclay pots (cp 12 em) at 20 days after sowing. A totalof 340 potted plants were placed into a greenhouse(Sx6 m) covered its roof with plastic vinyl, andwatered twice daily. A hexahedral structure (L'S>2xl.S m) as a control plot was constructed with steelframe and covered on all sides with fine mesh nylonnet, and filled with 48 potted plants. None of thetest plots were sprayed with any insecticide.

Rearing of parasitoids

A hundred cocoons of C. plutellae were kept in apetri dish, and placed it into acrylic cylinder. Also,absorbent cotton soaked in 10% honey solution ina small petri dish was placed inside the cylinder asa food source for emerging adults. After emergenceof the adults, they were allowed to mate for 2 - 3days before a fresh common cabbage leaf with 200second instar DBM larvae was introduced into thecylinder. After exposing the DBM larvae for 48hours for parasitization, the common cabbage leafwith parasitized larvae was removed from the cy­linder and placed on a fresh 6-week-old potted com­mon cabbage. After rearing parasited DBM larvae at23±2°C, 7S% RH, and a photoperiod of 16:8 (L:D)h for 2 weeks, cocoons of C. plutellae were col­lected and stored at 10°C in a refrigerator until usefor release into greenhouse.

Releasing scheme of insects

Just after transplanting host plants, totally 6S0 DBMadults were released three times into the greenhousefrom Ist, July. Totally 600 C. plutellae adults werereleased twice started from July zs" into only ex­periment plot where DBM had been introduced(Table 1). Only DBM adults were released threetimes into the check plot. For the introduction ofparasitoids into the greenhouse, a release stationwith A-type wooden house (l.Sm high) was builtinside the greenhouse for adult emergence. Stickyglue was pasted on the low part of the station polein order to protect parasitoid cocoons from beingattacked by harmful animals till emergence.

Evaluation of parasitism and control efficacy

To obtain parasitism to DBM, 30 (first, on July28th) and 120 (secondly, on August 3rd) 3rd or 4thinstar DBM larvae were collected randomly from

C. plutellae as a field control agent against DBM 57

Table 1. Releasing scheme of Diamondback moth and Cotesia plutellae into both greenhouse and control plot'

InsectsPopulation released at each date

21, July 23, July 25, July I, Aug. Total

In greenhouse

DBM adult 350 150 150 650

Cotesia adult 300 300 600

In control plot

DBM adult 50 20 20 90, Transplanting date; July I'" 2005

Table 2. Parasitism (%) to DBM by C. plutellae in the greenhouse

Survey time n C. plutellae DBM Died % Parasitism'

3 DAR l

9 DAR

30

120

17

97

10

9

3

4

56.7

80.8

, Days after release of parasitoids2 Collected DBM larvae were reared for 6 days until all pupated.

Results and Discussion

Parasitism to DBM by C. plutellae in fields

where, O=no damage; a=damage <5%; b=damage5- 20 %; c=damage 20- 60%; d=damage 60- 100%,and n=total number of plants.

A total of 961 fourth instar DBM larvae includingparasitized ones by C. plutellae was collected fromthe common cabbage fields in Kwanshan region, atownship of Taitong County located about 180 km

According to releasing scheme, we released 650DBM adults and 600 C. plutellae adults in theexperiment plot, and only DBM adults of 90 in thecheck plot. As a result, 56.7% of the DBM larvaehad been parasitized by C. plutellae adults till Julyzs" (three days after release). And, the parasitismreached 80.8% at nine days after release (Table 2).During final investigating inside the greenhouse,many parasitoid adults were observed hovering overthe plants. In general, the parasitism was affected inthe field by more factors than in the greenhouse.Although this experiment was practiced in the green­house, the parasitism obtained was lower than otherreport, where C. plutellae achieved rates of para­sitism as high as 90-95% in the field test (Waladdeet al., 2001). It can be supposed that high tem­perature inside the greenhouse caused a detrimentalinfluence on the parasitism rate of C. plutellae,because the temperature during daytime reaches upto 40°C, which is not suitable for activity of pa­rasitoids.

Parasitism to DBM in greenhouse

southeast from AVRDC Headquarter. During con­secutive four days, a total of 336 C. plutellaecocoons were formed from DBM larvae, whichshowed a parasitism rate of 35.0%. This result wassimilar to that reported by Waladde et al. (2001)which C. plutellae was active all year round andaccounted for 30 - 50% parasitism of DBM in thefield.

x 100la+2b+3c+4d

4nDamage degree (%)

each Pakchoi plot, and reared in the laboratory oncabbage plants until parasitoid cocoons were formed.Parasitism was calculated using the following equ­ation; No. of parasitoid cocoons/(Sum of parasitoidcocoons + normal DBM pupae) xl 00. The last in­vestigation was performed on August s". Totally 30Pakchoi plants were selected randomly, and thenumber of DBM and individual parasite pupae wascounted to determine DBM parasitism. Damage de­gree of Packchoi was assessed weekly on 30 randomplants for test plot and 15 plants for check plot. Forthe assessment of injury by the DBM larvae, thefollowing damage degree was employed;

58 J. Asia-Pacific Entomo!. Vo!. 9 (2006)

Control efficacy

At harvesting period, the damage degree of Pakchoiplants was evaluated by using a damage index me­thod as described before. With or without intro-

Fig. 1. Population densities of DBM on a Pakchoi plantafter release of parasitoids. Number of plant surveyed was30 for experiment plot and 15 for check plot. Test plantswere harvested 12 days after release of parasitoids, whenfell on August 9th

, 2005.

duction of parasitoids to control DBM resulted inmuch difference. Damage degree in experiment plotwas less than that in check plot (Table 3), whichrepresents C. plutellae played an excellent role incontrolling DBM on Pakchoi plants in the green­house. Converting the damage degree figures intocontrol efficacy, this species of parasitoids gave51.9%, calculated by (86.7-41.7)/86.7 x100.

This control efficacy suggested that the Pakchoiyields doubled in comparison to the check plotwhich DBM alone had been introduced. However,this yield could be still significantly less than in theDBM-free check, even though we did not performhere. To overcome these problems, many trials havebeen done till now. Talekar et al. (1992) stated thatthe IPM-DBM technology using C. plutellae as thecore component had to be supplemented with saferinsecticides such as microbial insecticide, Bacillusthuringiensis aizawai strains. This parasite, like otherparasites of DBM, is so highly susceptible to allchemicals pesticides that its activity drops wheneverchemical insecticides are used (Xu et al., 2001).Biological control methods maximizing the contri­bution of parasitoids and restricting pesticides shouldbe used only in situations in which it is absolutelynecessary, ought to be encouraged (Krishnamoorthy,2003). The combining use of two parasitoids speciesis also known to be promising as a biological controlapproach. Gharuka (2001) stated that combining useof C. plutellae and Microplitis plutellae should beable to exert adequate control of DBM in the tro­pical lowlands. It is, however, not advisable to mixC. plutellae and Oomyzus sokolowskii, a gregariouslarval parasitoid to DBM larvae (Ooi, 1988). SinceC. plutellae and 0. sokolowskii share similar ecolo­gical niches, and they both attack DBM larvae, it ispossible that these two parasitoids would competefor host larvae and that the weaker of the twospecies would not survive (Talekar and Hu, 1996).

One of possible alternatives to control DBM couldbe biological control, which involves principally theintroduction, augmentation, and conservation of na­tural enemies. It was reported that more than 90species of parasitoids attacked DBM but only 60 ofthem were important. Among them, 6 species attac­ked eggs, 38 attacked larvae, and 13 attacked pupae(Lim, 1986). The augmentative release of natural

., Release plot

3 6 12Days after release of parasltoids

.'o

12

.~ 8c:CD

""0

:::i::g 4

At harvest time, population density of C. plutellaecocoons in the experiment plot was 2.2/plant, andthat of DBMs was 0.5/plant. The density of DBMsin the check plot was much higher than in theexperiment plot, 7.7/plant (Fig. 1). The four timesdifference of population density between DBMs andC. plutellae cocoons in the experiment plot repre­sented that C. plutellae can give adequate control ofDBM in the cruciferous crops, the host plants ofDBM. On July zs", DBM larvae population infestedon a Pakchoi plant was relatively low, 0.3~ 0.8larvae. However, the DBM population increaseddramatically after six days later, 5.7~ 10.1 larvae/plant. It can be supposed that rapid increase of DBMpopulation could be caused by appropriate environ­mental condition inside the greenhouse. Talekar andShelton (1993) stated that the higher the temperaturethe faster is the completion of the life cycle ofDBM. Comparing the experiment plot and checkplot, the population density of DBM larvae showedclear difference. The DBM adult population doubledwithin a week.

Table 3. Comparison of damage degree by DBM larvae on Pakchoi plants between parasitoid-released plot and control plot

No. of plants with damage indexDamage degree, %Plot n

d0 a b c

Treatment 30 2 13 9 5 I 41.7

Control 15 0 2 7 8 3 86.7

1 Surveying date; August 9'", 2005, Damage degree ~ (la+2b+3c+4d)/4n xI00, where 0, no damage; a, damage <5%; b, 5-20% damage; c, 20-60% damage; d, >60% damage.

enemies can be an important component to any IPMprogram, and parasitoids such as C. plutellae, 0.sokolowskii and M plutellae are considered to beessential to any such program for DBM manage­ment. Therefore, further strategies in IPM of ve­getables have to focus on biological control andcultural control, with little emphasis on chemicalcontrol as suggested by Talekar and Yang (1993).Above all, the government and non-governmentalunits should support the transfer of environment­friendly technology utilizing parasitoids to all ca­bbage-growing areas.

Acknowledgement This research was supported by theAVRDC Special Project, KOREA/RDA 04-05, and parti­ally funded by the grant of Project No. 2005-03010301-09from RDA to M. Kwon.

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