J. Asia-Pacific Entomol. 6 (1) : 105-110 (2003)www.entomology.or.kr

Biological Control

Developmental Characteristics of Diadegma semiclausum Hellen(Hymenoptera: Ichneumonidae), a Larval Parasitoid of Plutella xylostellaL. (Lepidoptera: Vponomeutidae)Min Kwon, Kang-Ryong Park! and Hye-Jin Kwon

Highland Crop Research Div., National Highland Agricultural Experiment Station, Rural Development Administration,Pyeongchang 232-955, Korea

'Entomology Program, Graduate School of Agricultural Biotechnology, Seoul National University, Suwon 441-744, Korea

Abstract Some developmental characteristics of alarval-parasitoid ichneumonid, Diadegma semiclausumHellen, donated from Asian Vegetable Research andDevelopment Center (AVRDC), Taiwan, in 2001, wereinvestigated to utilize as a biological control agentof diamondback moth (DBM), Plutella xylostella L. Asa preliminary test for rearing the prey insect, weselected the most preferable host plant for oviposition.Among five host plants tested, DBM female laid thehighest number of eggs on the aluminum foil preparedwith radish leaf juice, followed by rape, Chinesecabbage, broccoli and common cabbage. The devel­opmental periods for the whole larval stage of D.semiclausum and DBM were 12.3 days and 14.2days at 23°C, respectively. Also, parasitoid cocoonscould be stored at 8°C for 24.5 days without a signif­icant loss of viability. The DBM pupa, however, lastedfor approximately four weeks at 8°C. The percentparasitism to DBM was 70.8% on average, and the sexratio of D. semiclausum adults was slightly male­biased, 0.94: 1. The seasonal fluctuation of DBMdensity showed apparently two generations, and thepeak periods occurred in middle June and middleJuly.

Key words Diadegma semiclausum, Plutella xylostella,Chinese cabbage, developmental period, parasitism,oviposition preference.

Introduction

The diamondback moth (DBM), Plutella xylostellaL. assumed to be originated in South-Southeast partof Europe, is a major pest of cruciferous vegetablesthroughout the world, especially serious in southeast

"Corresponding author.E-mail: mkwop@rda.go.krTel: +82-33-330-7850; Fax: +82-33-330-7715

(Received February 28, 2003; Accepted April 14, 2003)

Asia, and has been reported in at least 128 countriesorterritories (Salinas, 1972). In Korea, this insect hasbecame an important pest of Chinese cabbages,common cabbage and broccoli from the late 1980s(Kim, 1990), and it is found in most parts of theKorean Peninsular, from Jeju-do to Daegwallyeong(NIAST, 2000). If a suitable control is not undertaken,especially in the dry season, the yield loss and qualitydeterioration caused by DBM are extremely serious.The main reasons why DBM has became an importantpest to crucifers are the steady increase of mono­cultivation area of Chinese cabbage, the explosiveincrease ofDBM population by year-round cultivationin greenhouse, and the sudden decrease of naturalenemies due to indiscriminate use of pesticides. Inparticular, repeated and continuous spraying ofinsecticides has resulted in DBM rising as a majorpest to cruciferous vegetables (Hama, 1986; Kim etal., 1990; Zhao et al., 2002).

Despite the development of resistance to insecticides,worldwide vegetable growers continue to rely onpesticide application for control of DBM in the field(Talekar and Shelton, 1993). To control this insecticide­resistant DBM, many researchers have thereforetrying to develop effective methods; resistant cultivar(Dicksonet al., 1990),culturalpractices suchas splinklerirrigation (Talekar et al., 1986), synthetic sex pher­omone (Nemoto et al. 1992), microbial agents (Georgeand Thomas, 1992), selective insecticides having newmode-of-action (Schmutterer, 1992), and natural enemies(Lim, 1986; Talekar et al., 1992). Among them, oneof possible alternatives to chemical control is biologicalcontrol, which involves principally the introduction,augmentation, and conservation of natural enemies.

According to the field observation in Korea, fourspecies of parasitoids hosted DBM; Cotesia piutellaeKurdjumov, Diadegma sp.,Diadromus sp., and Oomyzussokoiowskii Kurdjumov (Oh et al., 1997). It is wellknown that two families of hymenopteran parasitoidsare promising to use biological control for DBM. Thefamily Ichneumonidae has Diadegma spp. and Hyposoter

106 J. Asia-Pacific Entomol. Vol. 6 (2003)

spp., and the other species belong to familyBraconidae (Flint and Dresitadt, 1998). Most speciesof hymenopteran parasitoids are larval endoparasitoidexcept Bracon spp., the exoparasitoids. Of which, D.semiclausum and Microplitis plutellae are famous forcool-climate adoptable parasites at highland area. Todate, there are many examples of suppression ofDBMpopulations usingparasitoids (DeBach, 1974; Sastrosiswojoand Sastrodihardjo, 1986; Ooi and Lim, 1989). Basedon these facts, researchers at AVRDC have tried todevelop integrated management based mainly onnatural enemies, cultivation of resistant lines, inter­cropping, modified irrigation, and microbial controlmethods (Lim, 1992). Many successful results byusing the natural enemies ofDBM have been reportedsince 1985.

ForDBMcontrol, wetestedonespecies ofIchneumonidae,Diadegma semiclausum Hellen, from Asian VegetableResearch and DevelopmentCenter (AVRDC) in Taiwan,2001. Some developmental characteristics on this larvalparasitoid as well as DBM were investigated in orderto evaluate the potential as a biocontrol agent underKorean conditions, first in the laboratory in 2001.

Materials and Methods

Maintenance of DBM colony

A colony of DBM was obtained from the NationalInstitute of Agricultural Science & Technology (NIAST),RDA, Suwon, Korea. Larvae were reared successivelyon Chinese cabbage cv. Gorangji Yeoreum at 23 ±2°C, 75% RH, and a photoperiod of 16:8 (L:D) hwithout any exposure to insecticides. The host plantswere maintained in the greenhouse until they had 10­12 leaf stage, at about 6 weeks after transplanting.This stage was so ideal for rearing DBM larvae thatwere transferred to the rearing room fitted withair-conditioner to maintain the temperature.

Selection of preferable crop for DBMoviposition

Before preparation of oviposition foil as CornellUniversity's method (Talekar, 1997), five cruciferouscrops; common cabbage, Chinese cabbage, radish, rape,and broccoli, were tested in order to select the mostpreferable host plant for DBM oviposition. About 130g of healthy and pesticide-free plant leaves was placedin a blender containing 1,000 ml of distilled water.The leaves were blended for' 2 minutes to form auniform slurry. The slurry was then transferred to a

2,000 ml grass flask and autoclaved (120°C, 1.05 kg!em' for 20 minutes). The autoclaved slurry was filteredthrough two layers of cheese clothes and plant debriswas discarded. Separately, several aluminumfoils werecut, wrinkled, and then flattened. Aluminum sheetswere dipped into the autoclaved juice in plastic trayuntil the sheets were uniformly wet. The sheets wereair-dried and cut into 2 X 10 em pieces and these werestored in a refrigerator at 8°C until use. Two hundredsof DBM pupae were placed in a 15 em diameter, 30ern long acrylic cylinder. Both opening ends of thecylinder were closed with a fine mesh cloth andtightened with several rubber bands. From the firstday of emergence, absorbent cotton soaked in 10%honey solution in a small petri dish was placed insidethe cylinder as a food source for emerged adults. Atthe same time, five oviposition foils formed roundtype by host plants were placed randomly in a cylinderbottom and the number of eggs oviposited werechecked every day for 6 days after emergence. Thisstudy was replicated 10 times with 200 DBM adultsper replicate.

Developmental periods of parasitoid

A hundred cocoons of D. semiclausum were kept ina petri 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 emerged adults. After emergenceof the adults (sex ratio, 0.94), they were allowed tomate for 2 - 3 days before a fresh Chinese cabbageleaf with 200 second instar larvae was introduced intothe cylinder. After exposing the DBM larvae for 48hours for parasitization, Chinese cabbage leaf withparasitized larvae was removed from the cylinder andput it on the fresh 6-week-old potted Chinese cabbagefor checking some developmental periods andparasitism at 23 ± 2 °C, 75% RH, and a photoperiodof 16:8 (L:D) h. These practices were continued everytwo days, 4 times, for 8 days with the same methodsabove mentioned. Finally, for optimal storage period,200 parasite cocoons and DBM pupae were groupedby 10 per container to make 20 containers and placedseparately in refrigerator at 8°C. The number of adultsemerged from cocoons and pupae was countedeveryday from 2 weeks after storage. This study wasreplicated with five times.

Monitoring DBM density in the field

As a preliminary study for field-release of D.semiclausum, though not practiced in this study,seasonal fluctuation in DBM density at Daegwallyeonghighland was surveyed in Chinese cabbage field by

using yellow sticky trap (15 X 30 em, 8 rep.) fromJune to September, 2001.

Results and Discussion

Oviposition preference of DBM adults

The DBM female began to lay eggs from the secondday after emergence, probably due to mating periodfor a day. Most oviposition occurred at the first fourdays, and 91.7% of the total eggs were laid in theperiod (Table I). Among five host plants tested, DBMfemale laid the highestnumber of eggs on the aluminumfoil prepared with radish leafjuice, followed by rape,Chinese cabbage, broccoli and common cabbage. Forfive days after emergence, the total number of DBMeggs oviposited on the radish-aluminum foil was2,220, but that on the common cabbage-aluminumfoil was 1,463 (F=16.04; df=4, 20; P<O.OOOI for totalegg number). Wakisaka et al. (1992) reported similarresults that the fecundity of DBM fed on broccoliand Chinese cabbage was significantly higher thanthose fed on common cabbage. This suggested thatradish and rape could be more preferable to rear theprey insect of parasite than common cabbage. Thereason why DBM female preferred radish and rapeamong cruciferous crops as oviposition host was notstudied here.

Developmental Characteristics of Diadegma semiclausum 107

Developmental periods of parasitoid and DBM

Table 2 illustrates the life statistics of D. semiclausumand DBM. The developmental periods for the wholelarval stage of D. semiclausum and DBM were 12.3days and 14.2 days at 23°C, respectively. The larvalstage period of D. semiclausum was shorter than thatof DBM.It should be favorable to utilize thisparasitoid as a biological agent for controlling DBM,because parasitoids could have more chances toparasitize the larvae. After formation of cocoons ofD. semiclausum inside DBM larvae, the parasitoidadults emerged from pupae within 8.6 days under thesame conditions as larvae, which was longer thanDBM pupae by 4.6 days. The difference of pupalstage period between parasitoid and DBM waseffective to discriminate the parasitized cocoons.Initially, pupae of DBM and D. semiclausum wereco-existed on the leaves, mainly other side of leavesand looked alike each other, which made selectingand collecting parasitoid cocoons difficult. Lee et al.(1996) reported that the developmental thresholdtemperature and total effective temperature of D.semiclausum were 8.4 °C and 267.8 degree-day (DD),respectively. Based on the developmental thresholdtemperature, our DBM colony needed more day­degrees for their development because the effectivetemperatures of our colony could be estimated as305.2 DD: 179.6 DD from egg to pupa and 125.6DD from pupa to adult.

Table 1. Number of daily DBM eggs laid after emergence on oviposition foil coated with each leaf juices extracted fromfive host plants

Crop testedNo. of eggs laid on each days after emergence

2ndTotal

3rd 4th 5th 6th

Radish 557 542 510 435 176 2,220 a

Rape 597 468 432 363 199 2,059 b

Chinese cabbage 477 469 407 352 189 1,894 c

Broccoli 406 346 389 321 94 1,556 d

Common cabbage 411 398 299 230 125 1,463 dMeans followed by the same letters are not significantly different (p<O.05, Duncan's multiple range test rSAS Institute, 1989]).

Table 2. Developmental period (days, Mean±SD) of egg, larvae and pupae of D. semiclausum and DBM in rearing room"

StageD. semiclausum

Penon Range Period

DBM

Range

Pupa 8.6±0.81

Egg

Larva 12.3± 1.59 10-15

8-10

3.3±0.48

14.2±1.84

4.6±O.82'Rearing condition: 23±2'C, 75% RH, 16:8 (L:D) h."Not tested.

108 J. Asia-Pacific Entomol. Vol. 6 (2003)

Storability of cocoons

After collecting pupae of D. semiclausum on theleaves and being stored in a refrigerator, the numberof emerged adults was checked everyday. As a result,parasitoid cocoons could be stored at 8°C for 24.5days without significant loss of viability (Table 3).The DBM pupa, however, lasted for approximatelyfour weeks under the same condition. The storabilityof parasitoid is very important and crucial factor forthe success of biological control, because a longstorage period can provide enough time to securesufficient parasitoids for field release. Considering thewinter period of highland region in Korea, the storageperiod (24.5 days) of D. semiclausum cocoons issomewhat short, so that much attempt to lengthenstorability of cocoons would be needed.

Parasitism of OBM by D. semiclausum

The parasitised pupae could be identified by the bodycolor. As the pupal period progressed, parasitoidpupae were darken, whereas non-parasitized DBMpupae remained light brown. In addition, parasitizedpupae were short with a blunt abdominal end,compared to the pointed abdominal end of the slightlylonger DBM pupae. The parasitism parameters werecalculated as shown in Table 4. These figures wouldbe changeable according to the sex ratio of inoculated

Table 3. Storability of D. semiclausum cocoons at 8°C

adults and test conditions. We inoculated parasitoidswith male-biased sex ratio of 0.94, which gave highparasitism of 70.8%. In Taiwan, the average parasitismwas only 13.1% at 4 weeks after the initiation ofparasitoid release into the field, but it reached 65.4%at 6 weeks (Talekar et al., 1992).

OBM density in the field

From June to September in 2001, the change inpopulation density of DBM adults was investigatedby using yellow sticky trap (15 X 30 em) in theChinese cabbage field at Daegwallyeong highland(Fig. 1). The seasonal fluctuation of DBM densityhas never been surveyed in a Chinese cabbage fieldat highland in Korea. There were apparently twogenerations of DBM, and the peak periods occurredin mid June and July. Considering larval stage forabout 2 weeks (Table 2), the period from late Juneto early July would be the most appropriate forinoculative release of the parasitoids into the fields.

D. semiclausum is widespread in Europe (Hardy,1938), and is believed to be one of the parasites thatsuppresses DBM population under control in thatcontinent. This parasitoid was first introduced intoTaiwan from Indonesia in 1985, and later becomeestablished practically in all major highland vegetableareas in Indonesia, Malaysia, Philippines, India andelsewhere(Talekaret al., 1992).Whereverthis parasitoid

Storage trialsPupal period, days

nMean±SD Range

March, 10 200 24.2±3.19 20-29

March, 25 200 26.6±1.14 25-28

April, 17 200 22.6±1.95 21-26

April, 27 200 25.2±3.11 20-28

May, 20 200 23.8± 1.79 22-26

Average 24.5±2.57 20-29

Table 4. Parasitism (%) of 2nd instar DBM larvae by D. semiclausum

DateNo. of cocoon or pupa Parasitism

Sex ratio"D. semiclausum DBM (%)

5 Oct. 128 65 66.3 0.94:1

7 Oct. 133 62 68.2

9 Oct. 143 50 74.1

11 Oct. 140 47 74.9

Total 544 224 70.8

"Female versus male for D. semiclausum adults inoculated.

Developmental Characteristics of Diadegma semiclausum 109

80

60::a:CO0....."5"'0 40III

'50z

20

o6/7 6/21 7/5 7/19 8/2 8/16

Date (Month/Day)

8/30 9/14 9/28

Fig. 1. The seasonal fluctuation in DBM density in a Chinese cabbage field at Daegwallyeong highland area in Korea, 2001.Migrated adults were trapped by yellow sticky trap (15 X 30 em, 8 rep.).

has been established well especially in the highlands,there are satisfactory control of DBM population andsignificant reduction in chemical insecticides use.Therefore,we introducedD. semiclausum from AVRDCin 2001, because D. semiclausum is well known forcool-climate adoptable, and can survive both in tem­perate regions where temperature does not go above26°C and cool highlands where temperature reachesbelow O°C. Our results indicated that this parasitoidcould be utilized to control DBM at highland regionif some following problems are solved. First, massrearing system at the level of laboratory should beestablished before inundant release of parasitoids.Second,alleviationof quarantineregulationson releasingexotic parasitoids introduced from foreign countriesinto the field is needed. Lastly, low toxic pesticidesto parasitoids should be selected to preserve thereleased parasitoids and the naturally occurred naturalenemies in the fields.

Acknowledgments We would like to thank Dr. N. S. Talekar,the entomologist at Asian Vegetable Research and DevelopmentCenter in Taiwan, for donating Diadegma semiclausum cocoonsas well as giving technical recommendations. Thanks are alsodue to Dr. Y. 1. Hahm at National Highland Agricultural Exper­iment Station, for reviewing the English of the manuscript.

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