DEVELOPMENT OF PALEXORISTA LAXA' IN HELJOTHIS ZEA' LARVAE FED PEST-RESISTANT SOYBEAN GENOTYPES'

D. J. Rogers' and M. J. Sullivan' Department of Entomology

Clemson University Clemson, South Carolina 29634-0365

Abstract: The development of the exotic parasitoid Palcxorista laxa (Curran) (DIPTERA: Thchinidae) nnd its host. Heliothis zea (Boddie) (LEPIDOPTERA: Noctuidae), was studied on artificial diet. and pest-resistant and susceptible soybean genotypes, These laboratory studies showed that H. zea l8lVsl dc\'ciopment was fastest on artificial diet and slowest on leaves of the plant introductions (PIs) 171451 and 229358. The H. zea lar...a1 growt.h rate on the cultivar 'Govan' was greater than on the (wo resistant Pis, while the gro.....1.h rate on 'Bragg' was greater than on PIl71451. The only significant effect on P. lara ''''as on larval development period; this was shortest on diet-fed H. zeu larvae and longest on P1229358-fed Ia.rvae. P. laxa larval development period on Pll71451-red hosts was not different from that on 'Bragg'- and 'Covan'-fed H. zea larvae.

I(ey \\lords: Soybean, Heliothis zea, Pnlcxorista taxa. parasitoid. plant resistant, 3-trophic level interaction, antibiosis.

J. Agric. Entomol. '1(3): 201-206 (July 1987)

Palexorisw, laxa (CWl'lm) is a lalval parasitoid of Heliothis armigem (Hubner) (LEPIDOPTERA: Noctuidae) in India and Afi·ica (Jackson et al. 1976). Females lay mernbranous·type eggs (Hagen 1964) on mid· to late·instar larvae. The eggs hatch within 15 seconds of oviposition and the larvae penetrate the host cuticle and begin to feed (Chauthani and Hamm 1967). The parasitoid larvae kill the host and pupate in 4 to 9 days at 20 to 35°C (Jackson et al. 1976).

Palexorista La.m has been imported into the United States [rom India (Chauthani and Hamm 1967; Bryan et al. 1969; Jackson et al. 1.976) and Arrica (B. M. Shepard, personal communication) for evaluation against Hcliothis species in cotton in Arizona, as well as cotton, soybeans and other crops in the southeastern United St.ates. The colony used in this study originated from parasitized H. armigera larvae collected on pigeonpea, Cajanus cajan (L.) Millsp., in Kenya, Africa, by B. M. Shepard in June 1982. Evaluations conducted with this colony have shown that P. taxa has good host-finding ability on soybean, good reproductive capacity. and may be able to overwinter in South Carolina in field cages (J. F. Grant, personal communication).

Host·plant resistance has been reported to a wide variety of soybean insect pests since the discovery of resistance to the Mexican bean beetle, Epilachna variuestis MuJsant (COLEOPTERA: Coccinellidae), by Van Duyn et al. (1971) in

I D1PTERA: Tachillidoe. 2 LEPIDOPTEIt.\: Noctuidne. ~ Technical Contribution Number 2582 of the South Carolina Agricultural Experiment StaLion, Clemson.

S.C. Accepted for publication 27 August 1987. • Present address: Emomolol,')' Branch, Department of PrimH!)' Industries. P. O. Box 23, Kingaroy,

Queensland. Austmlio 4610. l Edisto Research and Education Center, P. O. Box 247. H1uckville, S.C. 29817.

201

202 J. Agric. Entomol. Vol. 4, No. 3 (1987)

plant introductions (Pis) 171451, 227687 and 229358. These PIs exhibit resistance to several important noctuid pests of soybean, including corn eanvorm (CEW), H. zea (Boddie), soybean looper (SBL), Pseudoplusia includens (Walker), and velvetbean caterpillar (VBC), Aflticarsia gemmatalis Hubner (Turnipseed and Sullivan 1976; Sullivan 1985).

The effects of pest-resistant soybeans on the development of a Tachinid parasitoid, Varia ruralis Fallen, have been studied in the SBL by Grant and Shepard (1985). They found that parasitoid development was adversely affected in SBL fed the Pl229358-derived breeding line, ED73-371, but not in hosts fed other PI229358-derived line, D75-10230. The effects of pest-resistant soybeans on the development of hymenopterous parasitoids of lepidopterolls species have been studied by Yanes and Boethel (1983), Powell and Lambert (1984), Orr and Boethel (1985), and Beach and Todd (1986). Rogers and Sullivan (1986) studied the effects of two of the PIs on the Lygaeid predator Gercoris punctipes (Say). All of these studies documented adverse effects on the development of beneficial species when hosts or prey fed on pest-resistant soybean genotypes.

If P. laxa is to contribute to the reduction of CEW populations in soybean, then it should not be adversely affected by commercial pest-resistant soybean genotypes currently under development. The objective of this study was to compare the development of P. taxa in CEW larvae fed the pest-resistant PIs 171451 and 229358 with parasitoid development in host fed pest-susceptible cultivars and artificial diet.

MATERIALS AND METHODS

The study was conducted in a rearing chamber at 26.5 ± 1°C, 60% ± 10% and a photoperiod of 14:10 (L:D). CEW eggs and P. laxa adults used in this expedment were from laboratory colonies maintained at Clemson University by J. F. Grant and R. E. McWhorter. The source of the Clemson P. laxa colony was a colony maintained since 1982 by J. E. Powell at USDA Delta States Research Center, Stoneville, MS.

Diet-reared CE\V were fed on pinto-bean cliet modified fi'om that of Greene et al. (1976) by the substitution of alfalfa meal for soybean protein. Diet-fed larvae were reared individually in 30-ml plastic diet cups containing 10 to I5-ml of diet. Plantfed CE\V larvae were reared individually in 9-cm diameter petri dishes. Moistened filter paper was placed on the bottom of each petri dish to prevent dessication of the foliage. Leaves were replaced at 1 to 3-d intervals, depending on larval size. Soybean leaves for leaf-fed CEW were from the upper one-thiJ·d of field-grown plants in growth stages between Vll and R5 (Fehr and Caviness 1977). Before the leaves were used as food for the CE\V larvae, they were immersed in water to remove dirt, washed in 0.05% sodium hypochlorite to destroy surface-borne entomopathogens, and finally rinsed in water.

After CEW larvae moulted to the fifth instal', they wre exposed to ovipositing P. taxa females in a 31 X 31 X 41-cm Plexiglas cage. CEW larvae were removed and weighed when P. laxa oviposition had been observed on most CEW individuals. The CEW diet treatments were continued until CEW pupation or emergence of P. taxa larvae. CEW pupae and P. laxa puparia were weighed on the day of pupation. CEW age at parasitization, the lengt.h of the P. taxa egg + larval stage, the number of P. la:ca per host, and P. taxa pupal duration and adult sex were recorded

ROGEHS and SULLIVAN: P. taxa and H. zea. on Pest-Resistant Soybeans 203

for each individual. Experimental insects were monitored twice daily from parasitization to adult emergence.

CEW growth on the five diets was monitored using lalvae that had not been parasitized by P. laxa. The length of CEW latval life and the pupal weight 1 d after pupation were recorded. A growth rate index was calculated from these data by dividing pupal weight by the length of laNai life. This growth index provided a combined measure of the two aspects of antibiosis resistance.

Data were analyzed using analysis of variance (ANOVA). In this experiment it was impossible to precisely control either CEW weight at parasitization or the number of P. taxa. eggs laid per host. Because both of these factors could affect P. laxa performance, they wre considered as covariates in the A lOVAS. \Vhere one or both of these factors were significant (P < 0.05), adjusted means were used to test for differences among treatments using Fisher's LSD test (Steel and Torrie 1980).

RESULTS AND DISCUSSION

Because of antibiosis resistance effects, CEW were older at parasitization on the PIs than on the cultivars and artificial diet (Table I). Despite this age difference, mean Jruval weight at parasitization was not significantly different among treatments and aU larvae were stung dwing the first 24 h of the fifth instar.

Table 1. Weight and age of fifth-instar Heliothi$ zea laNae when parasitized by Palexorista laxa.

Weight at Age at Host parasitization parasitization food (mg) (days)

Artificial Diet 232.4 ± 5.32a' 7.6 ± 0.14a' 'Bragg' 227.9 ± 7.57a 12.9 ± 0.20c 'Govan' 228.7 ± 11.53a 11.4 ± 0.30b PI171451 209.8 ± 7.70a 13.8 ± 0.20d PI229358 227.6 + 8.32a 15.4 ± 0.22e • Mean ± SE. Within II column, means followed by different letters are significantly different (P < 0.05,

Fisher's LSD lest).

On all diets, the mortality of CEW larvne from causes other than P. taxa parasitization was minimal « 4%) after the larvae had reached the fifth instar. Because the study of CE\V mortality caused by plant resistance was not an objective of this experiment and had been documented previously (Hatchett et al. 1976), no formal data on mortality were recorded prior to the beginning of the fifth instar. However, it was apparent that mortality in instars 1 to 4 was higher on the PIs than on the susceptible cultivars and artificial diet.

Artificial diet was significantly (p < 0.05) better than the four soybean genotypes for CEW growth (Table 2). 'Govan' was the best soybean genotype for CEW growth while PI229358 was the worst. These data are similar to those reported by Beland and Hatchet (1976), in that they confirm the resistance of PI229358 compared to 'Bragg'. The difference in CEW pupal weight recorded between P1229358 and 'Bragg' (Table 2) is larger than that reported by Hatchett et al. (1976). In addition, our study shows the two Pis to be equal in antibiosis

204 J. Agric. EntomoJ. Vol. 4, No.3 (1987)

resistance, in contrast to the conclusion of Lambert and Kilen (1984) and Powell and Lambert (1984) that PI171451 is less resistant to the CEW than PI229358.

Table 2. Development of Heliothis zea reared on artificial diet and four soybean genotypes.

Larval Pupal Growth Host development weight rate index* food n time (days) (mg) (mg/day)

Artificial 32 13.4 ± 0.33et 414.0 ± 7.54a t 31.0 ± 0.57a t diet

'Bragg' 24 19.4 ± 0.38a 326.5 ± 8.71b 17.1 ± 0.65bc 'Govan' 15 18.1 ± 0.48b 329.5 ± 11.00b 18.5 ± 0.83b PI171451 26 20.1 ± 0.37a 292.3 ± 8.43c 14.7 ± 0.63d PI229358 7 19.9 + 0.70a 295.1 + 16.11bc 15.0 + 1.21ed

puplll weight • Gro..... th rate index = Inn-al development lime

t Mean ± SR. \\!ithin 1.1 column, 1lll'IlJlS folJO\'>'cd by different letters are significllntly different (P < 0.05, F'isher's LSD test).

Palexorista laxa larval development time was significantly (P < 0.05) shorter on diet-fed CEW than on 'Bragg'- or PI-fed CE\V, while pal1lsitoid larval development time on 'Bragg'- and 'Govan'·fed larvae was significantly (P < 0.05) shorter than on PI229358-fed larvae (Table 3). Palexorista laxa larval development period on PI171451-fed larvae was not different from that on 'Bragg'- and 'Govan'-fed larvae. No significant difference in larval development time was found between male and female parasitoids (F = 1.1; df = I, 268; P > 0.05). The number of P. laxa puparia per host and host weight at parasitization were significant covariates in this analysis. P. laxa larval development time declined as the number of puparia per hnst increased (F = 18.5; df ~ I, 268; P < 0.001). Larval development time increased as host weight decreased (F = 27.6; df = 1, 268; P < 0.001). The latter relationship had been observed previously by R. E. l\1c\Vhol'ter (personal communication).

Table 3. Effect of Heliothis zea larval diet on size and development of Palexon'sta laxa.

Larval Pupal Puparial Host development development weight food time (days) time (days) (mg)

Artificial 5.1 ± 0.07a' 9.0 ± 0.05a' 53.5 ± 0.97a* diet

'Bragg' 5.5 ± O.lOb 9.0 ± 0.07a 40.8 ± 1.47b 'Govan' 5.3 ± 0.15ab 8.9 ± O.l1a 40.9 ± 2.15b PI171451 5.6 ± O.lIbc 8.9 ± 0.08a 38.9 ± 1.53b PI229358 5.8 ± O.lle 8.9 ± 0.08a 37.3 ± 1.57b " Mean ± SE. Within u column. means folowed by different. let:t.crs are significantly dirferent (P < 0.05,

Fisher's LSD test).

The covariance analysis indicated that a lOO-rug reduction in CEW weight at parasitization increased P. taxa larval development time by only 0.4 ± 0.08 d. In

ROGERS and SULLIVAN: P. laxa and H. zea on Pest·H.esistant Soybeans 205

the field, the antibiosis of the pest-resistant genotypes would result in smaller CEW larvae than on susceptible cultivars. However, this size reduction should have relatively little impact on the total development time of P. laxa on resistant genotypes because of small rate increase of P. laxa larval development time as CE\V size declines.

No significant differences (P < 0.05) were found among treatments in either the number of P. taxa pupalia per host or the P. laxa pupal development time. However, the pupal development time of female parasitoids (9.3 ± 0.05 d) was significantly greater than that of male parasitoids (8.6 ± 0.5 d) (F = 98.7; df = I, 268; P < 0.001).

The mean weight of P. laxa puparia was significantly (p < 0.05) greater on diet-reared hosts that on the plant fed hosts. However, there was no significant differences in pupanal size among the four soybean genotypes (Table 3). Male and female P. laxa puparial weights were not different (F = 1.7; df = 1,267; P > 0.05). The number of P. taxa pupalia per host was a significant covariate in this analysis (F = 51.2; df = 1, 267; P < 0.001). On a,'erage, there was a decline in puparial weight of 5.2 ± 0.72 mg for each additional parasitoid larva developing in a host. This indicates that competition for food occurs among P. laxa larvae when two or more larvae develop within a single CEW host.

Out data indicate that the only effect on development of P. laxa in hosts fed pest~resistant soybeans is an increase in larval development time in hosts fed PI229358. Even though this increase in larval development time is statistically significant, it involves an increase of no more than 0.5 d over the 14 d larval/pupal development time. The effect of this increase in development time on the rate of population increase for P. laxa on PI229358 is likely to be minimal.

ACKNOWLEDGMENTS

Wc arc grflteful to J. Ji'. Grant and R. E. McWhorter for providing access to the insect colonies, and to B. M. Shepord for providing laboratory fucilit:ies. This material is based upon work supported, in part., by the USDA under Agreement No. 82-CRSR-2-IOOO tided "Development of Comprehensive, Unified, Economically, and Enviromnentally Sound Systems of Integrated Pest Management for f\.·lajor Crops."

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