Olfactory Response of Trichogramma chilonis to Capsicum annuum

J. Asia-Pacific Entomol. 1(2): 123-129 (1998)

Olfactory Response of Trichogramma chilonis to Capsicum annuum K. S. Boo* and J. P. Yang Abstract - Olfactory attraction of an egg parasitoid, Trichogramma chilonis, to the hot pepper (Capsicum annuum) and other host plants of its host insect species, Helicoverpa assulta, was studied in laboratory experiments. T. chilonis was attracted more to the hot pepper than the tobacco (Nicotiana tabacum), eggplant (Solanum melongena), carrot (Daucua carot var. sativa), or clean air flow. A higher attraction of T. chilonis was observed in combined leaf and green fruit of the hot pepper plant than leaf or fruit alone, and its red fruit had the least attractivity. But the combination of pepper odor and H. assulta sex pheromone did not attract more than the pepper odor or sex pheromone alone.

Key Words - Trichogramma chilonis, Kairomones, Capsicum annuum odor, Plant odor, Pheromones

Introduction Up to the last decade, insecticides were usually applied when the crop field was heavily infested by insect pests. This kind of practice caused serious adverse effects such as the mammalian toxicity and chemical residue in the environment. To reduce these problems, many researchers have studied new control methods such as biological control, use of semiochemicals, etc. (Nordlund et al., 1981). The Oriental tobacco budworm (Helicoverpa assulta) is widely distributed in Korea, Japan, China, Australia and Africa (Hardwick, 1965) and feeds on several crop plants, such as the hot pepper (Capsicum annuum), tobacco (Nicotiana tabacum), eggplant (Solanum melongena), carrot (Daucus carota var. sativa), cotton (Gossypium indicum), tomato (Lycopersicon esculentum), etc., with the most serious damage to hot pepper fruits in Korea (Hwang, 1987). Adverse effects of insecticides and the difficulty of its control led researchers to study using natural enemies such as Trichogramma parasitoids which are known to attack noctuid eggs on several crops (Rabb and Bradley, 1968). Trichogramma

pretiosum, for example, was claimed to be a candidate for inundative control against Heliothis spp. (Kingetal., 1986; King and Coleman, 1989). Among several approaches using Trichogramma spp. investigated, the most frequently used approach is the mass rearing and mass releasing of these parasitoids into the target area (King et al., 1986; Wei, 1987). In Korea the most important parasitoids on H. assulta were reported to be Trichogramma spp. on eggs and Campoletis spp. in larvae (Choe, 1968; Choi et al., 1975; Hwang, 1987). These species were identified as Trichogramma chilonis and Campoletis chlorideae, respectively (Choe, 1968; Nandhihalli, 1994). Their parasitism rates were variable depending on the season and host plants of H. assulta, but reached up to 85% in the case of T. chilonis on hot pepper leaves in early September (Choi et al., 1975) and 99% in the case of C. chlorideae on tobacco leaves (Hwang, 1987). The host-selection process by entomophagous insects is generally subdivided into five main steps: host-habitat location, host finding, host acceptance, host suitability and host regulation (Vinson, 1984). The success of each step depends on many environmental and host factors that involve both chemical

* To whom

correspondence should be addressed. Division of Applied Biology and Chemistry, College of Agriculture and Life Sciences, Seoul National University, Suwon 441-744, Korea. E-mail: [email protected]

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and physical cues. Especially, there is growing evidence that many insect species use chemical cues, such as pheromones by the host insects and kairomones produced by the plants or other hosts as a major tool in the selection of their feeding, mating and oviposition sites. The ultimate practical goal of research on semiochemicals is to find new environmentally safe methods for insect pest control. For achieving this goal, we must understand the chemistry and mechanisms that regulate insects' chemical communication system. Then we could be able to develop methods to exploit their weakness to control insects, for example mating disruption with synthetic pheromone (Carde and Minks, 1995). Because semiochemicals play such a role in various phases of host finding and selection by egg parasitoids of the genus Trichogramma (Noldus, 1989a, b), many researchers have studied kairomones of Trichogramma spp. These species use various semiochemicals including plant synomones and host kairomones in their search for host eggs (Noldus, 1989a, b). Besides sex pheromones and/or their components, plant volatiles increase parasitism rates by several Trichogramma spp., both in the field and laboratory tests (Altieri et al., 1981; Nordlund et al., 1985). For example, airborne stimuli from maize, the host plant of their host insect, elicited oriented responses in Trichogramma brassicae (Kaiser et al., 1989). We have been studying the source (s) of infochemical cues (kairomones) used by T. chilonis for locating their host, H. assulta, to evaluate their chemical communication in laboratory experiments. Some of sex pheromone components of its host insects, H. assulta and Ostrinia furnacalis, are apparently utilized as kairomones by T. chilonis , and short distance attractants/contact kairomones are also present in H. assulta scales and eggs (Boo and Yang, unpublished observation). This study is concerned with any kairomonal activity in the host plants for the host insect, H. assulta, of T. chilonis.

Materials and Methods Insects rearing H. assulta was reared on an artificial diet (Park, 1991) under a 15L/9D regime, 25±1°C and 50±5 % RH. Some of the moth eggs laid on gauze were

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supplied to T. chilonis, and others were allowed to develop. Some pupae were sexed and isolated to use for these studies. The adults emerging from the remaining pupae were maintained in 30 x 30 x 30 em acryl cages with 20% sugar solution to obtain eggs. T. chilonis used in this study were obtained from the Department of Plant Protection, National Agricultural Science and Technology .Institute, Rural Development Administration and reared on eggs of H. assulta in 500 ml beakers under a 15L/9D regime and 27 ± I"C. Adult wasps, emerged 8 days after parasitization of host eggs, were collected every day, transferred to glass tubes «(1) 2.5 x 14 em) and fed on 20% sugar solution. For these experiments, only 3 day-old females, having no previous oviposition experience, were used.

Odors We tested the odor of the pepper, tobacco, eggplant and carrot plants, which are usually more seriously damaged by H. assulta in Korea, since these plants may have the chemical cue (s) for finding the host insect, H. assulta, by their egg parasitoid, T. chilonis. These plants were obtained from the field. H. assulta sex pheromone used in this study is composedof(Z)-ll-hexadecenal (Zll -16Ald), (Z)9-hexadecenal (Z9-16Ald), (Z)-ll-hexadecenyl acetate (Zll-16Ac), and (Z)-9-hexadecenyl acetate (Z9-16Ac) at the ratio of 50: 1,000: 15: 300 (Cork et ai., 1992). Bioassay setup Bioassays of plant odors were conducted in a star-shaped olfactometer (Fig. 1) adapted from the device described by Vet et al. (1983) to the minute size of T. chilonis. The exposure chamber was divided into quadrants except for the central diaper region to create four distinct flow fields. Air was sucked out through the central hole at the flow rate of 1.2 L/min. One female of T. chilonis was introduced into the central spot of the olfactometer, and retention time and the frequency of entering were checked with a computer program for 5 min. The device was rotated by 90 between each replicate, and the chamber was cleaned out with methanol and distilled water after 4 replicates. Bioassays were conducted from 5th hr to 9th hr during the 0

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Statistical analysis Twenty or thirty replications were run for each. experiment. Since a preliminary analysis of the data obtained in this study, with Shapiro- Wilk test, indicated that they depart significantly from normality, arcsin (.,,!(yIlOO))*57.3 transformation or loge (y+ 1) transformation were applied to normalize some group of the data (Fry, 1993). And then their significance test was performed with Wilcoxon

1

rank-sum test (Snedecor & Cochran, 1989) or Duncan's multiple range test (SAS, 1995). Significant difference is indicated by an asterisk or alphabetical characters, respectively.

Results T. chilonis was definitely attracted more

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Fig. 3. Response of T. chilonis female adults to odors of various plant species in a star-shaped olfactometer. Data for retention time and the number of entering were normalized by arcsine square-root transformation. Bars indicated by the same letters are not significantly different by Duncan's multiple range test at the level of 5% for the same category (N=30). .

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Fig. 5. Response of T. chilonis female adults to various odors in a star-shaped olfactometer. Data for retention time and the number of entering were normalized by arcsine square-root and log transformations, respectively. (Pepper 5 g and H. assulta sex pheromone 10 ng of Zll16Ald, Zll-16Ac, Z9-16Ald and Z9-16Ac at the ratio of 50: 1,000: 15: 300 were used). Bars indicated by the same letters are not significantly different by Duncan's multiple range test at the level of 5% for the same category (N=30).

Table 1. Response of T. chilonis females to various plant odor sources in a star-shaped olfactometer. Experiment A (30')

B (30)

C (30)

Odor source Pepper Clean air Pepper Tobacco Eggplant Carrot Hot pepper leaf and fruit Hot pepper fruit Hot pepper leaf Hot pepper red fruit

Retention time (%)

Number of entering

47.2± 19.6a2 27.9± 18.6b 49.3±25.7a 12.7 ± 12.5b 1O.4± 13.0b 4.7±7.3b

2.8±0.9a2 1.2±0.9b 2.8± 1.0a 1 ±O.8b 1.5 ± 1.0b 0.7±0.7b

28.6±23.1a

6.8±1O.2a

19.4± 15.7b 12.6± 1O.3b 4.6±5.3c

5.4±7.2a 4.4±4.6a 1.6±2.2b

Experiment A: Data in each response were compared at 5% level of Wilcoxon rank-sum test. Experiments Band C: Data in both responses were normalized by arcsine square-root transformation. Data within a response were compared with Duncan's multiple range test at 5% level. 1 Number of replicates 2 Alphabets behind each value indicates significant difference among data in each response.

pepper than clean air (Table 1) or any other host plants of H. assulta, such as tobacco, eggplant or carrot (Table 1) in terms of retention time and the number of entering in a star-shaped olfactometer. Furthermore, combined leaf and green fruit of the

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Table 2. Response of T. chilonis female adults to various odor sources in a star-shaped olfactometer. Data of retention time was normalized by arcsine square-root and those of retention time/number of entering and number of entering were normalized by log transformation, respectively. (Pepper 5g. H. assulta sex pheromone lOng of ZII16Ald, ZII-16Ac, Z9-16Ald and Z9-16Ac at the ratio of 50: 1000: 15 : 300) (5% level of Duncan's multiple range test, N=30). Odor source Pepper and pheromone Pheromone Pepper Hexane

Retention time (%)

Number of entering

Retention time /No. entering

26.8±29.7a

2.1±1.4a

15.1 ±40.0a

21.3± 18.7a 18.5±20.1a 8.7±9.4b

2.5±1.7a 1.8 ± l.la 1.8 ± 1.8a

9.4±21.3a 1O.9±23.4ab 3.8±7.7b

hot pepper showed higher attraction than green fruit or leaf alone, and red fruit was least attractive (Table 1). Combination of pepper odor and sex pheromone had higher attraction than hexane, but did not attract more than the green pepper or sex pheromone alone (Table 2).

Discussion This study clearly demonstrates that T. chilonis responds to odors coming from the hot pepper plant, the host plant of its host insect, H. assulta, besides the sex pheromone of its host insect (Boo and Yang, unpublished observation). A parasitoid can greatly increase the efficacy of its search if it can identify and concentrate its efforts in those sites most likely to contain hosts. Such information can be supplied by odors from those plants preferred by its host insect and the sex pheromone of its host insect will greatly improve finding more appropriate area where the host insect may lay eggs. Trichogramma spp. have been considered generalists in their choice of hosts (Thomson and Stinner, 1989), and it has been suggested that plantgeneralist natural enemies use more general chemical cues than plant specialists (Vinson, 1976; Sheehan, 1986). This study shows that T. chilonis use odors of a specific plant, but not of other plant

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species, even though they may serve as host plants for its host insect. In India it was also reported that the high parasitism level on Helieoverpa armigera eggs on sorghum (Sorghum bieolor) by T. ehilonis was partly due to the attractivity of the host plant to the parasitoid females and the poor parasitism level on pigeon pea (Cajanus eajan) due to its repellency to the parasitoid female adults (Romeis et al., 1997). It means that T. ehilonis female adults respond to relatively specific chemicals from a particular part of a certan plant species or from such a host plant at a specific physiological stage. This point is not well matching to the generalization mentioned above. Of course H. assulta larvae prefer hot pepper fruits to any other plants including tobacco leaves (Boo, unpublished observation). The larvae also metamorphose into pupae after 5 instars when fed on hot pepper fruits, in contrast to 6 instars on tobacco leaves. And the larval period is shorter on hot pepper fruits requiring only two thirds to three quaters of the period, 41.0 to 18.9 days at 18-28°C, needed on tobacco leaves (Hwang, 1987). And no one has ever observed H. assulta larvae feeding on hot pepper leaves. It means that the hot pepper fruit is a better food to H. assulta than any other parts or plant species. In Korea, H. assulta adults emerged from overwintered pupae lay eggs on tobacco leaves and then their subsequent generation adults move to hot pepper fields a little later in season since hot pepper plant only start to bear fruits at this time of the season, usually in August. H. assulta population also start to build up only after this time (Hwang, 1987). Even though the hot pepper fruit is the preferred food to H. assulta larvae, the adults usually lay eggs on leaves, for example 70.4-91.2% of eggs found on leaves in comparison to 8.8-28.5% on fruits (Hwang, 1987; Han, 1993; Nandihalli, 1994). The number of eggs on stem or flowers are negligible. The parasitism of H. assulta eggs by T. ehilonis was also different, depending on the oviosition sites, with 83.3% of eggs on leaves against 44.9% of eggs on fruits (Hwang, 1987). This aspect is well reflected in the behavior of the egg parasitoid, T. ehilonis, showing a better attraction to green fruits of the hot pepper plant, but combined fruit and leaf is a little better than either fruit or leaf alone. After the egg parasitoid arrives at the hot pepper plant with these

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clues, sex pheromone of the host insect may come into play. Since the chemicals of the pepper and sex pheromone are volatile and effective in attractiing the parasitoid, we suspect that the association of pepper odor and sex pheromone is the main one eliciting the olfactory response. The higher attractiveness of the odorant combination may result from the juxtaposition of some volatile components that are cues for indicating presence of both the plant and the host insect. The association effect of volatiles related to the host insect and its environment may be the rule in most entomophagous species. Numerous studies conducted with parasitoids demonstrated that some odors released by the host-plant complex were volatile cues in the host location process (Thompson et al., 1983; Herard et al., 1988). The ability of Triehogramma to respond to the combination of the plant odor and sex pheromone of its host insect should increase its probability of finding a suitable host when searching in areas where adult moths are present and host eggs are likely to occur. But our result revealed that combination of sex pheromone and pepper odor did not significantly elicit an additive effect because they may affect different steps in locating host habitat and/or host. In larval parasitoids, the flying insect orients directly to the source of semiochemicals which is a feeding host and also the target of its searching effect (Drost et al., 1986, 1988; Elzen et al., 1987; Herard et al., 1988; Zanen et al., 1989). For an egg parasitoid, the sex pheromone of the host and the odor of plant may be indication for presence of reproducing adults, therefore of eggs in the area. The pheromone may be indication of more precise location of the host eggs in plant field, while odor of plant may be distributed continuously and regularly in the plant field. And thus the tactics of host finding of parasitoid may have been adapted differently between the two chemical cues. Prolonged searching in an area where host pheromone is perceived will increase the probability, and then the wasp may try other cues with a closer spatial correlation with oviposition sites (Noldus et al., 1991a). We suggest that this step is actual host location and T. ehilonis uses scale and egg chemicals (Boo and Yang, unpublished observation) for this step. Naturally occurring eggs of H. assulta

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would often be accompanied by a patch of scales left by the ovipositioning moth, thus the behavioral response exhibited by T. chilonis upon encountering kairomone patches of scale extract can be expected to substantially improve the parasite's searching efficiency. The localized nature of the response allows the parasite to concentrate its search in small areas likely to harbor hosts. At the same time, the tendency for the intensity of the response to wane allows the parasite to quickly delimit and search the area of the patch, and ultimately allows it to escape unproductive patches. It is interesting to see that mature, ripen hot pepper fruits are neither readily attacked by H. assulta larvae (Boo, unpublished observation) nor attracted to T. chilonis female adults. What kind of chemical (s) are involved in these processes remain to be solved. Although further isolation and identification of the kairomone (s) and other repellent (s) are necessary, their application at fields may improve or depress host-finding, parasitism rate and reproduction by the parasitoid, particularly in association with high host densities in insecticide-free agroecosystems. In spite of large vairation in this experiment result almost the same conclusion was obtained by several other statistical methods. These large variation could result from the following reasons. first, difficulty in manipulating T. chilonis due to their small size lessened exposing time in distinct four flow regions of the olfactometer, except for the central diaper region where T. chilonis is introduced abruptly. Since T. chilonis may need time for adaptation by walking around the central region, the central diaper region was excluded from distinct four flow regions although other researchers included it (Vet et al., 1983). Therefore, the time spent at fields of test odor sources and control chemicals is reduced, and these may be one factor of large variation. Second, the response of T. chilonis to air flow resulted in upwind locomotion even when the parasitoid was exposed to control chemicals. This response pattern could not tell which was the real cause for eliciting a positive reaction to control or odorant chemicals, from parasitoids. This was particularly so simply because the parasitoids were not individually tested for their vitality. In any event, however, the main conclusion seems to be alright.

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Acknowledgements - This study was financially supported with grants from the Ministry of Education, and Korea Science and Engineering Foundation through the Research Center for New Bio-Materials in Agriculture, Seoul National University, Korea.

Literature Cited Altieri, M.A., W.J. Lewis, D.A. Nordlund, n.c. Gueldner and J.W. Todd. 1981. Chemical interactions between plants and Trichogramma wasps in Georgia soybean fields. Prot. Ecol. 3: 259-263. Carde, R.T. and A.K. Minks. 1995. Control of moth pests by mating disruption: Successes and constraints. Annu. Rev. Entomol. 40: 559-585. Choe, B.M. 1968. Studies on the biological control of the oriental tobacco budworm. Theses Collection, Chongju Educational College 3: 45-54 (in Korean). Choi, K.M., E.H. Cho, J.S. So and c.v. Hwang. 1975. Studies on the seasonal occurrences of the tobacco budworm, Heliothis assulta H. (Lepidoptera: Noctuidae), and the parasitism rate of Trichogramma spp. on the eggs. Korean J. Plant Prot. 14: 137-140. Cork, A., K.S. Boo, E. Dunkelblum, D.R. Hall. K. JeeRajunga, M. Kehat, E. Kong Jie, K.C. Park, P. Tepgidagam and Liu Xun. 1992. Female sex pheromone of Oriental tobacco budworrn, Helicoverpa assulta (Guenee) (Lepidoptera: Noctuidae): Identification and field testing. J. Chern. Ecol. 18: 403-418. Drost, Y.C., W.J. Lewis, P.O. Zanen and M.A. Keller. 1986. Beneficial arthropod behavior mediated by airborne semiochemicals. 1. Flight behavior and influence of preflight handling of Microplitis croceipes (Cresson). J. Chern. Ecol. 12: 1247-1262. Drost, Y.c., W.J. Lewis and J.H. Tumlinson. 1988. Beneficial arthropod behavior mediated by airborne semiochemicals. V. Influence of rearing method, host plant and adult experience on host-searching behavior of Microplitis croceipes (Cresson), a larval parasitoid of Heliothis. J. Chern. Ecol. 14: 1607-1616. Elzen, G.W., H.J. Williams, S.B. Vinson and J.E. Powell. 1987. Comparative flight behavior of parasitoids Campoletis sonorensis and Microplitis croceipes. Entomol. Exp. App1.45: 175-180. Fry, J.e. 1993. One-way analysis of variance. pp.I-33 in Biological data analysis: A practical approach. Ed. J.C. Fry. 418pp. Oxford Univ. Press, Oxford. Han, Man-Wi, 1993. Studies on forecasting models of the oriental tobacco budworrn, Helicoverpa assulta (Guenee).

Vol. 1, No.2

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Ph.D. thesis, 97pp. Seoul National University, Korea (in Korean). Hardwick, D.F. 1965. The corn earworm complex. Mem. Entomol. Soc. Canada No. 40, 247pp. Herard, F., M.A. Keller, W.J. Lewis and J.H. Tumlinson. 1988. Beneficial arthropod behavior mediated by airborne semiochemicals. III. Influence of age and experience on flight chamber responses of Microplitis demolitor Wilkinson. J. Chern. Ecol. 14: 1583-1596. Hwang, C.Y. 1987. Studies on bionomics and parasitoids of oriental tobacco budworm, Heliothis assulta Guenee. Ph.D. thesis, 56pp. Chungnam National University, Korea (in Korean). Kaiser, L., M.H. Pham-Delegue, E. Bakchine and e. Masson. 1989. Olfactory response of Trichogramma maidis Pint. et Voeg.: Effects of chemical cues and behavioral plasticity. J. Insect Behav. 2: 701-712. King, E.G. and R.I. Coleman. 1989. Potential for biological control of Heliothis species. Annu. Rev. Entomol. 34: 5375. King, E.G., L.F. Bull, D.L.R.J. Coleman, W.A. Dickerson, W.J. Lewis, J.D. Lopez, RK. Morrison and J. R Phillips. 1986. Management of Heliothis spp. in cotton by augmentative releases of Trichogramma pretiosum Ril. J. Appl. Entomol. 101: 2-10. Nandihalli, B.S. 1994. Ecology of an egg parasitoid, Trichogramma chilonis Ishii, and a larval parasitoid, Campoletis chlorideae Uchida, of the oriental tobacco budworm, Helicoverpa assulta (Guenee). Ph.D. thesis, 106pp. Seoul National University, Korea Noldus, L.P.J.J., J.e. van Lenteren and W.J. Lewis. 1991a. How Trichogramma parasitoids use sex pheromones as kairomones: Orientation behavior in a wind tunnel. Physiol. Entomol. 16: 313-327. Noldus, L.P.J.J., RP.J. Potting and H.E. Barendregt. 1991b. Moth sex pheromone adsorption to leaf surface: bridge in time for chemical spies. Physiol. Entomol. 16: 329-334. Nordlund, D.A., RL. Jones and W.J. Lewis. 1981. Employment of kairomones in the management of parasitoids. pp. 137-150, In Serniochemicals, their role in pest control. Eds. H.R. Gross, Jr. 306pp. John Wiley & Sons, New York. Nordlund, D.A., RB. Chalfant and W.J. Lewis. 1985. Response of Trichogramma pretiosum females to volatile synomones from tomato plants. J. Entomol. Sci. 20: 372-376. Park, K.e. 1991. The composition and activity of female sex

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pheromone in the oriental tobacco budworm (Helicoverpa assulta (Gueneej). Ph.D. thesis, Seoul National University. Korea. Rabb, RL. and J.R Bradley. 1968. The influence of host plants on parasitism of eggs of tobacco hornworm. J. Econ. Entomol. 61: 1249-1252. Rorneis, J., T.G. Shanower and C.P.W. Zebitz. 1997. Volatile plant infochemicals mediate plant preference of Trichogramma chilonis. J. Chern. Ecol. 23: 2453-2465 SAS Institute. 1995. SAS user's guide: Statistics. version 6.11 ed. SAS Institute, Cary, NC. Sheehan, W. 1986. Response by specialist and generalist natural enemies to agroecosystem diversification: A selective review. Environ. Entomol. 15: 456-461. Snedecor, G.W. and W.G. Cochran. 1989. Shortcut and nonparametric methods. pp.135 -148, in Statistical methods. 8th ed. 503pp. Iowa State Univ. Press, Ames. Thomson, M.S. and RE. Stinner. 1989. Trichogramma spp. (Hymenoptera: Trichogrammatidae): Field hosts and multiple parasitism in North Carolina. J. Entomol. Sci. 24: 232-240. Thompson, A.C., J.P. Roth and E.G. King. 1983. Larviposition kairomone of the tachinid Lixophaga diatraea. Environ. Entomol. 12: 1312-1314. Vet, L.E.M., J.e. van Lenteren, M. Heymans and E. Meelis. 1983. An airflow olfactometer for measuring olfactory responses of hymenopterous parasitoids and other small insects. Physiol. Entomol. 8: 97 -1 06. Vinson, S.B. 1976. Host selection in insect parasitoids. Annu. Rev. Entomol. 21: 109-133. Vinson, S.B. 1984. Parasitoid-host relatonship. pp. 206-233, in Chemical ecology of insects. Eds. W.J. Bell and RT. Carde, 524pp. Sinauer Associates, New York. Wei, Z.G. 1987. A three-year experiment on releasing Trichogramma confusum against the tobacco bud worm, Heliothis assulta, on hot pepper. Chinese J. BioI. Control. 3: 78-80. Zanen, P.O., W.J. Lewis, RT. Card and B.G. Mullinix. 1989. Beneficial arthropod behavior mediated by airborne semiochemicals. VI. Flights responses of female Microplitis croceipes (Cresson), a braconid endoparasitoid of Heliothis spp. to varying olfactory stimulus conditions created with a turbulent jet. J. Chern. Ecol. 15: 141-168. (Received March 2, 1998; Accepted April 7, 1998)