Mechanisms of Olfaction in Parasitic Wasps: Analytical and Behavioral Studies of Response of a Specialist (Microplitis croceipes) and a Generalist (Cotesia marginiventris) Parasitoid to Host-Related Odor
Type of Degreedissertation
Entomology and Plant Pathology
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Parasitic wasps (parasitoids) are known to utilize as host location cues various types of host-related volatile signals. These volatile signals could be plant-based, originate from the herbivore host, or be produced from an interaction between herbivores and their plant host. The success of parasitoids in suppressing pest populations depends on their ability to locate hosts in a complex olfactory and visual environment. Despite the intense interest in host-parasitoid interactions, certain aspects of olfactory communication in this group of insects are not well understood. This study was conducted to characterize mechanisms of olfaction and response to host-related odor in two parasitic wasps (Hymenoptera: Braconidae) with different degrees of host specificity, Microplitis croceipes (Cresson) (specialist) and Cotesia marginiventris (Cresson) (generalist), using an integration of analytical, behavioral and electrophysiological techniques. Specific objectives are: (1) Electroantennogram (EAG) responses of M. croceipes and C. marginiventris and their lepidopteran hosts to a wide array of odor stimuli: Correlation between EAG response and degree of host specificity?; (2) Comparative GC-EAD responses of a specialist (M. croceipes) and a generalist (C. marginiventris) parasitoid to cotton volatiles induced by two caterpillar species; (3) Effects of plant growth-promoting rhizobateria (PGPR) on the induction of cotton volatiles and consequences for response of parasitoids; and (4) Sexual and species differences in the behavioral response of a specialist and generalist parasitoid species to host-related volatiles. In chapter II, studies were conducted in order to test whether the electroantennogram (EAG) response spectrum of an insect correlates to its degree of specificity. We recorded EAG responses of two parasitoid species with different degrees of host specificity, M. croceipes (specialist) and C. marginiventris (generalist) and their lepidopteran hosts (Lepidoptera: Noctuidae) (Heliothis virescens Fab. and Spodoptera exigua Hübner), to a wide array of odor stimuli. The compounds tested included green leaf volatiles (GLVs), herbivore-induced plant volatiles (HIPVs), ecologically irrelevant plant volatiles, and several types of host specific odor stimuli including synthetic host sex pheromones and extracts of host caterpillar and body frass. The specialist parasitoid showed greater EAG responses than the generalist to host-specific odor and one HIPV (cis-3-hexenyl butyrate), whereas the generalist showed relatively greater EAG responses to the GLVs and unrelated plant volatiles. There were no differences in the EAG responses of H. virescens and S. exigua to any of the tested odors. In Chapter III, a comparative study was done to determine similarities and differences in GC-EAD (coupled gas chromatography electroantennogram detection) responses of both parasitoid species to headspace volatiles of cotton plants damaged by H. virescens (a host species for both parasitoids) vs. S. exigua (a host species of C. marginiventris). Thirty volatile components were emitted by cotton plants in response to feeding by either of the two caterpillars, however, 18 components were significantly elevated in the headspace of H. virescens damaged plants. Sixteen components consistently elicited GC-EAD responses in both parasitoids. Cotesia marginiventris showed significantly greater GC-EAD responses than M. croceipes to most green leaf volatile components, whereas several herbivore-induced volatile components elicited comparatively greater responses in M. croceipes. Results suggest that differences in the ratios of identical volatile compounds between similar volatile blends may be used by specialist parasitoids to discriminate between host-plant and non-host plant complexes. In Chapter IV, studies were conducted to evaluate the potential of plant growth-promoting rhizobacteria (PGPR) on the induction of cotton volatiles and consequences for response of parasitoids. Three PGPR treatments were evaluated: i) Bacillus pumilis strain INR-7, ii) Blend 8, and iii) Blend 9. An untreated (water) control was also tested. There were quantitative and qualitative differences in headspace volatiles collected from PGPR treated and untreated cotton plants. A total of eleven peaks were detected from headspace of PGPR treated cotton plants but only three peaks were detectable in untreated cotton plants. Differences in root growth between PGPR treated vs. untreated plants were recorded, with Blend 9 recording the highest root growth. PGPR treated plants were also very highly attractive to parasitoids, with Blend 9 being the most attractive. In Chapter V, studies were done to determine if there were sexual and species differences in the behavioral response of a specialist and generalist parasitoid species to host-related volatiles. Y-tube olfactometer bioassays were conducted to compare responses of naïve females and males of both parasitoid species to select synthetic plant-based host-related volatiles; two GLVs (hexanal and (Z)-3-hexen-1-ol) and four HIPVs ((Z)-3-hexenyl acetate, linalool, (Z)-3-hexenyl butyrate, and (E,E)-α-farnesene). Linking previous reported electrophysiological responses (Chapter II) to behavioral observations, results revealed key differences in behavioral responses of both parasitoid species to the tested host-related plant volatiles. The specialist parasitoid (M. croceipes) was more responsive to most of the HIPVs, whereas, C. marginiventris (generalist) showed greater responses to the GLVs. Females of both parasitoid species showed greater behavioral responses than conspecific males. These results advance our understanding of mechanisms of olfaction, semiochemical-mediated responses, and foraging strategies in parasitoids with different degrees of host specificity.