CAPTIVE REARING AND SEMIOCHEMCIAL ECOLOGY OF TRICHOGRAMMA PAPILIONIS (HYMENOPTERA: TRICHOGRAMMATIDAE)
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2020
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This study addressed aspects of mass rearing of Trichogramma papilionis (Hymenoptera; Trichogrammatidae), including the effects of varied colony founder size on wasp fitness, and the exploitation of the wasps to locate egg hosts in which to deposit thereof progeny. Effects of initial founder female number of T. papilionis were investigated using fitness parameters (emergency rate, sex ratio and fecundity) to quantify the effects of a severe bottleneck (single founder female) on 10 subsequent generations. Results showed that no significant difference for eggs laid per female over ten generations, suggesting that the imposed bottleneck did not result in reduced female fecundity for any founder population size. However, founder numbers did affect both the emergence rate and sex ratio of T. papilionis. Further investigation of the impacts of inbreeding on field performance of the wasps was discontinued as extremely limited host finding ability of the wasps was observed in some habitats. The emphasis of the work was thus shifted to elucidating the searching behaviors of T. papilionis in relation to chemical cues. The hypothesis that T. papilionis are attracted to host habitat by host plant or egg-associated volatile chemicals was tested.
The response of T. papilionis females to olfactory cues from host eggs, host plants and induced plant volatiles were studied. The response of T. papilionis females to different info-chemical cues was tested in Y-tube olfactory assays. Wasps made a positive response to odors from corn earworm (CEW) eggs Helicoverpa zea (Lepidoptera: Noctuidae) compared with blank air, while there was a negative response to Ephestia kuehniella eggs (Lepidoptera: Pyralidae) compared blank air: T. papilionis females thus preferred odors from corn earworm eggs over the Mediterranean flour moth eggs. Further, the wasps were attracted to volatile emissions from sunn hemp Crotalaria juncea (L.) over maize Zea mays (L.), despite both plants infested with H. zea eggs. No preference was observed for plants not infested with H. zea eggs, suggesting T. papilionis showed a positive response to stimuli from sunn hemp plants that might be induced by H. zea oviposition. Chemical volatile collection and headspace analysis was conducted. Headspace analysis and thermal desorption and gas chromatography–mass spectrometry (TD-GCMS) was used to qualitatively and semi-quantitatively determine the difference in plant volatile organic components (VOCs) from Helicoverpa zea egg infested sunn hemp plants compared with intact sunn hemp plants and H. zea eggs only. TD is used as a preconcentration technique of VOCs for gas chromatography-mass spectrometry (GC–MS), making it useful to detect low-concentration analytes that would otherwise be undetectable. Results demonstrated that sunn hemp plants released 55 chemical volatiles with five compounds that were unique, or were emitted in higher concentrations, for plants infested with CEW eggs. These volatile compounds were consistent with linear alkanes, aldehydes, aromatics, polyterpene-related compounds, naphthalene derivatives, and ester-related compounds. High concentrations of anisole, β-myrcene, cis-butyric acid, trans-isoeugenol, and bis(2-ethylhexyl) phthalate were found in infested sunn hemp. The majority of GCMS peaks detected from H. zea eggs were consistent with phosphates, pheromone-related compounds, various natural products, a series of glycol-related compounds, and a series of fatty acid ester-related compounds. Several compounds were shared in sunn hemp samples and corn earworm eggs: anisole, β-myrcene, and bis (2-ethylhexyl) phthalate, but were detected in higher concentrations from the plants with H. zea eggs.
Evaluation of the response and the performance of T. papilionis females in y-tube olfactory bioassays to single compounds, and blends of synthetic chemical showed that the wasps were significantly attracted to only two of the assayed chemical volatiles (anisole and bis(2-ethylhexyl) phthalate). Some concentrations of anisole and bis (2-ethylhexyl) phthalate were attractant to the wasps, whereas some concentrations of the other tested chemical compounds repelled the wasps. Wasps were attracted to a blend of anisole and bis(2-ethylhexyl) phthalate (25μL /100μL ratio) which is similar to the ratio of anisole to bis(2-ethylhexyl) phthalate detected in the (GC-DMS) chromatograph for C. juncea plants infested with H. zea eggs. No significant attraction to any other blend ratios of anisole and bis(2-ethylhexyl) phthalate was observed.
Greenhouse and field experiments were conducted to determine whether the patterns observed in the y-olfactometer were consistent under less constrained conditions. The optimal blend identified above was initially tested in a greenhouse, and later in closed-canopy environments (under trees) and open habitat with no trees. The parasitism rate by T. papilionis wasps was significantly increased when the wasps were exposed to anisole and bis(2-ethylhexyl) phthalate blend in both greenhouse and outdoor trials (covered habitat), at least over short distance (up to 2m from the volatile sources).
The findings presented in this dissertation underscore the importance of improving our understanding of how tri-trophic interactions (natural enemies- herbivores and host plants) interact to influence insect behavior, as well as the impact of variable environments, impact parasitoid wasps. The results may also contribute to finding a way to improve natural enemy efficacy in augmentative and conservation biocontrol efforts. Semiochemical cues can positively or negatively affect the response of parasitic wasps. This may provide an understanding of ecology that could facilitate achieving successful field parasitism and thus enhanced pest management.
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Entomology, chemical volatiles, headspace analysis, olfactory bioassays, plant derived semiochemicals, searching behavior, Trichogramma papilionis
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158 pages
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