This Is AuburnElectronic Theses and Dissertations

Plasticity of Olfactory Response to Host-Related Plant Volatiles in The Parasitoid Microplitis croceipes




Burrows, Matthew

Type of Degree

Master's Thesis


Entomology and Plant Pathology


Plants release blends of volatile organic compounds (VOCs) in response to herbivore damage. Parasitoids use certain VOCs as indirect cues to locate their herbivore hosts. However, response of parasitoids to these chemical cues can be plastic, depending on their physiological state. Previous studies have demonstrated that various factors such as nutritional status, age, and mating may modulate response of parasitoids to host-related volatile compounds. In this study, the parasitoid Microplitis croceipes was used to test the hypothesis that the physiological state of parasitoids affects their response to host-related plant volatiles. Microplitis croceipes is a relatively specialized endoparasitoid of larvae of Heliothis virescens, which is a major pest of cotton. Specifically, the effect of nutritional status, age, and mating on olfactory response of M. croceipes to select cotton plant volatiles was investigated in a series of electroantennogram (EAG) and behavioral bioassays. The following VOCs were tested in this study since they have been previously shown to elicit olfactory response in M. croceipes: cis-3-hexenol (a green leaf volatile or GLV), α-pinene (a constitutive monoterpene), a 50/50 v/v binary mixture of cis-3-hexenol and α-pinene, and a complex natural odor blend collected from the headspace of H. virescens-damaged cotton plants. In chapter II, the effect of nutritional status on M. croceipes olfactory response to plant VOCs was investigated. Parasitoids were fed different diet treatments (i.e. 40%, 20%, 10%, or 0% sucrose/water solution (w/v)) using EAG recording and Y-tube olfactometer bioassays. Antennal and behavioral responses to select synthetic VOCs, 50:50 binary VOC mixture, and natural VOC blend were measured relative to their nutritional status. Parasitoids fed 20% or 10% sucrose solution showed significantly higher EAG response to the binary odor mixture (cis-3-hexenol + α-pinene) than sugar-starved wasps. Also, parasitoids fed 40% sucrose solution showed a significantly higher EAG response to H. virescens-infested cotton extract than sugar-starved parasitoids. In Y-tube olfactometer bioassays, female M. croceipes fed 40% sucrose solution were significantly more attracted to the natural odor blend than to solvent control. In general, M. croceipes showed relatively lower response to the test single compounds (cis-3-hexenol and α-pinene) compared to the binary odor mixture and the complex natural odor blend. These results suggest that nutritional status may modulate olfactory plasticity in M. croceipes, with consequences for its performance as a biological control agent. In Chapter III, effect of age and mating status of M. croceipes on their olfactory response to VOCs was investigated. Female parasitoids were first separated based on mating status and were either introduced into a cage with males or left in a cage with other females. Parasitoids were then arranged into age groups of 1-3, 4-6, 7-9, or 10-12 days old. Similar to chapter II, EAG recording and Y-tube olfactometer bioassays were conducted. No significant difference in EAG response was recorded relative to parasitoid age. With regard to mating status, only mated parasitoids aged 7-9 days old showed significant EAG response to single compound α-pinene and the binary odor mixture. In Y-tube olfactometer bioassays, mated female M. croceipes aged 4-6 days old were significantly more attracted to the binary mixture than to solvent control. In addition, mated female M. croceipes aged 1-3 days old were significantly more attracted to the natural odor blend than to control. The implications of these findings are discussed.