Chemical Ecology of Some Underexplored Aspects of Plant-Herbivore-Parasitoid Tritrophic Interactions
Type of DegreePhD Dissertation
Entomology and Plant Pathology
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Plants, herbivorous insects and their natural enemies are involved in an intricate tritrophic interaction in a complex chemical environment. Herbivore infested plants, as well as plant-fed herbivores release volatile organic compounds (VOCs) that can be used by natural enemies such as parasitoids to locate their hosts. This study investigated the chemical ecology of some underexplored aspects of plant-herbivore-parasitoid tritrophic interactions, mostly from the parasitoid’s perspective. These aspects include the synergy between plant-related experiences and herbivore-related experiences, parasitoid’s use of plant-associated volatiles emitted by herbivores, the abundance-relevance relationship of volatile compounds, and the dual nature of certain defensive compounds as both allomone and kairomone. In chapter I, I discussed existing literature on chemical ecology of parasitoids in tritrophic interactions, identified specific underexplored aspects of these interactions, and provided rationale for the current study. In chapter II, I tested the prediction that specialist parasitoids that utilize generalist herbivores rely more on herbivore-derived cues than plant-derived cues using a model system consisting of the parasitoid Microplitis croceipes (Hymenoptera: Braconidae) and its herbivore host Heliothis virescens (Lepidoptera: Noctuidae), which is a pest of cotton and soybean. It was hypothesized that: i) naïve parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity, and ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naïve) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting the first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting the second prediction. Further, associative learning of oviposition with host-damaged plants facilitated host location. Interestingly, naïve parasitoids attacked more soybean-fed than cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. I discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging. In chapter III, I hypothesized that certain compounds play key roles in the attractiveness of host-associated odor blends to parasitoids. The larval parasitoid, M. croceipes and its herbivore host, H. virescens, a major pest of cotton plant were used as model species to identify key compounds mediating attraction of parasitoids to hosts. Comparative GC/MS analyses of cotton-fed versus artificial diet-fed hosts indicated that 12 of 17 compounds in the headspace of H. virescens larvae were exclusive to plant-fed hosts, and thus considered to be plant-associated. In order to identify key attractive compounds, a full blend of 15 commercially available synthetic compounds was modified by removing each of the 10 plant-associated compounds emitted by host larvae. In Y-tube olfactometer bioassays testing parasitoid responses to modified blends, 1-octen-3-ol, decanal, (E)-β-caryophyllene, α-humulene, α-farnesene and β-pinene were identified as key compounds contributing to attractiveness of the natural blend of VOCs emitted by cotton-fed hosts. The results showed that while various host-associated compounds act in concert to serve as useful host location cues, only a fraction of the natural blend mediates attraction in parasitoids. Furthermore, the role of a compound is better assessed in the context of other compounds, and odor blends are better perceived as a whole rather than as individual components. In chapter IV, I investigated whether the relative abundance of the compounds emitted by cotton-fed H. virescens in chapter III is correlated with the level of antennal response in M. croceipes. In the present study, the olfactory response of female M. croceipes to synthetic versions of 15 previously identified compounds was tested in electroantennogram (EAG) bioassays. Female M. croceipes showed varying EAG responses to test compounds, indicating different levels of bioactivity in the insect antenna. Eight compounds, including decanal, 1-octen-3-ol, 3-octanone, 2-ethylhexanol, tridecane, tetradecane, α-farnesene and bisabolene, elicited EAG responses above or equal to the 50th percentile rank of all responses. Interestingly, decanal, which represented only 1% of the total amount of odors emitted by cotton-fed hosts, elicited the highest (0.82 mV) EAG response in parasitoids. On the other hand, (E)-β-caryophyllene, the most abundant (29%) blend component, elicited a relatively low (0.17 mV) EAG response. The results suggest that EAG response to host-related volatiles in parasitoids is probably more influenced by the ecological relevance or functional role of the compound in the blend, rather than its relative abundance. In chapter V, I summarized the major findings of my dissertation research and discussed areas of future studies in insect olfaction research. The effect of atmospheric pollution on the fidelity of plant-plant, plant-insect and tritrophic interactions was identified as one of the areas that merits further consideration. Future studies should investigate the effect of atmospheric pollutants such as ozone on signaling and metabolomic responses in plants, and to understand how plants may cope with these widespread abiotic stressors. In addition, the underlying behavioral, physiological and molecular mechanisms by which insects respond to the complex of plant odors and air pollutants should be studied. In the appendix, I introduced an on-going study that tracked behavioral responses of two parasitoids with varying degrees of host specificity, M. croceipes (relatively specialized) and C. marginiventris (generalist) to defensive secretion of a their larval host, H. virescens. Three hypotheses were tested: i) herbivore defensive secretion effectively deters parasitoids due to the presence of plant-derived chemicals, ii) M. croceipes (specialist) will show greater ability to circumvent host defensive secretion than C. marginiventris (generalist), and iii) self and conspecific defensive secretions will elicit behavioral responses in herbivores. A tracking software was used to analyze and visualize the behavioral responses of parasitoids to host defensive secretion.