Behavioral Responses of Two Parasitic Wasp Species with Different Degree of Host Specificity to Host-Related Plant Volatiles

Abstract

As an indirect defense to herbivore attack, plants release many types of volatile organic compounds (VOCs), which guide parasitoids to their herbivore hosts. Plants may release constitutive volatiles or synthesize new ones as an induced response to herbivore damage. Several studies have tested the attraction of various natural enemies to both synthetic VOCs and natural plant odors, but few have compared the behavioral responses of specialist and generalist parasitoids with varying degree of host specificity to various plant odors. This study was conducted to test the attraction of two parasitoids, Microplitis croceipes (specialist) and Cotesia marginiventris (generalist) to synthetic VOCs and natural plant odors. The goal of the study was to address the evolutionary and mechanistic question of whether specialist and generalist parasitoids differ in their use of plant volatiles for host location. Both species are solitary larval endoparasitoids in the same family (Hymenoptera: Braconidae) and are important parasitoids of Heliothis virescens (Lepidoptera: Noctuidae) and other caterpillar pests of cotton. In chapter II, VOCs were categorized as those released passively from undamaged plants (UD-VOC) and herbivore-induced plant volatiles (HIPVs). HIPVs were further categorized into: i) volatiles released by fresh damage plants (FD-VOC), and ii) volatiles released by old damage plants (OD-VOC). α-pinene (UD-VOC), (Z)-3-hexenol (FD-VOC) and (Z)-3-hexenyl acetate (OD-VOC) were selected as representatives of the different VOC types based on GC-MS and behavioral results from previous studies. The attraction of both parasitoid species to synthetic VOCs and a binary mixture were tested in four-choice olfactometer bioassays. Female M. croceipes (specialist) showed the greatest attraction to the HIPVs while female C. marginiventris could not discriminate among the VOC types. Comparing species, female M. croceipes were significantly more attracted than female C. marginiventris to (Z)-3-hexenol. In contrast, female C. marginiventris showed significantly greater attraction to α-pinene compared to female M. croceipes. Conspecific males showed similar responses with a few exceptions. When presented with the choices; α-pinene, (Z)-3-hexenol and a binary mixture (50:50v/v) of the two compounds, the specialist showed the greatest attraction to the mixture. The mixture did not elicit such an additive effect on the attraction of the generalist. Species and sexual (in the specialist) differences were recorded in the overall response latency (time taken to choose VOCs). The ecological significance and practical implications of these results are discussed. In chapter III, the responses of both sexes of the two parasitoid species to VOCs emitted by cotton plants infested by host H. virescens larvae were investigated using a headspace volatile collection system coupled with-four choice olfactometer bioassay. The advantage of this set up is that it allows for direct bioassay of parasitoids to the headspace volatiles emitted by treatment plants and subsequent analysis by GC-MS, thus providing a possible direct explanation for the observed responses. The treatments tested were undamaged plants (UD), fresh (6 hr infestation) damage plants (FD), and old (24 hr infestation) damage plants (OD). Both sexes of M. croceipes showed a preference for VOCs from host-damaged plants (FD- and OD-plants) over UD-plants, In contrast, female C. marginiventris could not discriminate among UD-, FD- and OD-plants, whereas the males showed a preference for FD-plants. GC-MS analyses showed qualitative and quantitative differences in the VOC profiles of UD-, FD- and OD-plants which may explain the behavioral responses of the parasitoids. The ecological significance and practical implications of the results are discussed.