Abstract
Herbivores emit volatile organic compounds (VOCs) after feeding on plants. Parasitoids exploit these VOCs as odor cues to locate their hosts. In nature, host-related odors are emitted as blends of various compounds occurring in different proportions, and minor blend components can sometimes have profound effects on parasitoid responses. In a previous related study, we identified and quantified VOCs emitted by cotton plant-fed Heliothis virescens (Lepidoptera: Noctuidae) larvae, an herbivore host of the parasitoid Microplitis croceipes (Hymenoptera: Braconidae). 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. Using M. croceipes as a model species, we further asked the question: does the relative abundance of a volatile compound match the level of antennal response in parasitoids? 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.
Highlights
Infested plants emit volatile organic compounds (VOCs) as an indirect defense against herbivore damage[1,2]
Comparing EAG results in the present study and gas chromatography-mass spectrometry (GC/MS) analyses in a previous study[5], we indicated the discrepancy between relative abundance of a volatile blend component and the level of antennal response in parasitoids
EAG responses of Micropiltis croceipes in the present study indicated variation in biological activity elicited by test compounds at the peripheral level, and revealed a discrepancy between relative abundance and level of antennal responses in parasitoids
Summary
Infested plants emit volatile organic compounds (VOCs) as an indirect defense against herbivore damage[1,2]. Informative volatile cues used by parasitoids for host location can be emitted by plants infested with herbivores[1,2] or emitted by herbivores that fed on plants[3,4]. Plant volatiles may initially lead parasitoids to the host patch, herbivore host-specific odors are important shortrange cues used in the later stages of host location[5]. The specific mechanism by which plant-fed host larvae emit these volatiles is not fully understood. It is evident that parasitoids use these plant-associated VOCs in the host location process[5]. Such odor cues are usually released as a blend of various compounds in nature. Are these plant derived volatile compounds emitted by herbivore itself (after feeding) or are they adsorbed into the hervivore body during feeding process (e.g. from frass)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
