Host specialization chemical ecology of a plant insect herbivore system

Abstract

Hostplant location and conspecific aggregation on the hostplant are the key behaviors of several herbivore insect species. The cues used by insects for host identification and aggregation initiation have been researched mainly using a single hostplant and herbivore species. It has been found that the chemical repertoire of plants, including volatile and non-volatile secondary metabolites, is critical in mediating these processes. In natural ecosystems, often several closely related plant species co-occur. Despite these related plant species similar chemical repertoires, insects proficiently locate their hosts. How they resolve such complex chemical cues is understudied. To study the basis of such resolution, we used five co-occurring Ipomoea spp. as hostplants and four Chiridopsis spp. (beetles) as their herbivores. In this wild sympatric system from the Western Ghats of India, monophagous, biphagous, and oligophagous Chiridopsis spp. are specialist herbivores of different Ipomoea species. We studied the chemistry of these beetles stringent host-specificity by determining the roles of different chemical cues in hostplant location and aggregation. We analyzed beetles hostplant preferences vis-à-vis hostplant volatile blends. We found plant volatiles as the primary hostplant identification cues. Using GC-MS/-FID and SPME headspace analyses we characterized odor blends of the five Ipomoea spp. and identified putative attractants and repellents for each Chiridopsis sp. using multivariate statistics. We determined their attractant or deterrent natures using behavioral assays and ascertained their perception by the antennal olfactory receptors using electroantennography. Beetles responded to these compounds only when they were delivered via their hostplant odor blends. The same compound delivered via differentially preferred hostplants odor blends generated differential responses. Beetles did not respond when these compounds were provided individually or via non-host odor blends.