Sniffer beetles: Odor imaging reveals congeneric herbivores identify their congeneric hostplants based on differential olfactory perceptions

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 species. 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 commonly 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 chemical cues in hostplant location and aggregation. We analyzed beetles hostplant preferences vis-a-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. Beetles did not respond when these compounds were given singularly or via non-host odor blends. We infer that these semiochemicals attractant, repellent, or neutral characters are associated with the hostplants volatile blend-the matrix. We integrated the multi-source data to visualize this in-flight odor perception by representing odor as color variations or odor images. Odor imaging revealed beetles differential olfactory perception of different hostplants and indicated how a beetle distinguishes between two closely-related plant species. Additionally, it showed a different olfactory perception of the same hostplant by different closely-related beetle species, suggesting they have evolved to recognize the same odor using different components. Our work demonstrates that the hostplants odor blend matrix is crucial; beetles do not respond to attractants/ repellents without it. Together, the closely-related plant species form an ideal system to understand how insects perceive subtle differences between hosts and non-host cues in nature. This investigation also underlines the relevance of studying entire odor blends over individual compounds.