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. We infer that these semiochemicals’ attractant, repellent, or neutral characters are associated with the hostplant’s volatile blend- the matrix. We integrated these multi-source data and performed odor imaging. Images revealed beetles’ differential olfactory perception of different hostplants and indicated how a beetle could distinguish between two closely related plant species. Moreover, odor images showed a differential olfactory perception of the same hostplant by different closely-related beetle species. Further, we investigated the conspecific aggregation of one of the beetle species. We observed that the first visitor(s) initiate the aggregation in nature. In controlled conditions, aggregation occurred similarly on beetle-devoured and mechanically wounded hostplants. Beetles did not aggregate on the unwounded plants suggesting that the signal was wound-induced. Behavioral assays revealed that the aggregation was olfactory cue-mediated. Beetle feeding induced the emission of three sesquiterpenes, α-copaene, β-copaene, and δ-cadinene, from the leaf. Using a complementation analysis, we found that α-copaene was the aggregation signal. Our work demonstrated that the Chiridopsis species’ host specificity is plant-odor-mediated. Hostplant’s odor blend matrix is crucial, and beetles do not respond to attractants or repellants without this matrix. Beetles differentiate closely related host species based on the number and concentrations of attractants and repellants in their blends. Since every odorant can have different effects on two beetle species, these beetle species have different olfactory perceptions of the same plant. Lastly, we showed that aggregation is also mediated by host-emitted olfactory cue, the wound-induced sesquiterpene. 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 multidimensional investigation also underlines the relevance of studying the entire odor blends over the individual compounds.