Behavioral and electroantennographic responsiveness of adult butterflies of six nymphalid species to food-derived volatiles

While adults of most species of butterflies visit flowers for food (nectar), those of certain species can feed on rotting food (e.g., exuded tree sap and rotting fruits). These food sources considerably differ in odor; flowers emit species-specific scents and rotting-food odors are characterized by fermentation products. To determine whether adult butterflies have different olfactory performances adaptive to their food selections, we examined the following responses of naïve adults of six nymphalid species, namely, Nymphalis xanthomelas, Kaniska canace, Vanessa indica, Cynthia cardui, Hypolimnas bolina, and Argyreus hyperbius: the proboscis extension reflex (PER) was evaluated as the behavioral criterion of preference for scent compounds, and the electroantennographic (EAG) response was assessed as the sensory criterion of antennal perception. All of these species are nectarivorous, but N. xanthomelas, K. canace, and V. indica also forage rotting food. The PERs to 52 tested compounds were in good agreement with the food habits of these butterflies. The six species showed high PERs to several aromatic compounds widely found in flower scents. Fermentation products such as ethanol and acetic acid elicited intermediate to high responses from the three species feeding on rotting foods. The other three species showed low PERs to these compounds, and acetic acid strongly inhibited the PER-eliciting activities of the other compounds. These results demonstrate that the olfactory preference for fermentation products is a characteristic of the rotting-food feeders. The spectrum of EAG responses to 37 tested compounds was relatively similar among the six species. Particular acyclic aliphatic compounds, including green leaf volatiles, elicited high EAG responses from all the species. Despite a high PER-eliciting activity, most aromatic compounds induced intermediate EAG responses; however, ethanol and acetic acid evoked very weak responses. The results suggest that in food selection, adult butterflies use a specific olfactory system to perceive these major food-derived volatiles.