Adult Vanessa indica and Argyreus hyperbius frequently forage on flower nectar, but the former also utilizes tree sap and rotting fruits. Compared to flower nectar, these rotting foods are characterized by low sugar concentrations and the presence of fermentation products (ethanol and acetic acid). We suspected that gustatory responses by the receptors on the proboscis might differ in these species. Among the three sugars tested, sucrose elicited the greatest probing (behavioral) responses and was followed by fructose and glucose. A. hyperbius showed higher sugar sensitivity than V. indica in probing responsiveness. In electrophysiological responses of the proboscis sensilla, V. indica was slightly more sensitive than A. hyperbius to glucose and lower concentrations of the other sugars. The sugar reception in A. hyperbius was strongly inhibited by fermentation products, particularly acetic acid at natural concentrations. In contrast, V. indica was noticeably less susceptible to them than A. hyperbius, and its behavioral and sensory responses to sucrose were enhanced by 5-20% (w/v) ethanol. Thus, V. indica not only possesses tolerance to fermentation products but may perceive them as synergists for sugar reception. To utilize rotting foods, such tolerance might be more necessary than high sugar sensitivity.