Priority of color over scent during flower visitation by adult Vanessa indica butterflies

Most flower visitors innately prefer a particular color and scent, and use them as cues for flower recognition and selection. However, in most cases, since color and scent serve as a combined signal, not only does the preference for an individual cue, but also the preference hierarchy among different cues, influence their flower visitation. In the present study, we attempted to reveal (1) the chromatic and (2) the olfactory cues that stimulate flower visiting, and (3) the preference hierarchy between these cues, using the naive adult butterfly Vanessa indica. When we offered 12 different-colored (six chromatic and six achromatic) paper flower models, V. indica showed a color preference for yellow and blue. When we examined the proboscis extension reflex (PER) of V. indica towards 16 individual compounds identified in the floral scents from two nectar plants belonging to the family Compositae, Taraxacum officinale and Cirsium japonicum, six compounds were found to have relatively high PER-eliciting activities, including benzaldehyde, acetophenone, and (E+Z)-nerolidol. When we combined color and scent cues in two-choice bioassays, where butterflies were offered flower models that were purple (a relatively unattractive color), the models scented with these active compounds were significantly more attractive than the odorless controls. In addition, synthetic blends mimicking the floral scents of T. officinale and C. japonicum (at doses equivalent to that of ten flowers) enhanced the number of visits to the scented models. However, the effect of odorizing was not conspicuous in parallel bioassays when yellow flower models were used, and the butterflies also significantly preferred odorless yellow models to scented purple models. These results demonstrate that V. indica depends primarily on color and secondarily on scent during flower visitation.