Synergy between visual and olfactory cues in nectar feeding by wild hawkmoths, Manduca sexta

We performed field experiments to measure the relative importance of olfactory and visual cues in nectar foraging by wild tobacco hornworm moths, Manduca sexta (Lepidoptera, Sphingidae) in the Sonoran Desert of Arizona, U.S.A. We manipulated flowers of sacred Datura (Datura wrightii; Solanaceae) to experimentally decouple floral scent from visual display and presented these cues to free-flying moths in mixed and homogeneous arrays. Moths did not feed from cloth-bagged fragrant flowers lacking strong visual contrast, nor did they feed from paper model flowers lacking plant odours. Unexpectedly, moths fed from paper model flowers that were associated solely with vegetative odours, albeit at lower levels than when floral scent was present. Subsequent experiments revealed that the combination of floral and vegetative odours did not incrementally increase nectar feeding and that floral scent without vegetation was sufficient to elicit feeding when paper flowers were present. Thus, wild M. sexta in our study did not show generalized feeding responses to natural or artificial flowers with single sensory stimuli; like naı̈ve laboratory-reared moths, they required a combination of visual and olfactory cues. However, given the prior foraging experience of our study population on Datura flowers, their lack of generalized feeding responses may reflect a learned preference for the full complement of floral cues, rather than the persistence of innate sensory constraints. Future efforts to distinguish between these hypotheses should focus on whether M. sexta can be conditioned to associate nectar with visual cues alone, and whether moths that feed ad libitum from nectar-rich plants learn floral attributes as a search image or generalize to single-modality stimuli.