SUMMARY Within an appetitive context, Manduca sexta, a nectivorous nocturnal hawkmoth, can be attracted by a range of stimuli including floral volatiles and visual display, carbon dioxide and water vapor. Several studies on this and other flower-visiting insects have shown how olfactory and visual stimulation play (or do not play) a role in attraction and feeding. Nevertheless, these studies have consistently manipulated stimuli in a`presence–absence' manner. Here, we experimentally decoupled the presentation of both stimuli spatially and temporally in a wind tunnel, rather than entirely eliminating either one, and found that the decision-making process based on these stimuli is more flexible and complex than previously asserted. Manduca sexta was most responsive when both cues were present and emanated from the same source. When stimuli were spatially separated, responsiveness levels were comparable to those elicited by a single stimulus. However, transient olfactory stimulation either before or after visually guided approach (temporal decoupling) enhanced responsiveness to an odorless visual target. Additionally, searching times were increased by either a transient olfactory stimulation before take-off or by having the flower model spatially separated from the odor source tracked by the moths. Finally,in a dual-choice experiment, moths showed a strong bias for the visual display over the odor plume, suggesting the former to be the ultimate indicator of a nectar source. Our manipulation of floral cues shows that the feeding behavior of M. sexta, and probably of other nectivorous insects, is based not only on the sensory stimulation per se but also on the temporal and spatial pattern in which these stimuli are perceived.

fragrance ͉ odor ͉ olfaction ͉ pollination ͉ Sphingidae A nthophilous insects use information from a variety of sensory channels to locate flowers and feed from them (1). Thus, a crucial task for studying insect-plant interactions is to identify which components of the environment provide the sensory inputs used by insects, and to what extent context and scale affect their information content (2). Whereas flower colors and patterns (3, 4), whole-flower and nectar odors (5), and even corolla shape and texture are known features used by foraging insects and other nectivorous animals (6, 7), few studies have explicitly addressed the role of flower respiratory carbon dioxide (CO 2 ) as a stimulus involved in plant-pollinator interactions.Our knowledge of how insects use CO 2 as a sensory cue is derived primarily from studies of insects that vector diseases or attack crop plants. For example, many haematophagous insects use CO 2 to locate their animal hosts from a distance (8). Discontinuous CO 2 plumes modulate host-seeking behavior by mosquitoes (9, 10), suggesting that CO 2 acts as a long-range orientation stimulus (11). Tsetse flies and biting midges also use CO 2 as a long-range attractant that can synergize the attractiveness of other host odors (12, 13). However, CO 2 also functions as a close-range feeding stimulus (on the host's skin) for mosquitoes (8). CO 2 synergizes the attractiveness of some human skin odors to female yellow fever mosquitoes (14) and has been suggested to synergize triatomine bug responses to L(ϩ)-lactic acid (15).Some herbivorous insects show similar responses to CO 2 , although typically at closer range to its source than haematophagous insects. CO 2 alone is sufficient to guide Diabrotica virgifera beetle larvae toward corn roots (16), and larvae of noctuid (Helicoverpa armigera) (17) and pyralid (Elasmopalpus lignosellus) (18) moths orient to above-ambient CO 2 sources. Females of the pyralid moth Cactoblastis cactorum search for CO 2 sinks on photosynthetic stem surfaces of Opuntia stricta, as indicators of high carbon fixation activity (19). In this case, CO 2 gradients are used as close-range stimuli, as is true for tephritid flies that oviposit on fruit wounds, which provide a localized source of CO 2 and other olfactory oviposition stimulants (20). Finally, responses to CO 2 may be context-dependent. Tephritid flies respond to CO 2 when presented with a fruit-like visual stimulus (20). Within a certain concentration range, Drosophila melanogaster adults and larvae are repelled by CO 2 , and it has been suggested that these responses depend on the olfactory context (21).It is clear from the studies reviewed above that CO 2 may function alone or in concert with host odors, at a distance or at close range, via several behavioral mechanisms. Recent studies suggest that CO 2 may also contribute to the interactions between flowers and insect pollinators. Floral CO 2 is primarily associated with elevated respiratory activity in thermogenic flowers, including deceptive flowers that mim...

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