Context- and scale-dependent effects of floral CO ₂ on nectar foraging by Manduca sexta

Abstract

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...

Honeybees prefer warmer nectar and less viscous nectar, regardless of sugar concentration

“…Floral warmth may act as a cue for pollinators (signalling improved nectar rewards) as well as a reward [39]. Other cues recently shown to be used by hawkmoths are CO 2 [40,41] and relative humidity [42]: like floral warmth, these will be effective only under certain environmental conditions.…”

Floral humidity as a reliable sensory cue for profitability assessment by nectar-foraging hawkmoths

“…Our previous studies of floral CO 2 (16,18) were motivated in part by the hypothesis that M. sexta and other foraging hawkmoths should attend to transient CO 2 plumes emitted by newly opened flowers of D. wrightii as more temporally accurate indicators of nectar availability. When elevated CO 2 is liberated as profitable flower buds open, it represents the same kind of condition-dependent cue defined by Hill (15) in the context of sexually selected ornaments in animal courtship and proposed by us for floral humidity gradients.…”

“…For example, above-ambient floral emissions of CO 2 peak at dusk, when nectar-rich flowers of Datura wrightii (Solanaceae) open, but floral CO 2 diminishes rapidly after visitation by Manduca sexta moths, whereas floral scent and visual display persist for hours after nectar removal (16). When present, flowers with elevated CO 2 should be preferred by nectar foraging Manduca as the most accurate indicator of profitability in Datura flowers, and they are (17,18). Whatever the physiological causes, elevated floral CO 2 in D. wrightii flowers provides moths with a more temporally accurate indicator of nectar than the visual and olfactory signals that guide hawkmoths to these flowers (19).…”

Analysis of cooking spices in natural waters

“…Flora typically produce spices and scents either to attract insects or animals for dispersion of pollen, or to chemically protect against predators (Goyret et al 2008;Raguso 2008). Humans use spices to enhance the flavor of food and mask bitter flavors in medicines (Dignum et al 2001).…”