Context- and scale-dependent effects of floral CO₂ on nectar foraging by Manduca sexta

Abstract

Typically, animal pollinators are attracted to flowers by sensory stimuli in the form of pigments, volatiles, and cuticular substances (hairs, waxes) derived from plant secondary metabolism. Few studies have addressed the extent to which primary plant metabolites, such as respiratory carbon dioxide (CO₂), may function as pollinator attractants. Night-blooming flowers of Datura wrightii show transient emissions of up to 200 ppm above-ambient CO₂ at anthesis, when nectar rewards are richest. Their main hawkmoth pollinator, Manduca sexta, can perceive minute variation (0.5 ppm) in CO₂ concentration through labial pit organs whose receptor neurons project afferents to the antennal lobe. We explored the behavioral responses of M. sexta to artificial flowers with different combinations of CO₂, visual, and olfactory stimuli using a laminar flow wind tunnel. Responses in no-choice assays were scale-dependent; CO₂ functioned as an olfactory distance-attractant redundant to floral scent, as each stimulus elicited upwind tracking flights. However, CO₂ played no role in probing behavior at the flower. Male moths showed significant bias in first-approach and probing choice of scented flowers with above-ambient CO₂ over those with ambient CO₂, whereas females showed similar bias only in the presence of host plant (tomato) leaf volatiles. Nevertheless, all males and females probed both flowers regardless of their first choice. While floral CO₂ unequivocally affects male appetitive responses, the context-dependence of female responses suggests that they may use floral CO₂ as a distance indicator of host plant quality during mixed feeding-oviposition bouts on Datura and Nicotiana plants.

• fragrance
• odor
• olfaction
• pollination
• Sphingidae