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

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   Fig. 1.

   Visualization of predicted outcomes for alternative hypotheses on the behavioral function of floral CO₂ in nectar foraging behavior by M. sexta, as tested in experiment 1. The vertical axis represents a unit-free relative measure of moth response (e.g., tracking and probing). The horizontal axis contrasts the predicted outcomes when moths are exposed to artificial flowers with different subsets of floral stimuli, including visual target (V) with floral odor (V+O), carbon dioxide (V+CO₂), or the combination thereof (V+O+CO₂). Hypothesis names reflect how CO₂ would interact with floral odor. Here, synergism is broadly defined as a nonadditive (+ or −) interaction with odor, as indicated by the two-headed arrow within the bar. The potential for strict synergism, in which only the combination of odor and CO₂ elicits the appropriate behavior, has already been falsified for tracking, floral approach, and probing in previous experiments (7, 43, 44).

   
   
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   Fig. 2.

   Behavioral responses of adult M. sexta in a wind tunnel with a single surrogate flower. (A) Responsiveness, expressed in percentage, of the recorded behaviors. Categories in the abscissa represent the different sets of stimuli offered by the surrogate flower: Different type of letters (uppercase, lowercase, and Greek) are used for comparisons between treatments for each behavior. Different letters indicate significant differences with an α-level of 0.05 (see Results). (B) Latencies to approach (i.e., time from takeoff to flower approach, in seconds) of male and female moths. Data points and error bars represent means ± SEM. Number of replicates for each treatment in parenthesis.

   
   
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   Fig. 3.

   Proportion of first flower choices by adult M. sexta in dual-choice assays between two scented surrogate flowers, of which only one emitted above-ambient CO₂ levels (V+O+CO₂). Assays were performed in a wind tunnel either without (experiment 2; A) or with (experiment 3; B) four tomato plants at the upstream end. * and **, significant departure from random probabilities with α-levels of 0.05 and 0.005, respectively.