Mate-location in most moths is mediated by a plume of female-emitted pheromone that induces flight by males along that plume. In wind, the principal mechanism governing the direction of flight along pheromone plumes is optomotor anemotaxis in which the upwind heading is gauged by transverse image flow. Maintenance of forward progress appears contingent on the rate of encountering individual filaments of odor within the plume. If the rate of filament encounter falls below about 10 Hz, moths head more toward the crosswind. If contact with odor is lost for a large fraction of a second, casting or zigzagging flight without upwind progress ensues. Rapid and often nearly straight flight toward upwind is prompted by rates of filament encounter > 10 Hz. Some intermittency of the odor signal appeared requisite for upwind movement, because two moth species have been found not fly upwind when they are in a homogenous plume. We have explored in a wind tunnel how the fine-scale structure of the pheromone plume dictates flight along the plume in the almond moth, Cadra cautella. To verify the plume’s structure we have used a tracer gas and monitored the concentration of a variety of plume types at 330 Hz. Plumes which were pulsed from 10 to 25 Hz produced many rapid flights upwind and no arrestment of upwind progress. Homogeneous plumes also produced rapid upwind flights over a wide range of concentrations. Together these findings suggest that a rapidly flickering pheromonal signal is not requisite for upwind flight in Cadra.

Index terms: optomotor, anemotaxis, pheromone, attraction, moths

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