Manoeuvres used by flying male oriental fruit moths to relocate a sex pheromone plume in an experimentally shifted wind‐field

ABSTRACT. In a wind‐field experimentally shifted in direction by 35d̀, flying male Grapholita molesta (Busck) zigzagging upwind either maintained contact with a pheromone plume and followed it across during the shift or lost it and commenced casting at c. 90d̀ across the shifting windline to locate it eventually in its new position. Males accomplished both of these results by integrating the previously described systems of optomotor anemotaxis and self‐steered counterturning, but with faster reaction‐times to pheromone on and off than heretofore calculated for this species. We found no evidence that males following the plume across used chemotaxis as proposed for another species, Rather, the sawtoothed‐shaped tracks were a result of the anemotactic and counterturning systems responding rapidly and reiteratively to each loss and gain of pheromone along the plume in the shifting wind. The response to an increase or decrease in pheromone concentration by males was to change their course angle to more upwind or more crosswind, respectively, on the very first reversal (within c. 0.15 s) after the concentration changed. Because males adjusted their airspeeds more slowly to changes in concentration, the groundspeeds along the more upwind‐orientated legs were lower than those along cross‐wind legs, contributing to the sawtoothed shape of tracks of plume‐followers. The self‐steered counterturning programme also reacted quickly to concentration changes, the reversal intervals tending to be shorter following each contact with pheromone than after each excursion into cleaner wind. Following casting after losing the plume, males relocating the pheromone plume exhibited an upwind ‘surge’ of narrow zigzagging flight because on the first leg in the plume they steered a course more directly upwind than on the previous leg and increased the frequency of counterturning to its highest value while maintaining the relatively high airspeed acquired while casting.