Pheromone-mediated optomotor anemotaxis and altitude control exhibited by male oriental fruit moths in the field

In the field over short grass, pheromone-stimulated oriental fruit moth males, Grapholita molesta (Busck), flying under high windspeeds tended to steer courses more into the wind and to increase their airspeeds compared with those flying in low windspeeds. Thus, optomotor anemotaxis enabled the males to steer relatively consistent upwind track angles and to maintain an upwind progress of between c. 50-100 cm/s despite variable wind velocities. Zigzagging flight tracks were observed at both 10 m and 3 m from the source, as were tracks with no apparent zigzags. Transitions from casting to upwind flight or vice-versa were observed. The durations of the intervals between reversals during both upwind zigzagging flight and casting were consistent with those observed in previous wind-tunnel experiments. The control of altitude was more precise during upwind zigzagging flight than during casting. In general, the side-to-side deviations in the tracks were greater than the up-and-down deviations, with both the side-to-side and vertical distances and their ratios being consistent with previous wind-tunnel studies of pheromone-mediated flight. One difference between the field and laboratory flight tracks was that males in the field exhibited much higher airspeeds than in the wind tunnel. Males occasionally were observed to progress downwind faster than the wind itself, and further analysis showed that they were steering a downwind course in pheromone-free air following exposure to pheromone, which is the first time this has been recorded in moths. We propose that such downwind flight may aid in the relocation of a pheromone plume that has been lost due to a wind-shift, by enabling the moth to catch up to the pheromone as it recedes straight downwind away from the source.