Influence of visual cues in the flight of male almond moths (Cadra cautella) within plumes of sex pheromone

Male moths find their way to females by following wind-borne pheromone plumes for distances as great as tens or even hundreds of meters. It is generally accepted that this is accomplished through the integration of positive optomotor anemotaxis (the use of wind-induced visual drift to orient upwind) and self-steered counterturning (crosswind zigzagginging the frequency of which is believed to be set by an internal counterturn generator). This usually results in zigzag-shaped upwind flight tracks. However, in turbulent plumes of appropriate pheromone concentration and blend quality, moths often fly nearly straight upwind tracks. Previous wind-tunnel studies with three other moth species have shown that when wind was stopped while males where in flight in a pheromone plume, the moths continued toward the odor sources. Under windless conditions there is no wind-induced visual drift and optomotor anemotaxis must be replaced with another orientation mechanism. I documented flight maneuvers of male almond moths (Cadra cautella) as they flew in a wind tunnel within a turbulent, 65-cm-wide pheromone plume in wind and after wind was stopped following the initiation of upwind-oriented flight. The males flew over visual floor patterns that were either aligned with wind or first aligned and then (further upwind) offset or skewed to the side. The flight tracks were videotaped, digitized and reconstructed in 3-D. Comparisons of mean crosswind locations and angles steered by the moths showed that males in wind and in zero wind that initially followed the aligned-with-wind portion of the pattern subsequently turned toward the floor pattern segment that was not aligned with the upwind heading. In still air, this may be explained by a 'memory' of visual flow consistent with the upwind heading experienced by the males before wind stopped. In wind, moths flying relatively straight tracks in a wide, turbulent pheromone plume may 'collimate' their flight trajectory with visual cues after setting a course with optomotor anemotaxis.