Male oriental fruit moths, Grupholithu molesta (Busck) (Tortricidae), flew at lower overall and net ground velocities when they flew toward higher concentration pheromone sources. Turning frequency was greater with increased pheromone concentration, while the distance of turns from the plume axis back towards the axis decreased. Turning magnitude and inter-reversal track angles remained constant at all concentrations tested. Concomitant with the changes in ground velocity but constant inter-reversal angles, were decreases in airspeed, decreases in the moths' course angles and increases in their drift angles. The significance of these changes is discussed in relation to their possible role in a longitudinal chemoklinotactic programme of turning operating in conjunction with anemotaxis to allow location of a pheromone source in wind.

No abstract

Male oriental fruit moths, Grapholitha molesta (Busck) (Tortricidae), continue to zigzag along a pheromone plume to the source in zero wind, if they have started flight with wind on. If the pheromone source is removed and the plume is hence truncated, moths flying in zero wind out of the end of the plume into clean air increase the width of their reversals and the angles of the straight legs of the tracks so they are more directly across the former wind line. Such moths reach the source less often than do those flying along a continuous plume. The males continue to zigzag up a plume in zero wind, apparently by a combination of sequential sampling of concentration along their path and the performance of an internal, self-steered programme of track reversals (zigzags) whose frequency increases with concentration. Visual feedback may aid in the still-air performance of the zigzags. We propose that both the sequential sampling (longitudinal klinotaxis) and self-steered counter-turning programme also are used in wind as well; anemotaxis apparently polarizes the direction of the zigzags to result in upwind displacement, and the narrow zigzags caused by the higher concentration in the plume keep the male 'locked on' to the odour.

Gypsy moth (Lyrnantria dispar L., Lepidoptera: Lymantriidae) males flying upwind to high concentration pheromone sources had lower ground speeds, shorter crosswind reversal distances and higher turn rates than males flying upwind to low pheromone doses. Casting flight, crosswind flight with no net upwind movement after loss of pheromone contact, developed gradually and was similar in fashion for males initially flying to the three pheromone doses tested; after plume loss, males' flight in the upwind direction decreased from 6cm turn-' to nearly Ocm turn-' by their fifth turn (crosswind reversal). After plume loss, upwind displacement decreased, crosswind distances and inter-turn duration increased. Ground speeds tended to increase after plume loss, but airspeeds decreased following loss of 10 and 100ng plumes and remained constant after loss of 1 ng plumes. Latency to casting was c. 1 s as measured by cessation of upwind progress and by timing the first reversal leg that was down or crosswind after plume loss. Though these measures of casting were independent of pheromone concentration, they are means that mask the fact that about 40% of males, rather than cast crosswind after loss of the plume, continued to move upwind, with wider and temporally less regular crosswind reversals. Such continued movement upwind is in sharp contrast to previous descriptions of casting flight after loss of odour-plume contact. In addition, some males engaged in 'regressive' casting, moving downwind after bouts of 'typical' casting. The value of casting and of the continued upwind flight after plume loss, both possible strategies for recontacting a pheromone plume, are discussed.