STRATEGIES FOR RECONTACTING A LOST PHEROMONE PLUME - CASTING AND UPWIND FLIGHT IN THE MALE GYPSY-MOTH

Gypsy moth (Lymantria dispar L., Lepidoptera: Lymantriidae) males flyi ng upwind to high concentration pheromone sources had lower ground spe eds, shorter crosswind reversal distances and higher turn rates than m ales flying upwind to low pheromone doses. Casting flight, crosswind f light with no net upwind movement after loss of pheromone contact, dev eloped 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 6 cm turn(-1) to nearly 0 cm tu rn(-1) by their fifth turn (crosswind reversal). After plume loss, upw ind displacement decreased, crosswind distances and inter-turn duratio n increased. Ground speeds tended to increase after plume loss, but ai rspeeds decreased following loss of 10 and 100 ng plumes and remained constant after loss of 1 ng plumes. Latency to casting was c. 1s as me asured by cessation of upwind progress and by timing the first reversa l leg that was down or crosswind after plume loss. Though these measur es of casting were independent of pheromone concentration, they are me ans that mask the fact that about 40% of males, rather than cast cross wind 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' castin g. The value of casting and of the continued upwind flight after plume loss, both possible strategies for recontacting a pheromone plume, ar e discussed.