Strategies for recontacting a lost pheromone plume: casting and upwind flight in the male gypsy moth

. Gypsy moth ( Lymantria 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 6 cm turn ‐1 to nearly 0 cm turn ‐1 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 100 ng 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.