INFLUENCE OF PLUME STRUCTURE AND PHEROMONE CONCENTRATION ON UPWIND FLIGHT OF CADRA-CAUTELLA MALES

The effects of pheromone plume structure and its concentration on the pheromone-mediated flight of male Cadra cautella (Lepidoptera: Phycitinae) were investigated in a laminar-flow wind tunnel. When two C. cautella males flew simultaneously along a ribbon plume of mixed smoke and pheromone, their in-flight behaviour was dependent on the instantaneous structure of the plume they encountered. When a male intercepted an intact ribbon filament, he sustained a crosswind course, whereas when he intercepted a turbulent filament (created by an upwind male fragmenting the ribbon plume), he adopted a flight course more due upwind. These results indicate that C. cautella males altered their in-flight manoeuvres in response to instantaneous changes in the fine structure of the pheromone plume. We also demonstrated that differences in the fine structure of the plume had more influence on the flight pattern of C. cautella males than a 1000-fold range in pheromone dose. The size of the plume was increased by adding wind deflectors upwind of the pheromone source. Independent of source dosage, males following ribbon plumes flew slow zigzag tracks, whereas males following large, turbulent plumes flew directly to the source in fast, straight tracks with less counterturning.