Effects of light levels and plume structure on the orientation manoeuvres of male gypsy moths flying along pheromone plumes

The upwind zigzag flights of male gypsy moths (Lymantria dispar L.; Lepidop tera: Lymantriidae) along narrow, ribbon-like and wide, turbulent plumes of pheromone were examined in a wind tunnel at light levels of 450 and 4 lux. Under all conditions tested males flew upwind zigzag paths. In 450 lux, ma les flying along turbulent plumes had the highest ground speeds and the wid est crosswind excursions between counterturns, compared to slow flight and a narrow zigzag of males along a ribbon plume. In a turbulent plume, males flew more slowly and had narrower zigzags in 4 than in 450 lux. Across most treatments of plume structure and light level, the rate of transverse imag e flow and the frequency of counterturning remained relatively constant. Th e effects of light levels on orientation are not readily reconcilable with a model in which moths in low light levels would head more towards crosswin d, thereby enhancing the rate of transverse image flow and the perception o f wind-induced drift.