BIOASSAYS OF INSECTICIDE SPRAY DRIFT - THE EFFECTS OF WIND-SPEED ON THE MORTALITY OF PIERIS-BRASSICAE LARVAE (LEPIDOPTERA) CAUSED BY DIFLUBENZURON

The environmental hazards of insecticide spray drift were examined at five different wind speeds. Two-day-old Pieris brassicae larvae were used in field bioassays to measure the effects of diflubenzuron (applied at 100 g ha-1) downwind of sprayed areas. Exponential and logistic models were used to describe the observed mortalities at different distances; the fitted models provided the estimates of distances at which the expected mortality would be 50%, 20% and 10%. At the lowest modal wind speed of 2.0 ms-1 (at 2 m height) there was negligible kill beyond 4 m, while at 5.3 m s-1 over 95% mortality occurred up to 16 m. The latter wind speed is above that recommended for spraying insecticides, but even at 2.5 m s-1 (equivalent to force 3 on the Beaufort Scale) 24% mortality was recorded at 24 m.