Bioassays of insecticide spray drift: the effects of wind speed on the mortality of Pieris brassicae larvae (Lepidoptera) caused by diflubenzuron

Abstract

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 m s −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.