The risks posed by deltamethrin drift to hedgerow butterflies.

The susceptibilities of Pieris rapae and P. brassicae (Lepidoptera: Pieridae) to topical treatment with deltamethrin were determined. LD(50)s continued to fall for an extended period after treatment and did not reach a clear end-point for P. rapae. This indicated a failure to excrete or metabolise the active ingredient, which may have continued to exert toxic effects throughout the insect's life cycle. Weight loss by P. brassicae larvae was detected at sublethal rates of exposure and treated insects exhibited feeding inhibition and produced smaller pupae and adults. Residual exposure bioassays over 72 h for P. brassicae exposed to deltamethrin deposits on cabbage leaves, detected toxic effects at rates as low as 1/520th of field application rate. LD(50)s again fell for an extended period after exposure. A model predicting mortality at given levels of drift indicated high levels of short-term risk to larvae exposed to rates of drift deposition recorded in the field. The potential of buffer zones to reduce the toxicological impact of spray drift in the field is discussed.