INSECTICIDE DRIFT FROM GROUND-BASED, HYDRAULIC SPRAYING OF PEAS AND BRUSSELS-SPROUTS - BIOASSAYS FOR DETERMINING BUFFER ZONES

Studies were made on insecticide spray drift from conventional hydraulic sprayers under typical agricultural conditions. A bioassay technique with 2-day-old Pieris brassicae larvae was used to assess drift of cypermethrin and triazophos from pea and brassica crops on six occasions. Earlier work had shown that this species was a suitable model for other butterflies. Replicated series of target larvae were placed downwind of the sprayed crops, and mortality data used to obtain profiles of spray drift related to physical features such as roadways, adjacent crops and hedges. Estimates of the distances at which 50%, 20% and 10% mortality occurred were obtained graphically or by fitting logistic models to the data. Buffer zones that limited mortality to 10% or less were between 16 and 24 m for cypermethrin and 12 m for triazophos. These estimates made no allowance for control mortality of about 5%, however, they were based conservatively on short-term effects. Hedges also probably limited the full extent of drift in two instances. The wider application of these results is discussed and comparisons are made with the use of unsprayed crop headlands.