PREDICTING AND MEASURING DRIFT OF BACILLUS-THURINGIENSIS SPRAYS

Drift of the pesticide Bacillus thuringiensis Berliner var. kurstaki ( Bt) was measured down-wind from a gypsy moth (Lymantria dispar L.) tre atment block in mountain terrain. The study was conducted in conjuncti on with a project to eradicate the moth along the Wasatch mountain ran ge of Utah. Objectives of the study were to quantitate drift, to evalu ate drift sampling methods, and to compare FSCBG (Forest Service Crame r-Barry-Grim) computer model predictions to field measurements of Bt b ased on colony-forming units. The treatment block was sprayed by helic opter three times, within a 12-d period during June 1991, which provid ed a study of three replicates. During each spray, downslope drainage winds carried the spray over an array of samplers that extended 3,150 m downwind. The FSCBG dispersion model, exercised after the treatments , used meteorological inputs measured during the treatments to predict spray deposition and airborne dosage of Bt. Results of the study demo nstrated a correlation among dosage, deposition and total flux sampler s; a potential of Bt drift of at least 3,150 m; and FSCBG model predic tions that generally followed the field recovery data.