Drift of the pesticide Bacillus thuringiensis Berliner var. kurstaki (Bt) was measured downwind from a gypsy moth (Lyrnantria dispar L.) treatment block in mountain terrain. The study was conducted in conjunction with a project to eradicate the moth along the Wasatch mountain range of Utah. Objectives of the study were to quantitate drift, to evaluate drift sampling methods, and to compare FSCBG (Forest Service Cramer-Barry-Grim) computer model predictions to field measurements of Bt based on colony-forming units. The treatment block was sprayed by helicopter three times, within a 12-d period during June 1991, which provided 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 demonstrated a correlation among dosage, deposition and total flux samplers; a potential of Bt drift of at least 3,150 m; and FSCBG model predictions that generally followed the field recovery data.

Drift of the pesticide Bacillus thuringiensis Berliner var. kurstaki (Bt) was measured downwind from a gypsy moth (Lymantria dispar L.) treatment block in mountain terrain. The study was conducted in conjunction with a project to eradicate the moth along the Wasatch mountain range of Utah. Objectives of the study were to quantitate drift, to evaluate drift sampling methods, and to compare FSCBG (Forest Service Cramer‐Barry‐Grim) computer model predictions to field measurements of Bt based on colony‐forming units. The treatment block was sprayed by helicopter three times, within a 12‐d period during June 1991, which provided 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 demonstrated a correlation among dosage, deposition and total flux samplers; a potential of Bt drift of at least 3,150 m; and FSCBG model predictions that generally followed the field recovery data.