Spatial and temporal distribution of airborne Bacillus thuringiensis var. kurstaki during an aerial spray program for gypsy moth eradication

We measured airborne exposures to the biological insecticide Bacillus thuri ngiensis var, kurstaki (Btk) during an aerial spray program to eradicate gy psy moths on the west coast of Canada. We aimed to determine whether stayin g indoors during spraying reduced exposures, to determine the rate of tempo ral decay of airborne concentrations, and to determine whether drift occurr ed outside the spray zone. During spraying, the average culturable airborne Btk concentration measured outdoors within the spray zone was 739 colony-f orming units (CFU)/m(3) of air. Outdoor air concentrations decreased over t ime, quickly in an initial phase with a half time of 3.3 hr, and then more slowly over the following 9 days, with an overall half-time of about 2.4 da ys. Inside residences during spraying, average concentrations were initiall y 2-5 times lower than outdoors, but at 5-6 hr after spraying began, indoor concentrations exceeded those outdoors, with an average of 244 CFU/m(3) vs . 77 CFU/m(3) outdoors, suggesting that the initial benefits of remaining i ndoors during spraying may not persist as outside air moves indoors with no rmal daily activities. There was drift of culturable Btk throughout a 125- to 1,000-meter band outside the spray zone where measurements were made, a consequence of the fine aerosol sizes that remained airborne (count median diameters of 4.3 to 7.2 mum). Btk concentrations outside the spray zone wer e related to wind speed and direction, but not to distance from the spray z one.