Effects of Sunlight and Simulated Rain on Residual Activity of Bacillus thuringiensis Formulations

The effect of sunlight and simulated rain on the residual insecticidal activity of Bacillus thuringiensis subsp. kurstaki formulations applied to field grown cabbage were measured. Leaf samples were collected 1,2,4, and 7 d after treatment and assayed against neonate cabbage looper, Trichoplusia ni (Hübner). Simulated rain applied by a sprinkler irrigation system washed B. thuringiensis from the plants, causing on average 20% reduction in insecticidal activity across all treatments. Sunlight intensity was manipulated by applying degrees of shade treatments consisting of no cover, clear plastic covers, and black plastic covers. Black plastic provided protection from sunlight degradation for 7 d, whereas both clear plastic and no cover treatments lost insecticidal activity equally beginning 2 d after application of B. thuringiensis. There was no interaction between simulated rain and shade treatments and thus no synergistic loss of insecticidal activity by the combination of both environmental factors. Formulations of B. thuringiensis differed in their ability to resist wash-off by simulated rain and degradation by sunlight. Formulations consisting of 1% wt:vol gluten or 0.5% wt:vol casein resisted wash-off better than flour/sucrose (2% wt:vol) and Dipel 2X. Resistance to sunlight degradation was greatest with the gluten formulation and progressively less for casein, flour/sucrose and Dipe12X formulations. Half-life of insecticidal activity in response to sunlight was calculated to be 7.1, 5.7, 4.8, and 4.3 d for gluten, casein, flour/sucrose and Dipel 2X formulations, respectively.