Persistence of bacillus thuringiensis deposits in oak foliage, after aerial application of foray® 48b using rotary and pressure atomizers

Abstract

A commercial formulation of Bacillus thuringiensis var. kurstaki (Btk), Foray® 48B, was sprayed aerially over blocks selected in a hardwood forest in the USA at the rate of 90 billion international units in 7.0 L/ha, using a helicopter and a fixed‐wing aircraft, each fitted with pressure or rotary cage atomizers. Droplet size spectra were assessed on foliage using two new parameters, viz., droplet‐spot number median diameter (S‐D_N.5) and protein mass median diameter (S‐D_M.5). The data indicated that when rotary cage atomizers were used, the droplet spectra were narrow with smaller S‐D_N.5 and S‐D_M.5 values, than when pressure atomizers were used. The protein mass deposits corresponded with the droplet sizes, i.e., the larger the droplet sizes, the greater the deposits. The initial bioactivity did not correspond with the deposits, because the greater the deposits the lower the activity. Persistence of bioactivity also showed irregularities, indicating the role of environmental transformation of formulation ingredients into products of greater toxicity via the dermal route. A controlled laboratory study provided a new methodology to determine oral and dermal toxicity separately, and showed that dermal toxicity of formulation ingredients increased with exposure to sunlight. The field observations were explained on the basis of the laboratory findings that when the applied droplets were too large, the spread area not only became too large to be effective, but also increased the probability of sunlight exposure of the formulation components present in the deposits.

The present investigation provided a novel method to develop two new parameters, S‐D_N.5 and S‐D_M.5, which were related to droplet spread area on target, and not to spherical diameters of the droplets. As a result, these parameters provided a better understanding of the extent of droplet spreading on surfaces, which played a more important role on rainwashing and photo‐conversion of the ingredients in deposits than the conventional spherical droplet parameters, i.e., the number and volume median diameters, D_N.5 and D_V.5 respectively.

Key words:

• Total protein
• formulation ingredients
• droplet size spectra
• bioactivity
• persistence of deposits
• oral and dermal toxicity