An insect bioassay method to determine persistence of bacillus thuringiensis var. Kurstaki (B.t.k.) protein in oak foliage, following application of a commercial formulation under field and laboratory conditions

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Abstract
A bioassay method was developed to determine field deposits and persistence of Bacillus thuringiensis var. kurstaki (B.t.k.) protein (in ng/g foliage, and ng/cm of foliar area), after aerial application of Foray® 48B formulation over a hardwood forest. Two blocks (B1 and B2) were sprayed at 50 BIU in 3.94 L/ha. initial deposits and persistence of protein were assessed by two force‐feeding bioassay methods, foliar‐extract‐bioassay (FE‐bioassay) and redissolved‐protein‐bloassay (RP‐bioassay), using fourth instar gypsy moth (Lymantria dispar L.) larvae. The initial deposits (490 and 424 ng protein/cm2 in both spray blocks. Among the two methods tested, the FE‐bioassay was more sensitive than the RP‐bioassay because the minimum detection limit (HDL) for Foray 48B was 40 ng protein/cm2, compared to 110 ng protein/cm2 for the RP‐bioassay. The protein could be detected on foliage even up to 8 d post‐spray by the former method, whereas it was detected only up to 5 d by the latter. The suitability of the two methods to detect differences in initial deposits was examined in a laboratory study. Foray 48B was sprayed on oak branches at two dosage rates (15 BIU in 1.2 L/ha and 30 BIU in 2.4 L/ha), and bioassays were conducted. With both bioassay methods, higher deposits were found on branches that received the higher dosage rate. Moreover, the initial deposits obtained at both dosage rates were greater than the MDL of the less‐sensitive RP‐bioassay. Therefore, both bioassay methods were equally suitable to detect the differences in the deposit levels.