Differing susceptibility to Bacillus thuringiensis formulations of Thaumetopoea wilkinsoni populations between forests with different Bt management in Israel

The susceptibility of Thaumetopoea wilkinsoni larvae to Bacillus thuringiensis (Bt) formulations was screened in 2003 and 2004. Eggs and larvae were collected from pine forests in 11 geographical locations in Israel. Larval mortality bioassays were conducted with commercial formulations (Delfin WG, Dipel DF and Foray 48B) at concentrations ranging from 0.001% to 0.1%. Significant differences in susceptibility to Bt were recorded among populations that were treated with Bt intensively, frequently, or never. The mortality recorded in a population that was never treated with Bt was twice that in an intensively Bt-treated population. The correlation between susceptibility to Bt and the possible resistance to the microbe is discussed.