Pine processionary moth, Thaumetopoea wilkinsoni Tams, is an important defoliating lepidopteran pest of pine trees. The aim of this study was to determine the required spore crystal concentration of local Bacillus thuringiensis (Bt) isolates, optimal ambient temperature, and larval stage to control this troublesome forest pest. The susceptibility of T wilkinsoni larvae decreased with older stage and lower temperatures. The optimum temperature was found to be 15 degrees C or higher for the control of early larval stages. At the highest spore crystal concentration (500 mu g g(-1)), the most effective isolate (SY49.1) caused 83% mortality for the second-stage larvae at 5 degrees C. However, an approximately 4-fold decrease in mortalities was observed in late-stage larvae for all isolates examined at this temperature. Nevertheless, other Bt isolates, excluding SY27.3, caused nearly complete mortality at 25 degrees C for early-stage larvae. Considering the distribution of seasonal temperature, Bt products should be applied at the highest ambient temperature to earlier stages for efficient control. We propose that local Bt isolates SY27.1, SY49.1, and SY62.1 could be used to develop environmentally safe bioinsecticides to control this important pest species. These results indicate that larval stage and environmental temperature should be taken into consideration for efficient control of T wilkinsoni using the spore crystal mixture of Bacillus thuringiensis isolates.