The Lethal and Sublethal Effects of Aerial Applications of Bacillus thuringiensis subsp. kurstaki on the Spruce Budworm and Its Parasitism

The bioinsecticide Bacillus thuringiensis subsp. kurstaki (Btk) is applied over large areas to reduce defoliation caused by the spruce budworm, an insect which affects millions of hectares of coniferous forests every 30 to 40 years in eastern North America. The aim of our study was to determine whether, in addition to its direct lethal effects, aerial spraying of Btk had sublethal effects on spruce budworm populations and their parasitism. Four sites were sprayed with Btk and compared to four control sites in two regions, one where the outbreak had started three years earlier and the other where it had been going on for 10 years. Insects were collected to compare budworm pupal mass and parasitism at different stages (L5, L6 and pupae). Budworm pupae were significantly lighter in Btk-treated sites than in controls, and in the older population than in the younger one. However, pupae collected from Btk-treated sites had the same mass in both regions, suggesting a minimum pupal mass threshold, which may affect population dynamics. Larval parasitism was low, but pupal parasitism was high and strongly influenced by an interaction between Btk and region, with a higher parasitism observed in Btk-treated sites of the younger population than in those of the older population. A significant interaction was observed between Btk treatment and region on the proportion of larvae that failed to complete development, which was particularly high in Btk-treated sites of the older population. Our study confirms the effectiveness of Btk in controlling spruce budworm populations directly but also indirectly through sublethal effects on budworm development, capacity to complete development, pupal size and parasitism. To maximize control efficacy, the timing of Btk applications could vary according to the age of populations during the budworm outbreak cycle.