Biennial aerial application of Bacillus thuringiensis Berliner var. kurstaki is the most cost-effective approach of protection against spruce budworm (Choristoneura fumiferana [Clemens])

Abstract Key message Aerial application of Bacillus thuringiensis Berliner var. kurstaki (Btk) every second year to stands of white spruce ( Picea glauca (Moench)Voss.), black spruce ( Picea mariana Mill.) and balsam fir ( Abies balsamea (L.) Mill.) is the most cost-effective spraying scenario for reducing the impact of spruce budworm ( Choristoneura fumiferana Clemens) on wood production, providing a similar level of forest protection, but at lower cost, to the standard scenario currently used in which 50% of current year’s foliage is protected every year. Context Insect outbreaks can have significant effects on forest productivity and various formulations of Bacillus thuringiensis Berliner var. kursta ki ( Btk ) are used to reduce their damage. In the Province of Québec, Canada, control programs aim to protect at least 50% of current-year foliage to limit tree mortality, but little information exists on the long-term cost-effectiveness of such programs. Aims Our goal was to evaluate the benefit/cost ratio and the efficacy of different Btk protection scenarios in reducing coniferous tree mortality and growth losses over a 11-year period. We hypothesized that less-intensive protection approaches ( Btk applications every 2 or 3 years) may provide similar levels of protection but with higher cost-effectiveness ratios than the standard program currently used in Quebec. Methods In 2007, we established nineteen 100-ha experimental units in Quebec’s Côte-Nord region to determine the efficacy and cost-effectiveness of different Btk spraying scenarios for reducing tree mortality and volume losses in coniferous stands dominated by mature balsam fir trees ( Abies balsamea (L.) Mill., with white spruce ( Picea glauca (Moench) Voss.) and black spruce ( P. mariana Mill.) as companion species. Tree mortality was monitored annually in three circular plots of 400 m 2 within each experimental unit. Growth losses were evaluated using stem analyses. Results Mortality was much higher in balsam fir than in black spruce and white spruce (respectively 74.4%, 13.8% and 5.9% in untreated stands) in all protection scenarios. The application of Btk every 2 years reduced balsam fir mortality to a level similar to the standard scenario (10.3% vs 7.15%, respectively) at a much lower cost. Growth losses have also been reduced but not to the same extent as in the standard scenario. Conclusion Spraying Btk every 2 years provides effective protection to balsam fir and is the most cost-effective scenario. A less intensive use of Btk would mitigate impact on non-target lepidoptera and allow protecting other areas, such as habitat of the woodland caribou, a threatened species which avoids disturbed areas.