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

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. kurstaki (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 m² 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. [ABSTRACT FROM AUTHOR]