Comparing the efficacy of various aerial spraying scenarios using Bacillus thuringiensis to protect trees from spruce budworm defoliation

Large-scale aerial spray operations against the spruce budworm (Choristoneura fumiferana (Clemens)) (SBW) with the biological insecticide Bacillus thuringiensis ssp. kurstaki (Btk) aim at maintaining trees alive during outbreaks. This objective is thought to be achieved when >= 50% of current-year foliage is preserved until the end of the outbreak. This protection target is associated with low balsam fir (Abies balsamea [L.] Mill.) mortality. However, it is unknown whether this approach is always needed or whether less frequent interventions could provide similar results at a lower cost. Between 2010 and 2016, we conducted field experiments in Quebec's Cote-Nord region to determine the efficacy of five different Btk spraying scenarios for protecting balsam fir, white spruce (Picea glauca [Moench] Voss) and black spruce (P. mariana [Mill.] BSP) in mixed stands. We used the residual photosynthetic capacity (RPC) to evaluate the efficacy of the five scenarios. RPC makes it possible to take into account the impact of SBW defoliation on foliage contribution to tree photosynthetic effort over several years, and can be used as a proxy of the risk of tree mortality. We hypothesized that less frequent Btk applications could maintain the required RPC level to keep trees alive. Our results show that areas not protected resulted in great losses of RPC in balsam fir and white spruce. Btk applications every 3 years kept RPC above 50% for 2 years in balsam fir and 4 years in white spruce. RPC losses were above 62% after 4 years in both species. The strategy currently employed in Quebec (spraying every year after a first year of moderate-severe defoliation) and the intensive protection scenario (Btk applications every year) meet the protection goals for these hosts. However, their cost prevents their application at a large scale. Btk applications every 2 years seems a relevant alternative to the current strategy to protect balsam fir and white spruce stands given the adequate level of protection provided (RPC above 39%) and the reduction in the number of Btk applications required (36% fewer applications over 7 years, resulting in 36% lower cost), particularly if the objective is to maintain trees alive. Black spruce maintained at least 54% of its RPC, even without protection. Btk applications every 3 years might be a valid alternative to reduce growth losses in black spruce-dominated stands. The use of different spraying scenarios may allow us to develop cost-efficient treatment strategies to protect Quebec's forests.