Pairing Spruce Budworm Control and Minimal Understory Perturbations: Effects of Btk Spraying Frequency in Boreal Forests

Since 1985, more than 10 million hectares of Canadian forests have been treated against spruce budworm (SBW) epidemics using the Btk biological insecticide. Still, no study has thoroughly evaluated the effects of these interventions on understory vegetation. Since the forest floor hosts most of the plant diversity in boreal forests and provides critical habitats for wildlife, it is crucial to determine the best treatment to reduce defoliation caused by SBW, while minimizing disturbances to understory plant communities. Using an experimental design established in 2007, we tested in 2022 the effects of different Btk spraying frequencies on tree defoliation, understory vegetation, and arboreal lichens. Our results show that reducing the spraying frequency increased light availability in the understory, leading to higher vascular plant richness, increased cover of shrubs, forbs, and shade-intolerant species, and a greater number of fruits produced by fleshy-fruit bearing plants, while arboreal lichen biomass remained unchanged. Although annual spraying most effectively limits SBW-induced defoliation, such an approach may not be necessary to maintain canopy closure. Biennial applications appear sufficient to preserve foliage during SBW infestation, while limiting understory shifts toward early-successional vegetation.