Effects of an early intervention management strategy for spruce budworm on balsam fir growth

When an outbreak of spruce budworm ( Choristoneura fumiferana ) occurs, larvae primarily defoliate balsam fir ( Abies balsamea ) trees in the boreal forest, leading to large-scale tree mortality and reduced growth that can cause forests to become carbon sources. In response to the current spruce budworm outbreak in eastern North America, managers have been trialling a new “early intervention strategy” (EIS). Under this strategy, areas are sprayed with insecticides ( Bacillus thuringiensis var. kurstaki (Btk)) to maintain larval population below outbreak levels, preventing defoliation and tree mortality. In this study, we investigated how the management of a spruce budworm outbreak is affecting the growth of fir trees on the island of Newfoundland. Specifically, we took cores from trees in sprayed (n = 60 trees) and unsprayed (n = 109 trees) sites and measured annual ring widths from 2010 to 2024. We then standardized the ring widths and performed a segmented regression to determine whether growth changed over time. We found that growth only changed in unsprayed sites with a significant breakpoint in the tree growth shortly after the outbreak started. Insecticide spraying as applied under the EIS appears to delay growth loss in balsam fir in the short-term. However, the benefits of spraying must be weighed against potential costs to assess the value of this active management approach over the long-term. • Spruce budworm can substantially defoliate balsam fir trees during an outbreak. • The early intervention strategy prevents tree mortality during spruce budworm outbreaks but effects on tree growth are unclear. • We conducted a field study to test effects of insecticide on tree growth. • Trees in unmanaged forests showed decreased growth after the outbreak began. • Trees treated under an early intervention strategy did not suffer growth loss.