Assessment of the effects of defoliation on freshwaters to inform insect outbreak control strategies in boreal and hemi-boreal forests

Defoliation by insects affects more of the North American boreal forest than wildfire and forest harvest combined, with Eastern Spruce Budworm (Choristoneura fumiferana) as the most prevalent defoliator. Control strategies vary in cost and degree of defoliation suppression, so informed decisions rely on understanding the broader impacts of defoliation. Little is known about how defoliation impacts downstream freshwaters – an important gap to address given growing recognition of the need to protect freshwater resources. We use a standardized benthic macroinvertebrate (BMI) bioindicator approach to compare streams in defoliated and undefoliated watersheds. We find dissimilarity in BMI community composition between streams in defoliated and undefoliated watersheds with a resulting influence on regional diversity. Despite differences in composition, diversity metrics were not correlated to defoliation nor outside of the natural range of variation (without defoliation). However, there was a loss of sensitive taxa and higher proportion of tolerant taxa in watersheds where substantial tree mortality was present (> 35% of watershed area) – indicating potential precursors to biodiversity loss. We therefore conclude that control strategies to suppress defoliation do not benefit nor impact freshwater BMI unless designed to prevent tree mortality, and so fully preventative control measures are not necessary.