In managed boreal forests, both plantations and natural regeneration are used to re-establish a cohort of conifer trees following harvest or disturbance. Young trees in open plantations generally grow more rapidly than under forest canopies, but more rapid growth could be compromised by greater insect damage. We compared insect damage on white spruce (Picea glauca (Moench) Voss, Pinaceae) growing in plantations with naturally regenerated trees under mature forest canopies in boreal forests (Québec, Canada). We selected ten sites in the naturally regenerated forest and in small, multispecies plantations and sampled ten young trees of 2.5–3 m (per site) in late summer 2020 and again in early and late summer 2021. We compared overall rates of herbivory, galls (adelgids), damage by the spruce budworm (Choristoneura fumiferana, Clemens), and defoliation from sawflies. Overall, insect herbivory damage remained at similarly low levels in both habitats; an average of 9.3% of expanding shoots were damaged on forest trees and 7.7% in plantation trees. Spruce budworm damage increased from 2020 to 2021 and remained higher in under-canopy trees, but damage rates were negligible at this early stage of the outbreak (1.5% in forest vs. 0.78% of buds damaged on plantation trees). While damage due to galls was higher in plantations, the overall low level of damage likely does not pose a significant impact on the growth or mortality of young trees.

With climate change projected to increase the frequency and severity of episodic insect outbreak events, assessing potential consequences for soil microbial communities and nutrient dynamics is of importance for understanding forest resilience. The forest tent caterpillar (Malacosoma disstria) is an important defoliator of deciduous tree species in temperate and mixed forests of eastern North America with an invasion cycle every 10–12 years and outbreak events that can last 3–6 years. Following a defoliation episode on trembling aspen (Populus tremuloides) from 2015 to 2017 in Abitibi-Témiscamingue, QC, Canada, we sought to test if defoliation resulted in changes to soil bacterial and fungal communities. We hypothesized an increase in soil microbial biomass due to increased caterpillar frass inputs and potential changes in community structure following the event. Soils were sampled in August 2018, May 2019 and July 2019 from sites that had been subjected to defoliation during the outbreak and from sites where no defoliation had been recorded. We assessed soil microbial biomass and fungal to total microbial activity ratio on all sampling dates, and Community Level Physiological Profiles (CLPPs) for 2018 only using a substrate-induced respiration method. Contrary to our hypothesis, we observed a significant 50% decrease in microbial biomass (μg biomass-C g−1 soil hour−1) in defoliated stands, suggesting tree carbon normally allocated towards root exudates was reallocated towards foliage regeneration. We noted a differentiated carbon-based substrate usage following defoliation, but no change in the fungal to total microbial activity ratio. The observed changes in the two years following the defoliation event suggest that defoliation episodes above-ground could trigger changes in soil chemistry below-ground with effects on soil microbial communities that may, in turn, feedback to influence forest plant dynamics.

Insect outbreaks are major drivers of natural disturbances in forest ecosystems. Outbreaks can have both direct and indirect effects on the composition of soil arthropod communities through canopy opening, nutrient addition and predator

Collapses of insect pest outbreaks are often attributed to delayed density dependence of predation. The forest tent caterpillar, Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), is an outbreaking pest species defoliating mixed wood boreal forests in eastern Canada. This species presents periodic population dynamics with peaks every 10 years and outbreaks lasting 1–3 years impacting the health of its host tree. We asked about the relative importance of various sources of early-instar mortality during and after the crash of an outbreak, testing for density dependence. We used a triad set-up of complete predator exclusion, partial exclusion, and free colonies to distinguish between intrinsic mortality (caused by, e.g., pathogens) and mortality caused by flighted and walking natural enemies, and compared defoliated outbreak sites with control sites. Overall, survival was lowest in the free colonies and increased with partial and complete predator exclusion. Survival was also higher in control than in outbreak sites and higher in the final year of the outbreak than in the following year. We observed no changes in mortality from walking enemies, but an increase in intrinsic mortality and mortality from flighted enemies. in the year following the outbreak. This increase is consistent with density dependence of these mortality sources, but its occurrence in the control sites as well was unexpected. These findings show that mortality from flighted natural enemies and intrinsic source increases after the outbreak, but do not contribute to the population crash. However, they help maintain low endemic levels between outbreaks.

Biocontrol of caterpillars by ants is highly variable, and we investigate how the strength of the trophic relationship between ants and an important outbreaking forest pest depends on phenological synchrony and on social foraging. We test the hypothesis that early spring foraging by ants, coupled with eusocial recruitment behavior, could undermine the caterpillar's strategies to achieve either enemy-free space or predator satiation.We use a series of field surveys and experiments in trembling aspen stands (Populus tremuloides) in the boreal forest of eastern Canada to assess the role of ants in early-instar mortality of the outbreaking, gregarious forest tent caterpillar (Malacosoma disstria). We also investigate individual-level mechanisms related to phenology and social behavior that underlie the effectiveness of ants as biocontrol on caterpillars. Our results show that ants climb trees early in the spring and harvest young forest tent caterpillars, suggesting that early phenology does not provide an entirely enemy-free space for caterpillars. Our findings further show that recruitment-based social foraging enables ants to deplete groups of gregarious prey, suggesting that these eusocial insects are particularly effective at generating predation pressure on gregarious herbivores since they do not satiate easily. Finally, a manipulative predator exclusion experiment confirms that ant predation is a significant mortality source for early-instar forest tent caterpillars. Taken together, these results suggest that phenology and sociality could modulate the role of ants as effective caterpillar predators and thus showcase the importance of considering natural history and behavioral traits when studying trophic interactions and their role in population dynamics.

Se peut-il que les peupliers profitent des services de prédation des fourmis, qui les débarrassent de fâcheux insectes herbivores?