Climate change is predicted to alter relationships between trophic levels by changing the phenology of interacting species. We tested whether synchrony between two critical phenological events, budburst of host species and larval emergence from diapause of eastern spruce budworm, increased at warmer temperatures in the boreal forest in northeastern Canada. Budburst was up to 4.6 ± 0.7 days earlier in balsam fir and up to 2.8 ± 0.8 days earlier in black spruce per degree increase in temperature, in naturally occurring microclimates. Larval emergence from diapause did not exhibit a similar response. Instead, larvae emerged once average ambient temperatures reached 10°C, regardless of differences in microclimate. Phenological synchrony increased with warmer microclimates, tightening the relationship between spruce budworm and its host species. Synchrony increased by up to 4.5 ± 0.7 days for balsam fir and up to 2.8 ± 0.8 days for black spruce per degree increase in temperature. Under a warmer climate, defoliation could potentially begin earlier in the season, in which case, damage on the primary host, balsam fir may increase. Black spruce, which escapes severe herbivory because of a 2‐week delay in budburst, would become more suitable as a resource for the spruce budworm. The northern boreal forest could become more vulnerable to outbreaks in the future.

The western bean cutworm, Striacosta albicosta (Smith), is considered a major corn pest in North America. In 2009, it started invading the province of Québec (northeastern Canada). To our knowledge, there has been no evidence that this pest could overwinter in this province. In the falls of 2017 and 2018, storage totes containing mature western bean cutworm larvae were placed in the soil in three agricultural regions, in Québec, where western bean cutworm invasions are frequent and crop damage is often significant. The goal was to evaluate if western bean cutworm could resist the province’s winter conditions. The storage totes were removed from the soil at different dates to estimate winter survival. Emergence cages were installed over the totes in the spring of the following year to determine whether western bean cutworm could complete its life cycle. In the spring of 2019, car shelters were also installed at two different sites to evaluate natural moth emergences in fields in which numerous mature western bean cutworm larvae and damage were reported in 2018. Western bean cutworm moth emergences occurred in both cases. These experiments constitute the first documented evidence that western bean cutworm can overwinter and complete its life cycle in Québec’s cold climate. It also represents the northernmost overwintering survival for this species ever documented in North America. The western bean cutworm’s ability to overwinter in Québec will have important implications for corn producers. Pest monitoring and management programs in the province of Québec will need to be adapted accordingly.