Climate warming disrupts tree phenological defenses, contributing to range expansion of an important boreal forest outbreaking pest.

Background/Question/Methods

Phenological shifts, induced by global warming, can lead to mismatch between closely interacting species or even form new matches. The eastern spruce budworm, Choristoneura fumiferana, an important outbreaking insect defoliator of the boreal forest, mainly feeds on balsam fir, Abies balsamea, which has historically been well synchronized with the insect. But as climate change pushes the northern range limit of the budworm further north into the boreal forest, the highly valuable black spruce, Picea mariana is suffering increased defoliation during the current outbreak. Budworm exploit a narrow window of opportunity in spring by feeding on expanding buds. Improved synchronization of this window on black spruce could explain the current range expansion of budworm onto black spruce.We first test how synchrony between budworm emergence and budbreak of the two host plants influences budworm performance with a 2-year field experiment rearing budworm larvae on both hosts at different lags with tree budburst phenology. We next tested the hypothesis that rising temperatures can lead to an improved match between the budworm and black spruce through differential phenological advancement with a 3-year experiment rearing budworm on both hosts in temperature enhancement mesocosms.

Results/Conclusions

Results show that, on both hosts, late-emerging larvae suffer both high mortality and poor development, presumably linked to decreasing foliar quality during the growing season. By contrast, early emerging larvae show good growth but high mortality relative to synchronized larvae. Further lab experiments suggest that the survival of early-emerging larvae is constrained by their ability to enter a needle and that this critical step in the life-cycle depends on needle toughness. Mesocosm results show that under higher temperature regimes, eastern spruce budworm will be as successful on black spruce as on balsam fir, as black spruce budburst becomes better synchronized with the insect's emergence from diapause. This could lead to critical changes in outbreak dynamics and severity with important ecological state shifts at the landscape level.