Spruce budworm (Choristoneura fumiferana) exploit a narrow window of opportunity in spring by feeding on expanding buds. An improved synchronization of this window on black spruce is one of the mechanisms suggested for the current northward migration of budworm outbreaks in Québec. We test how synchrony between budworm emergence and host plant budbreak influences budworm survival and development and examine the fate of early-emerging larvae. Field studies involved placing 7 cohorts of budworm larvae on black spruce (Picea mariana) and balsam fir (Abies balsamea) branches at different lags with tree budburst phenology over 2 years.
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. Lab experiments show that larvae placed on shoots with closed buds have better success at mining year-old needles and are less likely to disperse on balsam fir than on black spruce. Together, our results 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. Previous studies show that budworm emergence advances more with climate warming than does host budburst; hence, the fate of these early-emerging larvae will become increasingly important to budworm dynamics.