The big chill: quantifying the effect of the 2014 North American cold wave on hemlock woolly adelgid populations in the central Appalachian Mountains.
The ability to survive winter temperatures is a key determinant of insect distributional ranges and population dynamics in temperate ecosystems. Although many insects overwinter in a state of diapause, the hemlock woolly adelgid [Adelges tsugae (Annand)] is an exception and instead develops during winter. We studied a low density population of A. tsugae, which undergoes two generations per year, in a forested area in which its only available host plant, eastern hemlock (Tsuga canadensis), was patchy and scarce. In January 2014, this area also experienced an exceptionally cold winter due to a southward shift in the North Polar Vortex. We used 3 years of systematic sampling prior to the 2014 cold wave, and 1 year following, to quantify the effect of the 2014 cold wave on A. tsugae population dynamics. We observed a strong negative correlation between the number of days below sub-zero temperature thresholds and A. tsugae, and estimated that the 2014 cold wave resulted in at least a 238% decrease in its population growth rate. However, we also observed that the detrimental effect of the 2014 cold wave to A. tsugae was short-lived, as populations measured in the late summer of 2014 rebounded to pre-2014 cold wave densities. This study highlights the effect that cold winter weather events can have on a winter active insect species, and the speed at which populations can recover from stochastic mortality events.