Mortality and Recovery of Hemlock Woolly Adelgid (Adelges tsugae) in Response to Winter Temperatures and Predictions for the Future

Abstract

Eastern (Tsuga canadensis) and Carolina hemlocks (T. caroliniana) of eastern North America have been attacked by the non-native hemlock woolly adelgid (Adelges tsugae Annand) (HWA) since the first half of the 20th century. Unlike most insects, HWA develops through one generation from fall to late winter, exposing this insect to the lethal effects of winter temperatures. The mortality inflicted by winter temperatures on HWA determines the surviving population density as well as its ability to spread to uninfested areas. With the ongoing changes in climate, knowledge of this species' ability to survive and spread in the future can help land managers prepare for its management. This study began during the winter of 2014 and ended in the spring of 2017. During this period, winter mortality of HWA was recorded at 100 sites from Maine to Georgia (n = 209). Changes in population density from the sistens to the succeeding progrediens generation were recorded at 24 sites (n = 35). Models were developed to predict HWA mortality using the lowest minimum temperature prior to the mortality assessment date, the number of days with mean temperature <−1 • C, and the mean daily temperature of the three days preceding that minimum. Models were also developed to predict population density changes from the overwintering sistens generation to the following progrediens generation. Future projections under climate change showed increases in winter survival and population growth rates over time. Especially towards the northeastern edge of T. canadensis' distribution as minimum temperatures are predicted to increase at a greater rate. This will result in an increase in density throughout its current distribution and expansion northward causing an increase in its impact on eastern Tsuga spp.

First Report on Establishment of Laricobius osakensis (Coleoptera: Derodontidae), a Biological Control Agent for Hemlock Woolly Adelgid, Adelges tsugae (Hemiptera: Adelgidae), in the Eastern U.S.

“…With the lag effect in HWA and predator numbers as they relate to tree health [18], additional years of monitoring are needed to capture the lag effects we suspect are at work. Extreme cold temperatures associated with the polar vortex reduced HWA densities at our study sites [27], which gave trees an opportunity to rebound from pest attack.…”

“…We found a 29% increase in odds of finding beetles with each increase in plant hardiness zone (i.e., 5b to 6a), suggesting that warmer plant hardiness zones are more suitable for Laricobius spp. Sites in the warmer hardiness zones did not get the same extreme winter temperatures during the polar vortex [27], which led to a smaller decline in HWA density and in Laricobius spp. populations.…”

“…The branch clipping sampling method for Laricobius larvae was much more successful at finding the target predator. The low number of recoveries in the first year was likely due to the colder than normal winter temperatures in the 2014 and 2015 seasons, which resulted in a significant decrease in adelgid density throughout the range of HWA in the eastern U.S. [27]. The low adelgid populations made it difficult to collect L. osakensis, since other than their occurrence as pupae and aestivating adults, this beetle can only be found directly associated with HWA [11].…”

Forest health monitoring in transition: Evaluating insect-induced disturbances in forested landscapes at varying spatial scales

“…In the HWA MaxEnt model, mean January and February precipitation were observed as important predictors. In previous HWA studies, minimum winter temperatures have been identified as a major limiting factor for HWA distribution (Paradis et al 2008;McAvoy et al 2017;Tobin et al 2017). However, only the mean minimum October temperature was found among the used 27 climatic variables in the final models and it was not an important predictor.…”

“…The high susceptibility of the host species combined with lack of natural enemies, rapid reproduction, and several vectors have provided for the successful performance and fast spreading of HWA in the eastern USA (McClure 1987;McClure and Cheah 1999;Trotter and Shields 2009). The species was recorded for the first time in the eastern USA in Richmond of Virginia, in 1951 (Stoetzel 2002), although the introduction has most likely been much earlier (McAvoy et al 2017). In the beginning, HWA spread slowly in ornamental settings and was not considered as a pest.…”