Thu, Aug 05, 2021:On Demand
Background/Question/Methods
The exotic invasive forest pest, hemlock woolly adelgid (HWA; Adelges tsugae) is the cause of widespread mortality of Carolina and eastern hemlock (Tsuga caroliniana and T. canadensis) throughout the eastern United States. Since its arrival in the northeastern U.S. in the 1980s, HWA has steadily spread throughout eastern hemlock stands. However, in 2018, anecdotal evidence suggested a sharp, widespread decline in HWA populations in the northeastern U.S. following a summer and fall of heavy rainfall. To quantify this decline in HWA density and investigate its cause, we re-surveyed HWA in eastern hemlock stands along a long-term hemlock monitoring transect from northern Massachusetts to southern Connecticut that was established after the initial wave of HWA invasion in New England. As previous research has documented presence of native fungal entomopathogens on HWA in New England and rainfall is known to facilitate the propagation and spread of fungus, we hypothesized that heavy rainfall may facilitate fungal infection of estivating nymphs leading to a decline in HWA density. We tested this hypothesis by applying a rain-simulation treatment to hemlock branches with existing HWA infestations in western MA and evaluating the proportion of diseased and dead adelgid nymphs on experimental vs. control branches.
Results/Conclusions Surveys of long-term HWA-monitoring sites indicate HWA density significantly dropped in 2019 compared to 2011 levels (Wilcoxon = 248.5, p < 0.01) but rebounded in 2020 to levels higher than in 2011 (Wilcoxon = 54, p < 0.01). Precipitation during Jul-Nov of 2018 was negatively correlated with HWA density at each site in both 2019 (F1,21 = 4.04, p = 0.06, R2 = 0.16) and 2020 (F1,21 = 7.33, p = 0.01, R2 = 0.22) suggesting effects of high rainfall (on average, 38 cm above normal) on HWA density persist for at least 2 years, even as the population rebounds. Examination of winter temperature data ruled out high winter mortality as a factor contributing to the decline in HWA density. Experimental rain simulations led to higher proportions of diseased and dead estivating nymphs when HWA density was high (Z = 2.46, p = 0.01). This observational and experimental evidence of a rainfall-mediated HWA decline in conjunction with no evidence for alternative sources of mortality implicate heavy rainfall as the cause of the regional-scale drop in HWA density. Isolation of the fungal pathogen(s) responsible for this HWA mortality is underway and may lead to identification of novel HWA biocontrol agents.
Results/Conclusions Surveys of long-term HWA-monitoring sites indicate HWA density significantly dropped in 2019 compared to 2011 levels (Wilcoxon = 248.5, p < 0.01) but rebounded in 2020 to levels higher than in 2011 (Wilcoxon = 54, p < 0.01). Precipitation during Jul-Nov of 2018 was negatively correlated with HWA density at each site in both 2019 (F1,21 = 4.04, p = 0.06, R2 = 0.16) and 2020 (F1,21 = 7.33, p = 0.01, R2 = 0.22) suggesting effects of high rainfall (on average, 38 cm above normal) on HWA density persist for at least 2 years, even as the population rebounds. Examination of winter temperature data ruled out high winter mortality as a factor contributing to the decline in HWA density. Experimental rain simulations led to higher proportions of diseased and dead estivating nymphs when HWA density was high (Z = 2.46, p = 0.01). This observational and experimental evidence of a rainfall-mediated HWA decline in conjunction with no evidence for alternative sources of mortality implicate heavy rainfall as the cause of the regional-scale drop in HWA density. Isolation of the fungal pathogen(s) responsible for this HWA mortality is underway and may lead to identification of novel HWA biocontrol agents.