Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers

Abstract

Previous research demonstrated that elevated sunlight improves carbon balance and growth of eastern hemlock (Tsuga canadensis (L.) Carrière) seedlings infested with the invasive hemlock woolly adelgid (Adelges tsugae Annand). 2 This study examined the hypothesis that elevated visible and ultraviolet sunlight directly affects the settling behaviour of A. tsugae crawlers. 3 Hemlock seedlings were manually infested with equal densities of A. tsugae and placed into artificial shade treatments (0%, 40% and 80% shade). The density of settled nymphs of the ensuing generation on shaded seedlings was three times higher than the density on unshaded seedlings. 4 Adelges tsugae ovisacs were also attached to hydrated hemlock branches placed beneath acrylic filter treatments that selectively allowed transmission of various sunlight wavelengths. The proportion of adelgid crawlers that settled on the branch (vs. dropping from it) was higher beneath filters that blocked the widest spectrum of solar radiation (opaque and amber filters) than beneath full sun. In the March-April trial, exposure to full sun and the full visible spectrum only (clear-UV filter) significantly increased the proportion of crawlers that settled on the bottom (vs. top) side of the branch. 5 Our results suggest that A. tsugae crawlers are negatively phototactic and/or thermotactic, with behaviour influenced more by visible light than ultraviolet light.

Interaction of hemlock woolly adelgid populations with host tree health drives population oscillations

“…Another limitation to our modeling approach is in representing other factors that may be important to the T. canadensis and A. tsugae system such as temperature, seasonality variability, environmental stochasticity, and site characteristics such as elevation, drought stress, or level of shade are not represented in our model. Although T. canadensis is extremely shade‐tolerant (Havill et al, 2014), A. tsugae density may be impacted by light levels and silvicultural strategies are among the management techniques for T. canadensis (Mayfield et al, 2019). The variation between the parameter estimation results for Groups I and II and the resulting differences in model simulation results may be in part due to these site characteristics we do not represent in the model.…”