Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers.
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). This study examined the hypothesis that elevated visible and ultraviolet sunlight directly affects the settling behaviour of A. tsugae crawlers. 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. 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. Our results suggest that A. tsugae crawlers are negatively phototactic and/or thermotactic, with behaviour influenced more by visible light than ultraviolet light.