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Stylet bundle insertion site, path traveled, and feeding site were examined for the hemlock woolly adelgid, Adelges tsugae Annand, on needles from current and previous years of eastern hemlock, Tsuga canadensis Carriere. The stylet bundle is composed of 4 individual stylets—2 outer mandibular stylets and 2 inner maxillary stylets. A. tsugae inserts its stylet bundle on the adaxial side of eastern hemlock needles, proximal to the twig with respect to the leaf abscission layer. Insertions are primarily intracellular through epidermal cells. Once inserted, the stylet bundle then follows a mixed intracellular and intercellular pathway, the latter predominating, when penetrating the plant tissue to the feeding site, the xylem ray parenchyma cells. We observed evidence of salivary secretions, tracks, and sheaths produced by A. tsugae in the plant similar to those produced by aphids and other adelgids in their host plants. Unlike other adelgids studied, which feed on cortical parenchyma cells and on solutes from phloem of their spruce hosts, A. tsugae appears to feed only on storage cells, the parenchyma cells which comprise the xylem rays. This suggests that the intense effect of A. tsugae on eastern hemlock may be caused by factors other than its food consumption, that is, a possible toxin effect, or altered host plant response to environmental conditions.
Young et al. (Wed,) studied this question.