Biophysical characteristics of the stem and petiole surface of six hemlock (Tsuga) species and a hybrid: implications for resistance to Adelges tsugae

Characteristics of the plant surface significantly affect host-plant selection by phytophagous insects. Surface morphology of six hemlock species (Tsuga spp.) and a hybrid was investigated using low-temperature scanning electron microscopy. Observations focused on trichome presence and placement and cuticle thickness. These characteristics were studied in the context of species-level host-plant resistance to the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), an exotic insect causing massive mortality to eastern hemlocks (Tsuga canadensis (L.) Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.) in the eastern United States. Hemlocks in the native range of the insect do not succumb to infestations and the mechanism of resistance is unknown. We addressed the potential role of plant surface morphology in the adelgid–hemlock interaction by comparing four adelgid-resistant hemlock species and a hybrid with the two adelgid-susceptible hemlock species. We found that trichomes are likely not involved in conferring resistance to A. tsugae. Cuticle thickness may be involved in insertion site selection by A. tsugae and may therefore have implications for resistance. The cuticle is thinnest at the point of A. tsugae stylet insertion and thus may affect A. tsugae feeding.