Allothrombium mitchelli (Acari: Trombidiidae) in the Great Smoky Mountains National Park: Incidence, seasonality, and predation on beech scale (Homoptera: Eriococcidae)

Abstract

American beech gaps in the southern Appalachian mountains are currently threatened by an insect-mediated disease complex known as beech bark disease. Cryptococcus fagisuga Lindinger, or beech scale, a major component of beech bark disease, wounds trees through feeding on vascular tissue through the outer cambium. This feeding method leaves numerous wounds and provides entryways for infection by fungal pathogens. Allothrombium mitchelli Davis, a large red velvet mite, was found in the Great Smoky Mountains National Park, where it was observed to feed on beech scale. First-instar larvae of A. mitchelli are quiescent, and deutonymphs are the only mobile immature stage. A. mitchelli probably has one generation per year with adult population peaks in late spring and early fall; deutonymph populations also peaked in early fall within the areas studied. Both adults and deutonymphs feed on beech scale. In 1996, mites were found on 22.1% of beech trees (n = 453) in selected plots; incidence was greatest at Sweat Heifer on 9 May (81%, n = 29). Mite incidence was significantly influenced by categorized aspect (Chi;2 = 57.92, P < 0.0001, n = 453) and categorized elevation (Χ2 = 101.33, P < 0.0001, n = 453). A strong significant (0.4528, P < 0.0001) correlation was observed between mite incidence and elevation. A weak but significant negative correlation (-0.1341, P = 0.0001) was found between numbers of A. mitchelli on individual trees and beech scale ratings at Sweat Heifer and Indian Gap plots. Although their status as a biological control agent of beech scale is uncertain, A. mitchelli is one of only a few species that has been observed to feed consistently on beech scale. © 2001 Entomological Society of America.