Spatial and Seasonal Distribution of Disseminating Stages of Fiorinia externa (Homoptera: Diaspididae) and Natural Enemies in a Hemlock Forest

Abstract

Numerous developmental stages of elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae), disseminating on the wind were trapped during 1978 on sticky plates positioned throughout the canopy of a forest of Eastern hemlock, Tsuga canadensis Carr., located in Westport, CT. Although crawlers (mobile first-instar nymphs), the primary stage for dispersal, were much more abundant than other stages, eggs, gravid adult females, and hemlock needles infested with all stages were sufficiently numerous to implicate these as important agents for local dispersal. All stages able to establish new infestations were most abundant in the air during late spring when the nutritional status of the young foliage favors successful establishment. Adults of two aphelinid parasites, Aspidiotiphagus citrinus Craw. and Aphytis nr. aonidiae (Mercet) occurred in significantly greater numbers both in the air (from sticky plates) and on the foliage (from emergence cages) at the lower (5 m) and middle (10 m) heights of the canopy where host densities were greatest. Both species attacked a greater proportion of the scales feeding on the foliage at these heights for two generations, suggesting density-dependent parasitism. The predominant predators, a mirid bug, Atractotomus magnicornis nr. buenoi Knight, and the lady beetle, Chilocorus stigma (Say), were distributed uniformly among 30 hemlocks supporting a range of scale densities indicating that predators do not respond numerically to host density. Because the seasonal occurrence of adults of these natural enemies coincides with peak abundance of scale crawlers, chemical control efforts might better be directed toward adult scales as part of an integrated program for managing hemlock scale populations.