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Two exotic armored scales, Fiorinia externa Ferris and Tsugaspidiotus tsugae (Marlatt), native to Japan occur sympatrically in the northeastern United States where they often attain high population densities on eastern hemlock, Tsuga canadensis Carriere. Both scales preferentially colonize the young needles of the lower hemlock crown, which intensities competition between them when food and space become limiting at high scale densities. Studies conducted for 2 yr in the greenhouse and in a hemlock forest located in Darien, Connecticut, demonstrated that summer generations of F. externa and T. tsugae compete intraspecifically and interspecifically at densities occurring in the field. F. externa had a greater advderse affect (reduced survival and fecundity) on itself and on T. tsugae than did T. tsugae have either on itself or on F. externa. The superior competitive ability of F. externa over T. tsugae was likely due to its earlier colonization time. Hatching in early June some 2—4 wk prior to its competitor, F. externa is afforded a distinct spatial and nutritional advantage (1) by colonizing young, nitrogen—rich needles at a time during the year when concentrations of foliar nitrogen and water are high, (2) by reducing the amount of foliar nitrogen available to T. tsugae nymphs, and (3) by forcing T. tsugae crawlers to colonize older, nitrogen—poor foilage where they subsequently suffer significantly greater mortality. Aspidiotihagus citrinus Crawford (Hymenoptera: Aphelinidae), a primary internal parasitoid of F. externa and T. tsugae, exhibited a density dependent response to both scales in single—species infestations in the Darien forest. In mixed infestations, however, the parasitoid did not selectively attack the more abundant species but parasitized each scale in proportion to its frequency. Because it did not discriminate between hosts, A. citrinus did not alter the outcome of the competition between summer generations of the two scales. The geographical distributions of the two scales in Faifield County, Connecticut, and their population trends for 3 yr at 20 sites where they occur sympatrically indicate that competition has excluded T. tsugae. At 18 to 20 sites coinhabited at different densities during 1976, T. tsugae was excluded or nearly excluded within 3 yr. Only at two sites, where T. tsugae was abundant and F. externa was sparse, was T. tsugae able to maintain its initial relative density.

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