Seasonal Development of Fiorinia externa, Tsugaspidiotus tsugae (Homoptera: Diaspididae), and Their Parasite, Aspidiotiphagus citrinus (Hymenoptera: Aphelinidae): Importance of Parasite-Host Synchronism to the Population Dynamics of Two Scale Pests of Hemlock

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Abstract
The seasonal development of Fiorinia externa Ferris, Tsugaspidiotus tsugae (Marlatt) (Homoptera: Disaspididae), and their parasite, Aspidiotiphagus citrinus (Craw.) (Hymenoptera: Aphelinidae) on hemlock was studied at two sites in southwestern Connecticut. Descriptions of the parasite life stages developing within each scale and of the host stages attacked are given. Although A. citrinus completed 2 generations on both scales during 1977, its seasonal development on each differed considerably in response to the sharply contrasting life histories of univoltine F. externa and bivoltine T. tsugae . Evidence is presented that synchronism between ovipositing adult parasites and suitable life stages of their hosts had an important influence on percent parasitism and on the dynamics of scale populations. On T. tsugae , where the seasonal abundance of parasite and host were synchronous, percent parasitism and scale population growth remained relatively constant for 3 generations. However, on F. externa where seasonal abundance of parasite and host were asynchronous, parasitism and population growth fluctuated greatly from generation to generation. Several features of the scale insect community on hemlock which influence the permanency of the parasite-host relationship and the probable success of biological control are discussed.