Resurgence of the Scale, Fiorinia externa (Homoptera: Diaspididae), on Hemlock Following Insecticide Application

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Abstract
When thoroughly applied to the entire crown, dimethoate 2E insecticide can provide excellent control of Fiorinia externa Ferris (Homoptera: Diaspididae), a destructive scale insect of Eastern hemlock, Tsuga canadensis (L.) Carr., in the northeast (U.S.). However, incomplete application of this insecticide, a likely occurrence when treating forest hemlock, favors the resurgence of scale populations. On sprayed trees, F. externa had a faster rate of development and was more fecund than on unsprayed trees. Also, application of the insecticide, even on trees where only one side of the crown was treated, virtually eliminated the parasite, Aspidiotiphagus citrinus Craw., and 3 species of predators from the entire lower crown. Increased rate of growth and fecundity of F. externa and reduced mortality from natural enemies following insecticide treatment will facilitate the resurgence of populations of the scale on hemlock. The relationship between nutritive quality of the host plant and resurgence of post-spray populations of F. externa is discussed.

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