Self-regulation in populations of the elongate hemlock scale, Fiorinia externa (Homoptera: Diaspididae).

Abstract

The presence of numerical self-restraints in populations of the elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae), was investigated by determining the effects of scale density on development rate, fecundity, mortality, and dispersal. Evidence indicates that populations are, to some extent, numerically self-regulated on eastern hemlock, Tsuga canadensis (L) Carr. Development rate, fecundity and survival for two consecutive years were all negatively correlated with scale density on 10 hemlocks in a Ridgefield, CT, USA forest. On hemlocks which had supported the higher densities, the summer, 1977 generation developed more slowly, produced significantly fewer eggs, and suffered up to four times greater mortality than it did on less heavily infested trees.Percent parasitism by Aspidiotiphagus citrinus (Craw.) (Hymenoptera: Aphelinidae) was positively correlated with scale density for two years, reconfirming the density-dependent nature of this parasite-host interaction.First instar nymphs (crawlers) readily dispersed on the wind during both the spring and autumn hatching periods. However, the proportion of crawlers which dispersed from trees supporting the higher densities was no greater than from trees supporting lower densities, indicating that dispersal is density-independent.Self-restraints on F. externa populations imposed at high densities are related to changes in the quality and availability of essential resources for feeding nymphs.