Parasitism of the Scale Insect, Fiorinia externa (Homoptera: Diaspididae), by Aspidiotiphagus citrinus (Hymenoptera: Eulophidae) in a Hemlock Forest: Density Dependence

Abstract

The spatial distributions of the scale insect, Fiorinia externa Ferris (Homoptera: Diaspididae) and its parasite, Aspidiotiphagus citrinus (Craw.) (Hymenoptera: Eulophidae), on Eastern hemlock, Tsuga canadensis (L.) Carr., were studied during 1976 in a mixed conifer-hardwood forest in Westport, Conn. The highest numbers of adult female scales and immature parasites occurred in the lower crown (0–4 m above ground) where 49 and 70% of the total number of each were collected, respectively. The upper crown (9–12 m) supported the least numbers (19 and 6% of the total scale and parasites, respectively). Percent parasitism of 2nd-instar females by A. citrinus was significantly correlated with the density of F. externa females on branches collected within the crown and from the lower crowns of 30 trees sampled in May and Sept. Increases from May–Sept. in percent parasitism were significantly correlated with increases in scale density over the period, indicating that A. citrinus responds density-dependently to changes in host population levels. The density-dependent response of A. citrinus to F. externa within the crown and among trees indicates the potential value of the parasite as a biological control agent of a most destructive insect pest of hemlock in parts of the northeast United States.