Dispersal of the Scale Fiorinia externa (Homoptera: Diaspididae) and Effects of Edaphic Factors on Its Establishment on Hemlock

Abstract

Crawlers of Fiorinia externa Ferris (Homoptera: Diaspididae) were able to disperse on the wind and establish new infestations on Eastern hemlock, Tsuga canadensis (L.) Carr. A significantly greater number of airborne crawlers was trapped on sticky slides along a transect leeward of infested hemlocks than along transects at 3 other directions from these trees. On each transect more crawlers were collected at 15 m (ca. one tree height) from infested trees than from directly beneath trees or at distances beyond 15 m. Living crawlers on slides at the furthest distance examined (105 m) indicated the ability of F. externa to colonize hemlocks at considerable distances from infested trees. There were significant differences between the numbers of F. externa colonists which survived on hemlock planted in 7 soil conditions characteristic of sites where hemlock grows naturally. Survival of 1st and 2nd instars and the rate of development of females were higher on trees least adversely affected by edaphic conditions. Only on fertilized hemlocks was there less mortality and a higher percentage of fecund female scales than on control trees. On control trees, ca. 39% of dispersing crawlers colonized the youngest needles (1976 growth) while only 26, 20, and 15% settled on 1975, 1974, and 1973 foliage, respectively. Possible limitations on the spread of F. externa imposed by the effect of edaphic conditions on the host plant are discussed in the light of hypotheses relating quality of the food and vigor of the host plant to the dynamics of populations of phytophagous insects.