Natural Enemies and Plant Water Relations Influence the Distribution of an Armored Scale Insect

Abstract

In urban settings, the armored scale insect Pseudaulacaspis pentagona infests mulberry trees in disturbed landscape habitats (roadsides, parking lots) but not trees in forested habitats (woodlots). Populations of P. pentagona are contagiously distributed among mulberry trees in landscape habitats. We investigated how two factors, natural enemies and plant water potential, interact to affect the spatial distribution and abundance of P. pentagona. Survivorship of P. pentagona (caged with enemies excluded) was significantly lower on scale—free landscape trees than on scale—infested landscape trees or forest trees. Differences in scale survivorship disappeared, however, when scales were raised on greenhouse—grown clones of these same field trees, suggesting that variation in survivorship was due to the habitat in which trees grew. The survivorship of caged scales (enemies excluded) on field trees was positively related to midday water potential, an index of plant moisture stress. Scale—free landscape trees had significantly lower midday water potentials than did either scale—infested landscape trees or forest trees. Patterns of water potential and scale survivorship across trees suggested that the absence of P. pentagona on landscape trees with a history of scale absence was attributable to plant water relations. Moreover, highly significant differences in water potential occurred between infested and scale—free mulberry trees located <10 m apart, suggesting that moisture deficit influences the performance and abundance of scale insects over small distances. When cohorts of P. pentagona were raised on field trees and exposed to natural enemies for one generation, mortality was significantly higher on forest trees than on either scale—free or scale—infested landscape trees. High scale mortality in the forest habitat was attributable to abundant generalist predators. (e.g., phalangids, earwigs, and tree crickets). Scale mortality in the landscape habitat, due to specialized coccinellid predators and aphenlinid parasitoids, was much lower than in the forest habitat. The patchy distribution of P. pentagona in Maryland is best explained by the combined influences of natural enemies and plant water relations. P. pentagona infests a small subset of mulberry trees in the urban landscape that are without water deficits and free of generalist invertebrate predators.