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Abstract
The white peach scale, Pseudaulacaspis pentagona (Targioni-Tozzetti), is a severe pest of woody ornamental and fruit trees throughout the world. In urban Maryland settings, the scale infests mulberry trees primarily in disturbed landscape habitats while trees in forested habitats invariably remain free of scale populations. Furthermore, populations of P. pentagona are contagiously distributed among mulberry trees in landscape habitats. Local adaptation of P. pentagona to individual mulberry trees (Morus alba L.) as a potential cause of patchy distributions was investigated by conducting within- and between-tree transfers of eggs and measuring the survivorship of scales. Survivorship of P. pentagona was significantly higher when raised on their natal hosts, suggesting that scale populations were adapted to individual trees. However, adaptation did not greatly limit the ability of scales to colonize novel host trees and therefore is unlikely to cause patchy distributions of P. pentagona populations.
The influence of plant water deficit and natural enemies on the spatial distribution and abundance of P. pentagona was also investigated. The survivorship of caged scales on field trees was inversely related to host plant moisture deficit. Patterns of moisture deficit and scale survivorship across trees suggested that the absence of populations of P. pentagona on some trees in the landscape habitat was attributable to water deficit. High rates of scale mortality in the forest habitat were attributable to abundant generalist predators that decimated populations. In the landscape habitat, specialized parasitoids inflicted inversely density-dependent mortality, while predation by coccinellids was directly density dependent. These natural enemies failed to suppress P. pentagona populations in the landscape habitat.
The patchy distribution of P. pentagona was best explained by the combined influences of generalist natural enemies and moisture-stressed host plants which relegate populations to a narrow spectrum of mulberry trees in the urban landscape, to those trees not experiencing a water deficit.
I also review information on the life history, host plant relationships, and natural enemies of P. pentagona as well as present a model for forecasting its phenology using degree-day methods.
