Density-Dependent Parasitism of the Hackberry Nipplegall Maker (Homoptera: Psyllidae): A Multi-Scale Analysis

Abstract

The spatial scale employed in field studies investigating density-dependent parasitism of insect hosts may influence the results of such investigations. As an empirical test of this observation, the relationship between the density of hackberry nipplegall makers, Pachypsylla celtidismamma (Fletcher), on hackberry, Celtis occidentalis L., and parasitism by the specialist parasitoid Psyllaephagus pachypsyllae Howard was examined at 3 spatially nested scales: leaves, branches, and trees. The distribution of galls among leaves on trees was highly clumped, and gall density varied considerably among leaves, branches, and trees, regardless of how it was measured (absolute number of galls, galls per leaf, or proportion of galled leaves). At the individual leaf level, parasitism was higher for galls on leaves with multiple galls than for single galls. However, the incidence of parasitism did not increase with increasing gall densities on the multiply galled leaves, indicating that parasitoids, although attracted to high-density patches, do not appear to remain in these patches. Parasitism was found to be density independent at the branch level, regardless of how density was measured, indicating that parasitoids do not respond to host density at this spatial scale. However, at the whole-tree level there was a significant positive relationship between density and parasitism for 2 measures of gall density: absolute gall number and galls per leaf. This pattern may result from the establishment of highly localized populations of parasitoids around trees with consistently high galling levels. Overall, the relationship between gall density and parasitism was found to be complex and scale dependent.