Refuge Dynamics and Metapopulation Dynamics: An Experimental Test

Abstract

Red scale, an insect pest of citrus, is under control by the parasitoid Aphytis melinus in many areas, and in our study area the interaction appears dynamically stable. The bark on the interior branches and trunk of trees provides a partial refuge for red scale, which are rarely attacked there by the parasitoid. In a grapefruit grove, we carried out a two-way experiment in which we manipulated the refuge population (present or removed) and either left trees connected with the rest of the grove or isolated individual trees with cages to test for metapopulation effects. The experiment ran for 17 mo, encompassing three generations of scale. Scale density in the exterior of refuge-removed trees decreased by about 60%. However, neither removal of the refuge population nor isolation of individual trees increased the temporal variability of the scale population in the exterior or led to drift in population density. Indeed, removal of the refuge population caused a decrease in temporal variability. We conclude that stability in the control population was not maintained by either refuge or metapopulation dynamics. Reduced scale recruitment and density in the exterior of trees lacking a refuge population were associated with increased (i.e., density-dependent) scale survival that did not reflect a change in parasitism.