Biological Control by the Parasitoid Aphytis melinus, and Population Stability of the California Red Scale

Abstract

(1) The California red scale, a citrus pest, is under successful biological control by its parasitoid Aphytis melinus. Scale abundance is relatively stable, in comparison to other insect populations, and the rate of change of total scale abundance is density dependent. (2) Using data from a lemon grove, we have examined the scale-Aphytis system for several potential stabilizing mechanisms. These were: (a) density-dependent parasitism by Aphytis, in which parasitism rates vary in response to changes in scale density through time, (b) density-dependent sex-ratios in Aphytis, where the proportion of female progeny is influenced by both scale and parasitoid density, and (c) a spatial refuge from parasitism. (3) No evidence was found for the first two mechanisms. Parasitism rates were influenced only by rainfall, while sex-ratio was not significantly affected by any of the variables examined. (4) Strong evidence exists for a refuge from parasitism, which may account for the apparent stability of the system.