Biological Control of the Cottony‐Cushion Scale: Experimental Tests of the Spatial Density‐Dependence Hypothesis

Abstract

According to the spatial density—dependence hypothesis, successful biological control is stabilized by parasitoid foraging behavior, such that increasing rates of parasitism accompany increasing local host (pest) density. Three field experiments were conducted to try to detect spatially density—dependent parasitism in the biological control of the cottony—cushion scale, Icerya purchasi, by the parasitic fly Cryptochaetum iceryae. Experimental colonies of the scale were established on terminal shoots of one of its host plants, mock orange (Pittosporum tobira). These colonies, consisting mostly of second— and third—stage nymphs, were exposed to parasitism for 11—14 d, depending on the experiment, and the resulting spatial patterns of parasitism rates recorded. In the first experiment the developmental stage of each scale was recorded; in the remaining two experiments, the length of each individual scale was also recorded. Weighted regression analysis was used to study the relationship between spatial density and parasitism rate, using mean length of scales (when available) as a covariate. The relationship was studied on two spatial scales, first by terminal shoots and then by individual leaves nested within shoots. No spatial density dependence was found in any of the three experiments on either spatial scale. Rates of parasitism increased with scale body length, indicating parasite preference for larger hosts. The results suggest that spatial density dependence may not be necessary to the success of biological control. The results should, however, be interpreted with caution because of the limits imposed by finite statistial power; also, it is possible that spatial density dependence may express itself on other host plants or on another spatial scale. Alternative stabilizing mechanisms, such as partial invulnerability of adults and individual variation in risk, are discussed with reference to the cottony—cushion scale system.