Spatial heterogeneity in risk of secondary parasitism in a natural population of an aphid parasitoid

Abstract

Summary 1 The relationship between primary parasitoid (Aphidius matricariae) density and the risk of secondary parasitism (chiefly by Alloxysta tcheki) was studied in a system based on the aphid Capitophorus carduinis feeding on Cirsium palustre. 2 The densities of healthy and parasitized aphids were sampled fortnightly over seven growing seasons. Population densities were relatively stable and parasitism rates were low (∼6%). 3 Rates of secondary parasitism were high (∼60%), which probably accounts for the low impact of primary parasitoids on aphids. 4 Density-dependent and density-independent secondary parasitism was assessed at the level of the host plant at a long-term study site over a 2-year period (22 samples) to test the hypothesis that these factors may be significant enough to influence population processes. 5 Significant density-independent parasitism was observed in all but four samples. Significant positive density-dependent parasitism was observed nine times, and negative density-dependence once. The strength of density-dependent parasitism was strongly influenced by the variance in host (mummified aphid) density across patches.