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

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.