Predatory activity and spatial pattern: the response of generalist carabids to their aphid prey

1. The spatial distribution of cereal aphids infesting a field of winter wheat during the population establishment, development and decline phases were studied using a field-scale grid of sampling locations. 2. The distribution of two generalist predators, Pterostichus melanarius and P. madidus, were sampled contemporaneously. 3. Using spatial analysis by distance indices (SADIE), spatial pattern in the aphid population, predator activity-density, predator hunger and aphid predation was detected and mapped. 4. We tested the hypothesis that carabids and aphids were spatially associated with one another through predation. Aphid predation by individual beetles was detected using enzyme-linked immunosorbent assays (ELISA) and hunger was assessed by measurement of foregut weights. 5. Initially, there was a strong spatial dissociation between aphids and P. melanarius activity-density. While the aphid population was increasing there was a strong spatial association between aphids and both P. melanarius and P. madidus activity-density. During aphid population decline there was no measurable association between aphids and predatory activity-density. 6. Predation of aphids was strongly locally associated with predator activity-density on all sample dates for both predator species, regardless of the association with aphid spatial pattern. 7. Areas within the field most isolated from P. melanarius predation had the highest rates of aphid population increase. 8. Although the proportion of P. melanarius individuals consuming aphids was much lower compared to P. madidus, it was concluded that it was a more effective biological control agent due to its comparative abundance.