The relative effects of natural enemy abundance and alternative prey abundance on aphid predation rates

Several studies show increased generalist predator abundance because of specific landscape features or farm management, but it is not evident that enhanced biological control will always result. High abundance of prey species other than the target pest (alternative prey) may decrease generalist predator predation on the pest via prey substitution. In this paper, I have recorded the removal rate of experimentally added aphids, and abundance of generalist predators and alternative prey in spring barley fields in Sweden. I hypothesized that the aphid removal rate would increase with generalist predator abundance and decrease with alternative prey abundance at given alternative prey- and generalist predator abundances, respectively. The aphid removal rate increased with generalist predator abundance, whereas there was no clear relationship with alternative prey abundance. Because of a positive correlation between generalist predator- and alternative prey abundance, a positive correlation between alternative prey abundance and aphid removal rate was detected when not adjusted for generalist predator abundance. This suggests the effect of generalist predators substituting alternative prey for the pest was small, and other factors may confound the putative relationship between generalist predator abundance and biological control efficacy.