Alternative prey impedes the efficacy of a natural enemy of mosquitoes

Adaptive foraging behaviour in the presence of multiple prey types may mediate stability to predator-prey relationships. For biological control agents, the presence of alternative prey may thus reduce ecological impacts towards target organisms, presenting a key challenge to the derivation of agent efficacies. Quantifications of non-target effects are especially important for generalist biocontrol agents in their regulation of pests, vectors and invasive species. We examined the predatory impact of the notonectid Anisops debilis towards larvae of the vector mosquito complex Culex pipiens in the presence of varying densities of alternative daphniid prey. Experimentally, we quantified functional responses of A. debilis towards target mosquito prey under different background daphniid compositions, and also tested for prey switching propensities by the notonectid predator. Increasing background densities of daphniids significantly reduced the predatory impact of notonectids on mosquitoes, characterised by reductions in attack rates, lengthening of handling times and lessening of maximum feeding rates. Furthermore, notonectids displayed a significant selective preference in favour of daphniid prey over mosquitoes, irrespective of prey proportions in aquatic environments. Accordingly, notonectids did not display a prey switching pattern. We thus demonstrate that the presence of alternative prey can dampen predatory impacts of notonectids towards mosquitoes, as compared to more simplistic pairwise systems with singular prey choice. Accordingly, the effects of stabilising mechanisms, such as adaptive foraging, should be further integrated in biocontrol agent assessments.