Polyphagy complicates conservation biological control that targets generalist predators

1. While evidence suggests that undisturbed refuges within agricultural fields conserve natural enemies, few studies have examined whether pest control does actually improve following predator conservation. When the targets of conservation are generalists, polyphagy may complicate the impact of the conserved predators on agricultural pests. 2. We examined the use of beetle banks to conserve predatory beetles for the control of pest Diptera. Locally, the community of predatory beetles included several species of small carabid and staphylinid beetles (< 1 cm in length), which eat fly eggs, and one common larger carabid beetle (> 1.5 cm), Pterostichus melanarius, which rarely eats fly eggs but does eat smaller beetles. 3. Predator beetle activity densities, but not rates of fly egg predation, increased in fields including beetle banks. A series of field experiments was conducted to examine whether two types of polyphagy, intraguild predation and feeding on non-target prey, could be preventing increased egg predation following successful predator conservation. 4. The putative intraguild predator Pterostichus melanarius reduced activity densities of smaller beetles, and thus weakened fly egg predation. The strength of fly suppression increased with increasing densities of small beetles in the absence of Pterostichus melanarius, but not when aphid alternative prey were readily available. In the presence of abundant aphids, egg predation rates did not increase at higher small beetle densities. 5. Synthesis and applications. Overall, our results suggest that both intraguild predation and the presence of alternative prey could limit conservation biological control that targets generalist predators. Thus, higher predator densities will not necessarily lead to improved pest control. Ecologists must consider the impact of predator manipulations at multiple trophic levels when assessing the success or failure of conservation biological control.