Polyphagy complicates conservation biological control that targets generalist predators.
The use of beetle banks to conserve predatory beetles for the control of pest Diptera was studied in Washington, USA. 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 eats smaller beetles. Predator beetle activity densities, but not rates of fly egg predation, increased in fields including beetle banks. A series of field experiments was conducted in vegetable (including radish and broccoli) farms during 2002-04 to examine whether the 2 types of polyphagy, intraguild predation and feeding on non-target prey, could be preventing increased egg predation following successful predator conservation. The putative intraguild predator Pterostichus melanarius reduced the activity and densities of smaller beetles (Bradycellus congener, Bembidion tetracolum, Bembidion lampros, Amara spp., Aleochara bilineata, Philonthus politus and an Aleocharine morphospecies) and, thus, weakened fly (Musca domestica) egg predation. The strength of fly suppression increased with increasing densities of small beetles in the absence of Pterostichus melanarius, but not when aphid (Myzus persicae) alternative prey were readily available. In the presence of abundant aphids, egg predation rates did not increase at higher small beetle densities. Overall, the results suggested 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.