Alternative prey disrupt biocontrol by a guild of generalist predators

We examined the potential of a guild of generalist predators to control Colorado potato beetles (“CPB,” Leptinotarsa decemlineata) on potato. We were interested in determining whether beetle suppression would change with varying predator density, and with varying background densities of green peach aphids (“GPA,” Myzus persicae), which are common alternative prey for the predators in the field. We conducted two field experiments where we manipulated predator densities and measured the impact of these manipulations on CPB and GPA densities. In the first experiment, with low aphid densities, the predator guild significantly reduced beetle and aphid densities. In the second field experiment, with higher aphid densities, predators did not reduce beetle densities and only slightly depressed aphid increase. In both field experiments, we were unable to maintain elevated predator densities in cages where we added more predators, possibly due to intraguild predation. In laboratory microcosms, we further examined beetle predation by two common predators, Nabis spp. and Geocoris spp., in the presence versus absence of aphids. The two predators responded differently to a choice in prey. Geocoris spp. preyed upon aphids and eggs in rough proportion to the abundance of each, whereas Nabis spp. appeared to switch more readily to feeding on aphids. Overall, in both the field and the laboratory, we found evidence for a positive prey–prey interaction, with the presence of aphids reducing predation of potato beetles.