The Role of Alternative Prey in Sustaining Predator Populations

Generalist predators are widely acknowledged to contribute valuable levels of biological control in agroecosystems throughout the world. Although their feeding habits can result in the rejection of target pests in favor of preferred and often more nutritious non-pest prey, these natural enemies are capable of colonizing habitats prior to the arrival of pests by subsisting on alternative sources of food. The effect of consuming non-pest species on rates of pest predation by a generalist predator can be twofold; feeding upon these nutritious food items generally enhances fecundity thus improving their population growth, but the presence of alternative prey, especially during times when pest regulation is required, can result in reduced levels of pest consumption per individual predator. However, an increased density of natural enemies can counteract this reduction in pest consumption and exert significant levels of biological control. The role of alternative prey in sustaining predator populations has been widely reported in laboratory studies and field trials examining the fecundity, feeding behavior and growth rates of species subjected to diets of varying quality. Recently, the application of monoclonal antibody and molecular technology to study predation rates in the field has revealed the extent to which many predator communities rely on alternative prey before, during and after the immigration of pests into crops. In this study we examine the role of key species of alternative prey to generalist predators and discuss their impact in the context of biological control. The importance of these prey items to sustaining linyphiid spider and coccinellid communities will also be examined. Microsite sampling of arthropod populations in alfalfa indicated that the overlap in availability of pests (Acyrthosiphon pisum and Empoasca fabae) and alternative prey to linyphiid spiders is likely to reduce the ability of these generalist predators to restrict the growth of pest populations.