An intra-guild predator simultaneously feeds upon herbivores and other predators of those herbivores. In the ponderosa pine forests of Colorado, canopy foraging birds feed upon the aphid Essigella fusca, but they also benefit the aphid by feeding upon this herbivore’s arthropod predators. Because of these simultaneous positive and negative effects, food web theory has predicted that intra-guild predators should be poor agents of biological control. I predicted that the ability of birds to control E. fusca populations would depend, in part, on the identity of the canopy arthropod predator community. To test this hypothesis I conducted a multi-year bird exclusion experiment on pine trees with and without the predatory ant Formica podzolica. On trees without ants, bird exclusion led to a mean increase in E. fusca abundance of 360% (P<.01), despite a simultaneous increase in arthropod predators including coccinelid and neuropteran larvae (P<.01). On trees with ants, bird exclusion led to a mean decrease in E. fusca abundance of 10% (P>.05), while arthropod predators, including ants, again increased (P<.01). These results suggest that in systems with intra-guild predation, whether or not the intermediate predator community includes effective and abundant predators such as ants plays a central role in determining when top predators can serve as effective agents of biological control.
Species 1: Homoptera Aphididae Essigella fusca
Species 2: Hymenoptera Formicidae Formica podzolica
Keywords: intraguild predation, biological control
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