Intraguild predation among biological-control agents: theory and evidence

Theoretical and empirical evidence developed in four subdisciplines of biological control (biocontrol of plant pathogens, weeds, nematodes, and arthropods) is brought to bear upon a shared question: the significance of intraguild predation. Intraguild predation ("IGP") occurs when two species that share a host or prey (and therefore may compete) also engage in a trophic interaction with each other (parasitism or predation). We describe the prevalence of IGP and its role in the population dynamics of biological-control agents and target pests. IGP is a widespread interaction within many, but not all, communities of biological-control agents. IGP appears to be pervasive among communities of control agents associated with nematode or arthropod pests. Common forms of IGP include pathogens that infect both herbivores and parasitoids of the herbivore; facultative hyperparasitoids, which can parsitize either an herbivore or a primary parasitoid of the herbivore; predators that attack herbivores that harbor a developing parasitoid; and predators that attack each other. In contrast, IGP appears to be relatively uncommon among biological-control agents of plant pathogens because trophic interactions are less important than competition or antibiosis. Likewise, biological-control agents of weeds interact primarily through competition alone because host ranges are mostly restricted to plant taxa. Empirically based simulation models and general analytical models of interactions involving arthropod pathogens or facultative hyperparasitoids yield variable and often conflicting predictions for the influence of IGP on the success of biological control. Models for predator-predator interactions, however, consistently predict that IGP disrupts biological control. All the field-documented cases of IGP leading to disruption of biological control stem from studies of predators, including mites, insects, and predatory fishes. IGP between two predators or between a predator and an adult parasitoid does not require mortality of the shared prey/host (i.e., the target pest); thus, IGP can be intense, resulting in high levels of mortality for one or both of the natural enemies, while the total mortality imposed on the target pest population is minimal. For this reason, we hypothesize that IGP by predators is particularly likely to influence the efficacy of biological control. Our ability to develop successful programs of biological control will be enhanced by field studies that address the complexity of trophic interactions occurring in agroecosystems. There is a critical need for additional manipulative experiments conducted in the field that test not only population ecology theory for two-species interactions, but also community ecology theory for multispecies interactions.