Does a natural enemy limit the diet breadth of a generalist predator?

Abstract

Abstract This dissertation explores the ways in which predators respond to novel prey, and in particular, how interactions among predators shape the habitat and prey use of predatory insects. Although predator habitat and prey use are commonly described in terms of responses to prey quality and density, interactions among predators may also be very important. In many systems, predators consume not only herbivorous prey, but also predators within their feeding guild. This interaction, termed intraguild predation, can alter the habitat use of predators that also act as prey, and thus limit access to some prey. Using a system in which a predator is simultaneously confronted with a novel prey and an aggressive intraguild predator, I consider how these forces work together to shape predator prey use. Aphid-feeding lady beetles have been extensively studied due to their importance in suppression of agricultural pests. Moving from habitat to habitat over the course of a growing season, they prey upon diverse aphid (and other) species. Although many species seem to aggregate in the areas with the densest aphid populations, others deviate from this expectation, particularly in responding to a novel resource. For example, the predatory lady beetle Coleogilla maculata, native to the Americas, seldom feeds on the soybean aphid, established in the Midwestern United States in 2001. To better understand the factors limiting the prey use of the native predator, I conducted a series of field experiments in maize and soybean, as well as laboratory predation trials, emphasizing the potential role of intraguild predation in limiting prey use by the native. This work establishes that use of soybean habitats by the native coccinellid is very limited, particularly with regard to laying eggs, and that contact between the native species and an aggressive intraguild predator, Harmonia axyridis, is extensive. However, studies of field patterns in survivorship and predation on sentinel eggs suggest that predation by the exotic does not exclude the native from soybean habitats. Lastly, I examine the potential for cannibalism to explain the coexistence between these two species.