The effects of predator presence on insect behavior: Consequences for individual fitness, population growth and pest suppression in agroecosystems

Organisms respond to the presence of foraging predators with a variety of antipredator strategies including escape responses, microhabitat shifts, and the restriction of conspicuous activities such as movement, foraging, and reproduction. Although these effects have been well-documented in aquatic systems, relatively little information is available about their impacts in terrestrial systems. In addition, because conceptual models of predator-prey interactions form the basis of our understanding of pest suppression in agroecosystems, information regarding the impact of predator presence on economically important pest insects may help us to better understand the qualities embodied by successful biological control agents. In this two-part study, I examined the impacts of predator presence on the feeding activity of insects in garden test systems. In the first part, a series of experiments paired pest insects with single, spider species to measure the relative contributions of direct mortality and predator-induced reductions of feeding activity to reductions in crop damage. Direct mortality accounted for approximately 15% of the observed reduction in crop damage while predator-presence effects accounted for an additional 10--40% depending on the system. In the second part of this study, I present a detailed examination of a single system in which predator-presence effects proved to be important. An individual-based computer simulation of the fifth larval instar of the small, white cabbage butterfly, Pieris rapae L. was developed in conjunction with Dr. Gary Huxel of the University of California at Davis. This simulation incorporated experimentally-derived estimates of feeding activity and growth under predation pressure. The simulation was run at three different predator densities to evaluate the impact of predator-presence effects on population growth. At high densities, predators significantly lengthen the maturation time for fifth-instar P. rapae larvae resulting in probable losses of fitness and slowed population growth. These effects are discussed within the context of their ecological and evolutionary significance, as well as their implications for the biological control of insect pests in agroecosystems.