Knowledge of how individual organisms behave in their environment can provide a greater understanding of population dynamics. In a predator-prey system, the choices made by predators when foraging for prey are important aspects of behavior. Particularly in the case of a stage-structured prey population, how the predator selects prey stages once a prey patch has been located can have implications for prey population growth, predator development and fecundity, and predator-prey system dynamics. Predaceous mites of the family Phytoseiidae are important biological control agents of Tetranychus spider mites in agricultural settings worldwide. Phytoseiid species range from specialists that require Tetranychus spider mite prey in order to develop and reproduce to generalist omnivores. In studies with 13 phytoseiid species, specialized species tended to prefer T. urticae eggs as prey or have no prey-stage preference while more generalist species tended to have no prey-stage preference or prey more often on mobile immatures. Further testing with a subset of these species suggested variability among species with regard to genetic and environmental influences on prey-stage preference. The specialist Phytoseiulus persimilis also preferred to forage and oviposit in patches containing egg-biased stage distributions rather than in adjacent mobile immature-biased patches. No benefits to offspring developing in either type of patch were found in terms of developmental time or subsequent adult fecundity. However, the results of both manipulative experiments and nonlinear population models indicated potential benefits of egg-biased predation in terms of current adult female fecundity, a less severe impact of predation on the prey population, and an increased number of predator descendents during the predator-prey interaction. Considering both subjective factors and discriminant analyses, prey-stage preference performed well as an indicator for the ecological classification of phytoseiid species. The classifications of four phytoseiid species occurring on apple in central and eastern Oregon, USA, were evaluated accordingly. / Graduation date: 2004