Generalist predators may play an especially important role in the biological control of crop pests in annual agroecosystems. Nevertheless, little attention has been focused on the ecological functions of generalist predators, and in particular there have been few studies investigating predator-predator interactions under field conditions. Of the predators common in mid- and late-season cotton fields in California's San Joaquin Valley (Orius tristicolor, Geocoris, Nabis, Zelus renardii, crab spiders, and lacewings), lacewing larvae are the most effective in suppressing aphid populations when they are the only predators present. However, natural populations of immature stages of lacewings are comprised almost entirely of eggs; most fields have very low densities of larvae. Observational and experimental evidence suggests that this is a result of intense predation on lacewing larvae by other generalist predators (especially Nabis and Zelus renardii). In small cage trials, successful biological control generated by lacewing larvae alone broke down, resulting in rapid aphid population growth, when Zelus renardii was added. The ecological role of generalist predators in agroecosystems may be more varied and more complex than is recognized by existing biological control theory.