Generalist biocontrol agents feed not only on pests, but also on other prey species. The presence of alternative prey might improve biological control by providing additional prey and thus bolstering predator densities, or disrupt biological control by distracting predators from attacking the target herbivore. Working in Washington state potato fields, we have been investigating the impact of alternative prey on control of the Colorado potato beetle (Leptinotarsa decemlineata) by a guild of generalist predators. In short-duration field and laboratory experiments, we have found that the impact of predators on potato beetles is disrupted when aphid alternative prey are abundant, apparently because predators prefer to feed on the aphids. However, over the longer term aphids might attract and sustain more predators than would otherwise be present, eventually improving potato beetle control. In a large-scale field experiment we are separately manipulating densities of two alternative prey taxa, aphids and detritivores, and measuring the impact of these manipulations on season-long suppression of potato beetles by the predator guild.