Nymphs of Perillus bioculatus F., a specialist pentatomid predator of the Colorado potato beetle, Leptinotarsa decemlineata (Say), are smaller than comparable instars of their prey. In laboratory experiments, P. bioculatus fed small prey exhibited greater capture success, increased survivorship, and faster development to adulthood than individuals fed large prey. An increase of potato beetle prey stage by one instar lowered capture success and survival by 82 and 80%, respectively. Although multiple P. bioculatus are commonly found feeding on a single prey in the field, laboratory survival rates, and development times did not differ significantly between Perillus bioculatus reared individually and in groups. P. bioculatus presented with two prey sizes most commonly attacked the instar that was easiest to capture and that maximized survival. In the absence of augmentative release programs, P. bioculatus is unable to regulate L. decemlineata populations. At the study site in Yakima, Washington, L. decemlineata completes one to three generations per year in potato fields, never reaching stable age distribution before the end of the growing season. Consequently, small, easily captured prey are present only episodically. Developing P. bioculatus must therefore feed on large prey during much of the growing season, limiting their capacity to grow and survive, which is hypothesized as one reason for their failure to regulate L. decemlineata populations. Typically, invulnerable prey stages stabilize predator–prey dynamics. The evidence presented here suggests that prey refuges can also destabilize predator–prey interactions by limiting the capacity of predators to numerically respond and to regulate prey populations.