none Little attention has been given to indirect nontarget effects of classical biological control, those effects that do not involve control agents directly attacking nontarget species. We combined experiments, theory, and historical information to show that the decline of a native parasitoid, Praon pequodorum Viereck (Braconidae: Hymenoptera), in alfalfa cropping systems may be caused by competition with Aphidius ervi Haliday (Braconidae: Hymenoptera), an exotic parasitoid introduced as a biological control agent of pea aphids. We conducted studies to examine competition for hosts at the population and plant levels, compare parasitoid searching behaviors, and model the interactions between two parasitoids on their single host. At the population level, P. pequodorum achieved relatively higher parasitism than A. ervi when the overall level of parasitism (and hence competition) was higher; this is explained by P. pequodorum's superior within-host competitive ability. A negative correlation between A. ervi and P. pequodorum parasitism per plant suggested strong competition by larvae within hosts or behavioral avoidance of previously parasitized hosts by P. pequodorum. Comparison of searching behavior revealed that A. ervi is a superior among-host competitor, searching longer on a plant after an aphid is encountered, moving more rapidly within plants, and attacking and parasitizing more aphids per unit time than P. pequodorum. Using a simple model, we show that high searching efficiency such as that exhibited by A. ervi could lead it to extirpate a superior larval competitor such as P. pequodorum. This competitive exclusion is facilitated by severe or frequent disturbances (such as regular harvesting of alfalfa) that cause mortality of both hosts and parasitoids. Disturbances place a premium on searching efficiency that allows rapid parasitoid population growth. These findings suggest that indirect nontarget effects of biological control may depend on agricultural practices and the consequent disturbance regime of human-dominated systems. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Competition; Land and freshwater zones; Nearctic region; North America; USA Aphidius ervi (Aphidiidae); Interspecific competition; Indigenous hymenopteran parasite species on hemipteran host; Wisconsin Praon pequodorum (Aphidiidae); Hemipteran hosts; Acyrthosiphon pisum; Competition with introduced parasite species; Interspecific competition; Wisconsin; Arlington; Competition with introduced parasite on hemipteran host Acyrthosiphon pisum (Aphidoidea); Hymenopteran parasites; Praon pequodorum; Competition with introduced parasite species; Wisconsin; Arlington; Hymenopteran parasite competition with introduced parasite