The population dynamics, persistence, and efficacy of Heterorhabditis bacteriophora Poinar (Oswego strain) applied to control the clover root curculio, Sitona hispidulus (F.), were investigated in a Pennsylvania alfalfa (Medicago sativa L.) field. Nematodes established and persisted following application rates of 2.5, 7, and 15 billion infective juveniles per hectare. Significant differences in nematode densities between treatments were not observed beyond 43 d after application, indicating that application rate likely did not affect long-term persistence. In the third field season, plots that received the original 15-billion nematodes per ha treatment were split into clover root curculio-excluded and clover root curculio-present subplots to assess the effect of this insect on nematode persistence. Nematode populations were significantly lower in the curculio-excluded plots by October, suggesting that the nematodes recycled through that host. However, nematode populations were not significantly different by April of the fourth field season. Nematode efficacy throughout the study was inconsistent. In the first field season, emergence of clover root curculio adults was significantly reduced in plots receiving the 15 billion nematode per ha treatment. However, alfalfa taproot ratings for clover root curculio feeding injury indicated that scarring was reduced only in the 7 billion nematodes per ha treatments. No significant reductions in taproot injury were observed in the second field season, but by the third year, significant reductions in root injury were evident in the plots that originally received 7 and 15 billion nematodes per ha.