We investigated the ability of entomopathogenic nematodes to infect diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) on a leaf surface. In a leaf disk assay, mortality of late stage DBM larvae ranged from <7% caused by Steinernema kushidai Mamiya to >95% caused by S. carpocapsae (Weiser) All strain. LC50 values for S. carpocapsae, S. riobravis Cabanillas, Poinar & Raulston, and Heterorhabditis bacteriophora Poinar NC1 strain were 14.6, 15.4, and 65.4 nematodes/larva, respectively. S. carpocapsae, S. riobravis, and H. bacteriophora caused 29%, 33%, and 14% mortality of DBM pupae, respectively. DBM mortality caused by S. carpocapsae on radish declined at low (<76%) to moderate (76–90%) RH, because nematode survival and infectivity declined at low (76%) to moderate (76–90%) RH. However, DBM mortality caused by S. riobravis did not decline with RH. S. riobravis survival declined with RH, but infectivity did not. Overall, nematode survival and infectivity to DBM larvae were lower for S. riobravis than for S. carpocapsae. In addition, DBM mortality was higher on radish plants (pubescent leaves) than on cabbage plants (glaborous leaves).