The primary objective was to identify potential nematode and fungus pathogens for control of plum curculio (Conotrachelus nenuphar) larvae. Initial bioassays were conducted in the laboratory. Seventeen isolates (13 wild-types and four color mutants) of the entomopathogenic fungus Metarhizium anisopliae var. anisopliae were screened. Seven of the isolates were highly virulent for plum curculio larvae (LT50 = 4.0–5.4 days at a concentration of 107 conidia ml−1). Of these isolates, four were color mutants induced by UV-B irradiation exposure (DWR 180, 145, 142, and 62), and three of these showed significantly higher virulence than their parental wild-types (ARSEF 2575 and 23). The most virulent wild-type was the isolate ARSEF 1187. Surprisingly, higher virulence was not correlated with higher conidial adhesion to the cuticle of plum curculio larvae. A Utah-collected isolate of Heterorhabditis bacteriophora was as effective as a commercial strain of Steinernema feltiae in killing plum curculio larvae in laboratory bioassays. Larval mortality ranged from 21 to 89% at inoculum concentrations of 0.125 to 4.0 × 106 infective juveniles (IJs) per m−2. A diapausing (northern) population of plum curculio was less susceptible to S. feltiae (LC50 = 8.6 × 105 IJs m−2) than a non-diapausing (southern) population (LC50 = 3.6 × 105 IJs m−2). A time delay in adding plum curculio larvae to sterilized soil treated with S. feltiae significantly reduced insect mortality after 2 days and reduced mortality below 50% after a delay of 7 days. In field tests, S. feltiae killed 22–39% of northern plum curculio larvae at concentrations of 0.5–2.0 × 106 IJs m−2.