The Colorado potato beetle (CPB) is a serious pest of potatoes worldwide. Insecticides are the primary control method against CPB even though the insects are increasingly resistant to many of the most commonly used chemicals. In Washington, organic potato acreage is increasing with few cost-effective CPB control options, while conventional growers rely on broad-spectrum insecticides that may be taken off the market due to changing federal regulations. Entomopathogenic nematodes have shown potential as biological control agents for CPB. We have been examining the diversity and density of entomopathogens in conventional and organic potato fields in the Columbia Basin of Washington. Fields of three types were sampled: organic following a winter cover crop, conventional, and conventional following a cover crop. We hypothesized that organic fields and fields following cover crops would have a higher diversity and density of entomopathogens. The greater wax moth, Galleria mellonella, is highly susceptible to entomopathogens and was used as sentinel prey in field bioassays. Groups of five larvae were placed in mesh sacks (10 sacks/field) and buried at a depth of 10-cm for 48 hrs. Laboratory bioassays used Petri dishes filled with soil from each field (10 dishes/field), with larvae and prepupae, the most susceptible life stages of CPB, placed in these dishes for 48 hrs. The larvae and prepupae from both bioassays were examined for mortality and infection by entomopathogens. The response variables were the number of larvae and prepupae dead, the number with nematode emergence, and the overall taxonomic diversity of entomopathogenic nematodes.