Survey of entomopathogens attacking larval western corn rootworm (Diabrotica virgifera virgifera) in Iowa cornfields

Natural populations of entomopathogenic fungi and nematodes are ecologically important pathogens of soil-borne pests such as the western corn rootworm Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae). Feeding by larval D. v. virgifera on the roots of corn Zea mays L. can cause severe economic losses in both the United States and Europe. We surveyed corn fields in Iowa, USA for naturally occurring populations of entomopathogenic nematodes and fungi. During early October 2008, we sampled five cornfields throughout Iowa that had been heavily infested by naturally occurring populations of rootworm (Diabrotica spp.) during the summer. Entomopathogens were recovered by baiting with three insect species: D. v. virgifera, Tenebrio molitor L. (Coleoptera: Tenebrionidae) and Galleria mellonella L. (Lepidoptera: Pyralidae). Significant differences were found in mortality of the three baiting species, with G. mellonella and T. molitor displaying higher mortality than D. v. virgifera. More D. v. virgifera were killed by nematodes than by fungi, suggesting that nematodes are the more lethal pathogen to this soil-borne pest. We report a high prevalence of both fungi (Beauveria bassiana and Metarhizium anisopliae) and nematodes (Steinernema spp. and Heterorhabditis spp.) in cornfields, but only a subset of these pathogens appear to kill D. v. virgifera.