| The western corn rootworm Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a major soil-borne pest of corn Zea mays L. in both the United States and Europe, where larval feeding on corn roots can cause severe economic losses. Two factors that can affect survival of larval D. v. virgifera are natural populations of soil-dwelling entomopathogens and transgenic corn, Zea mays L., expressing genes from Bacillus thuringiensis Berliner (Bt). During the fall of 2008 and 2009, we measured incidence of entomopathogenic fungi and nematodes in corn fields that displayed a high abundance of D. v. virgifera by exposing three insect species: D. v. virgifera , Tenebrio molitor L. (Coleoptera: Tenebrionidae) and Galleria mellonella L. (Lepidoptera: Pyralidae) to field soil. Entomopathogenic activity of select field isolated nematode strains and fungal isolates was tested in follow up laboratory bioassays. We conducted laboratory, greenhouse and field experiments to examine the effects of Bt corn (producing Cry34Ab1/35Ab1) and isoline (non-Bt) corn on behavior, survival, and development of larval D. v. virgifera. We also tested effects of soil entomopathogens on D. v. virgifera development in the presence of Bt and isoline corn. We report a high prevalence of naturally occurring entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae in corn fields and nematodes were present, but at lower abundance than fungi. Of the entomopathogens observed, M. anisopliae appears to have the greatest potential to control D. v. virgifera in temperate cornfields, as follow up bioassays showed that select field isolates were more efficacious against D. v. virgifera larvae than a standard commercial isolate. Results from laboratory feeding experiments indicated that larvae prefer to feed on isoline corn more than Bt corn. In a greenhouse experiment, larval survival at 17 days did not differ significantly among isoline corn, Bt corn and a blend of the two. However, larval development was hindered on Bt corn compared to either isoline or a blend. Additionally, later instar larvae on isoline corn had lower survival at high concentrations of entomopathogenic nematodes. In a field experiment that tested larval survival and movement, the number of larvae recovered was higher from isoline than from Bt corn, and recovery decreased as distance from the infested plant increased. These data suggest that Bt corn influences larval movement and development, which can influence the efficacy of entomopathogens against D. v. virgifera larvae. |
