Virulence of Some Isolates of Entomopathogenic Fungus, Beauveria bassiana on Ostrinia nubilalis (Lepidoptera: Pyralidae) Larvae

The European Corn Borer (ECB) is one of the most important insect pests of corn and some other crops such as rice in Iran. This pest is one of the most important hosts of the entomopathogenic fungus Beauveria bassiana, a well known fungal entomopathogen with high host range and considerable potential in insect pest control. In an isolate selection program of fungal isolates against ECB, ten isolates consisting of eight isolates from Iran and two from other countries were assayed using the dipping method on third instar larvae of ECB. Inoculum concentrations were 104, 105, 106, 107, and 108 conidia ml-1. For each concentration, 30 larvae were dipped into the conidial suspension for 30 secends. Control larvae were treated with distilled water containing 0.03 percent Tween-80 as surfactant. Each experiment was repeated three times. Results showed that BEH isolate which was isolated from the soil of insects living in the field, caused the highest mortality in larvae in comparison with other isolates with a mean of 57.67 percent mortality using 108 conidia ml-1. Other isolates, such as DEBI007 and EVIN I, were scored in the lower position with producing 53.43 and 42.67 percent mortality, respectively. EVIN II, DEBI002, and DEBI008 caused the lowest mortality in assayed larvae. A decrease in larval feeding was detectable a few days before death. Possible causes for the low mortality in isolates are discussed.