Susceptibility of ostrinia nubilalis to bacillus thuringiensis var. kurstaki

Baseline susceptibility of European corn borer larvae, Ostrinia nubilalis (Hübner), to a commercial formulation of Bacillus thuringiensis var. kurstaki was determined for different strains in 7 geographic locations in the corn area of North Italy (Po Valley). For the bioassay 7 different doses, raised on a meredic diet, were used and the data was analysed with the Probit analysis. Differences in the susceptibility among the northern Italian populations of O. nubilalis were recorded, which can be attributed to natural variability. The ECB collected near Como and Torino was the most susceptible, respectively with LC50 of 0.23 and 0.57 mg/ml. The most tolerant ones were the larvae coming from the area of Vicenza (LC50 = 3.40 mg/ml). Baseline susceptibility was also determined for one population originally from Iowa but reared for more than 20 years in the laboratory and which had never experimented Bt. The LC50 calculated for this last one was compared to the Italian situation.