Susceptibility of wild northern corn rootworm, Diabrotica barberi Smith & Lawrence, to Cry3Bb1 protein in diet toxicity assays

The northern corn rootworm (NCR), Diabrotica barberi, is an important pest of maize in the U.S. Corn Belt. Recently, resistance to Bacillus thuringiensis (Bt) corn was reported in North Dakota, and is probably evolving in other states. In this research, susceptibility of wild NCR populations from six fields collected in five different states (Minnesota, Missouri, Nebraska, Iowa, and North Dakota) plus a susceptible non-diapause NCR strain to Cry3Bb1 protein, was evaluated in diet overlay toxicity assays. The assays were conducted using 96-well plates containing 200 µl of artificial diet. Three concentrations of Cry3Bb1 protein (0.4; 4.0; and 40.0µg/cm²) plus buffer as control, were used. Five replicates were used in each assay (except Iowa-2019, three reps). Mortality and number of larvae that reached 2^nd instar were recorded after 10 days. Surviving larvae from each concentration were harvested and dried in the oven to estimate dry weight. At the highest Cry3Bb1 concentration (40.0 µg/cm²), the Minnesota population had the lowest mortality (14%), the highest percent 2^nd instar (81%), and the largest dry weight (4-fold higher than susceptible) compared to any colony. Similarly, the Minnesota population had the lowest mortality (6%) and the highest percent 2^nd instar (94%) compared to other strains at 4.0 µg/cm² of Cry3Bb1. NCR populations from North Dakota and Missouri along with susceptible strain are still susceptible to Cry3Bb1 protein. More studies such as field assays should be conducted to confirm if NCR are becoming more tolerant to Bt protein