Bt-resistant Corn Rootworm Poses a New Threat

Dubbed the “billion dollar bug”, corn rootworm (CRW) is now more resistant to corn crops engineered with Bacillus thuringiensis (Bt) genes in several states. The first documented case of CRW resistance to the gene Cry3Bb1, a seed trait that produces insecticidal toxins, was reported in Iowa in 2011. Cry3Bb1-resistant CRW has since appeared in Illinois, Minnesota, Nebraska, and North Dakota. Farmers who plant corn for multiple years without rotating to other crops are at the greatest risk for fostering Bt-resistant CRW. Studies show it only takes four years of corn-on-corn planting for resistance to develop. Seed companies have attempted to limit the possibility of CRW resistance by combining multiple Bt genes within one seed, a process commonly known as “stacking traits”. All stacked corn traits currently available use Cry3Bb1 or mCry3A in combination with a second gene, either Cry34/35Ab1 or eCry3.1Ab.

Bt-resistant CRW popping up in key seed markets poses a challenge for an industry searching for solutions. Depending on regulatory approval, Monsanto plans to release SmartStax Pro which features three modes of action against CRW in either 2019 or 2020. Monsanto’s third CRW defense gene, MON89034, will soon also be offered in Corteva Agriscience™’s line of seeds in hopes of better CRW management. However, controlling CRW resistance can only be achieved when new technologies are coupled with proper cultural control methods. Gro Intelligence provides subscribers with the insight and analytics necessary to stay ahead of agronomic trends across the United States.