Western Corn Rootworm Resistance to Final Bt Trait Confirmed

November 7, 2018

Corteva Agriscience recently confirmed resistance to the Cry34/35Ab1 Bt protein in Delaware County. 

Three of four Bt traits targeting corn rootworm (Cry3Bb1/Yieldguard rootworm; mCry3A/Agrisure RW; and eCry3.1Ab + mCry3A/Agrisure Duracade) are not reliable for rootworm control due to widespread resistance. Due to a reduction in effectiveness of the Cry3 proteins, farmers have come to rely upon Cry34/35Ab1 for rootworm protection, placing significant selection pressure on this trait.

Cry 34/35Ab1 is found in Herculex RW, SmartStax, Agrisure 3122, Intrasect Xtra, Intrasect Xtreme, QROME, and certain AcreMax hybrids including AcreMax Xtra and AcreMax Xtreme. While the Cry34/35Ab1 protein is still effective in managing corn rootworm in most areas, the importance of stewardship of this trait is clear. 

The best way to preserve the effectiveness of this crucial technology is by diversifying management practices. Options to consider include rotating to a non-host crop (e.g. soybean) at least every 5 years, use of hybrids with pyramided Bt traits, and rotation with non-Bt hybrids along with soil-applied insecticides. For recommendations regarding rootworm management, please see “Approaches for Managing Corn Rootworm in Iowa” by ISU Entomologists Erin Hodgson and Aaron Gassmann.

Farmers may have heard of a new technology—RNAi—that will be available for rootworm control in the future. However, stewardship of Bt proteins is crucial even after the RNAi technology is expected to become available early next decade, as full efficacy of RNAi only comes when it is paired with an effective Bt protein.

Western corn rootworm is one of four pests targeted by the Iowa Pest Resistance Management Program. Development of a pilot project to address Bt resistance in eastern Iowa is currently underway.

Evan Sivesind is the program manager for the Iowa Pest Resistance Management Program (IPRMP). Please contact him by email or phone (515-294-7990) if you would like to join the effort or learn more.