With the efficacy of Bt corn to combat damage from western corn rootworm eroding, researchers are recommending production changes to control the damaging insect.

Concern about the effectiveness of the Bt trait is becoming clear in Iowa fields. Most recently Corteva Agriscience found a corn rootworm population in Delaware County that was resistant to the Cry34Ab1/Cry35Ab1 protein. The other three Bt proteins/toxins (Cry3.1Ab, mCry3A, and Cry3Bb1) that are effective in managing corn rootworm have been found to have resistant populations also.

Yield losses due to western corn rootworm can reach 20 to 40...