A cold, wet winter and late corn planting in some Northern states could spell trouble for many cornfields, especially those planted with single-trait Bt corn year after year Of the four Bt proteins registered in the U.S. for corn — Cry3Bb1, Cry34/35Ab1, mCry3A and eCry3.1Ab — field-evolved resistance has been documented for both Cry3Bb1 and mCry3A traits, and cross-resistance has also been confirmed between those two traits. Cry3Bb1 is the protein found in YieldGard VT3 and YieldGard VT3 Pro hybrids, while mCry3A is the protein used in some Agrisure hybrids. Five Illinois counties have reported field-evolved WCR resistance to Cry3Bb1 corn, and researchers in Nebraska, Minnesota, Wisconsin and Iowa have also reported WCR resistance issues. During a recent webinar, several university experts shared tips on how farmers can improve long-term management of rootworms and keep resistance at bay. Evolution Of Resistance. Diversified techniques are needed to manage corn rootworms effectively while reducing the risk of resistance developing. But that’s a difficult task due to the strong adaptation ability of WCRs, says Iowa State University entomologist Aaron Gassmann. Gassmann says his laboratory began studying Bt resistance in corn rootworms in 2009 after getting complaints from farmers about fields with severe larval feed injury to Bt corn. His research group began by studying the effectiveness of Cry3Bb1 (YieldGard VT3 and VT3Pro); Cry34/35Ab1 (Herculex); and mCry3A (Agrisure). Researchers visited problem fields and sampled WCR adults, bringing them back to the lab to obtain eggs and use bioassays to check survival on Bt and…