Western corn rootworm is a highly adaptable insect, and it was just a matter of time before we saw resistance to Bt traits designed to protect against root damage.
In the Western Corn Belt, growers have noticed many field failures due to heavy rootworm feeding. Most of this research was led by Dr. Aaron Gassmann’s laboratory at Iowa State University. In 2011 they discovered resistance to Cry3Bb1 (which may be present in Yieldgard or Genuity traits). In 2014 they discovered resistance to mCry3A (which may be present in Agrisure traits).
Now, in 2016, they have discovered resistance to Cry34/35Ab1 (which may be present in Herculex or Optimum traits). For a full list of Bt traits see: http://www.msuent.com/assets/pdf/28BtTraitTable2016.pdf
Remember that Bt against rootworm has only been available since 2003, and, in just 13 years, most of our major tools have been compromised. Currently there is only one trait, eCry3.1Ab (present in Duracade traits), without any published reports of resistance.
Luckily for Ohio growers, all these products are still effective against Western rootworms in our state. We have only heard of a few, scattered reports of field failures. Observations from the Western Corn Belt indicate that a lack of rotation greatly increases the risk of Bt resistance.
Any field with corn grown from more than 3 straight years should be inspected for root feeding and proper trait performance. Dig 5 roots in 10 locations and use the 0-3 node injury scale to rate feeding (see this guide by Dr. Chris DiFonzo @ Michigan State University: http://msuent.com/assets/pdf/42CRWRating.pdf).
Now is the perfect time to perform root digs — if you suspect field failures, please contact us at firstname.lastname@example.org email@example.com or contact your extension educator. Also remember that crop rotation remains our single, best tactic to prevent Bt resistance from occurring in Ohio.
If crop rotation is not possible, the next best alternative is to rotate different Bt traits each year or consider soil insecticides which are still quite effective. However, we do not see a need nor a benefit for combining both soil insecticides with Bt in Ohio.