Last year, research led by Dr. Aaron Gassman at Iowa State University documented the presence of resistance to the protein Cry3Bb1 in western corn rootworm populations. So far, resistance has only been documented in the western corn belt. Most importantly, Dr. Gassman’s research shown a strong correlation between resistance and continuous corn fields that have used corn with Cry3Bb1 for at least 3 consecutive years. In a previous CORN issue (2013-11), we mentioned the risk of resistance in Ohio, and the necessity of remaining vigilant in finding any suspected cases of resistance. Over the next few weeks is the time that growers should dig corn roots and inspect them for rootworm feeding. Dig at least 5 plants in 10 different locations in your field. To determine the level of injury, use the Node Injury Scale—this scale ranges from 1 to 3, were 0.5 is half of a node of roots damaged, 1 is a full node of roots damaged, 2 is 2 full nodes damage, etc. Any rating more than 1.0 in corn fields containing Cry3b1, or any rootworm trait for that matter, might suggest potential resistance (remember, if you are using a blended refuge and find a damage root, use Bt test strips to make sure you have inspected a Bt plant). Based on the pattern in the western corn belt, continuous corn fields should be our first priority. If you suspect any cases of resistance please contact state entomology specialists.
(Source – http://www.farms.com/news/check-corn-roots-for-presence-of-bt-resistance-in-western-corn-rootworm-64657.aspx )