Source: Ohio State University Extension
By Andy Michel, Entomology Specialist
The good news is all this rain has likely caused substantial mortality in corn rootworm larvae. However, growers should still be mindful of our most important corn insect.
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 1-3, 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. For more help in rating roots, see this handout from Michigan State.
Economic damage occurs from anywhere starting at 0.25 and above, depending on weather conditions — usually any ratings above 0.75 indicates substantial damage. See this fact sheet for more information.
Growers should also keep in mind the risk of Bt resistance. Although we haven’t had any official cases of Bt resistance, observations from the Western corn belt suggest fields with corn for more than 3 straight years is at high risk for developing resistance. 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 for more than 3 years should be our first priority. If you suspect any cases of resistance, please contact me, the state entomology specialist at firstname.lastname@example.org, or your local extension educator.