Three Indiana certified crop advisers answer a farmer’s question about rootworm protection in first-year corn. The CCAs are Danny Greene, Greene Ag Consulting, Franklin; Andy Like, agronomist, Amvac, Knox County; and Jeff Nagel, agronomist, Ceres Solutions Cooperative, Lafayette. 

Question: I live in central Indiana. My crop consultant found a few rootworm beetles in one soybean field in late August. I haven’t been applying an insecticide for rootworm or using Bt corn borer protection on first-year corn. He says the few beetles he found weren’t enough to worry about and suggested I stay with my system. It bothers me that he found beetles. What do you think?

Greene: Rootworm beetles are mobile, and finding a few in a soybean field isn’t uncommon and may not be an economic concern. Purdue University Publication E-218 suggests setting up yellow sticky traps in a soybean field to capture rootworm beetles. The bulletin indicates that an average of five western corn rootworm beetles caught per day is the threshold for preventative treatment for the following year.  

Additionally, you could plan to dig some roots this next season to look for rootworm feeding in corn that follows beans. If it bothers you too much, then provide rootworm protection. That can be done through purchase of seed with a seed treatment at rootworm control rate, a soil-applied insecticide or a Bt rootworm trait in the genetics.   

Like: I agree with your consultant. If you haven’t had a problem, I think the risk is minimal. However, if you don’t feel comfortable with his decision, you could use a partial rate of a soil-applied insecticide to ease your fear of root-lodged corn. With the insecticide, you will get the benefit of controlling seed- attacking secondary pests in addition to rootworms. This may help you justify the cost of the application and give you peace of mind during the growing season.

Nagel: There is probably no need to change the system for rootworm in the short term. Overall, rootworm pressure has decreased, even in historically high rootworm pressure areas like west-central Indiana. Lower beetle counts translate to fewer beetles for egg laying. 

Reduction in beetle numbers might be attributed to a few seasons of heavy rainfall at rootworm egg hatch time and the widespread adoption of insect-protecting traits. There has been a shift to more hybrids without rootworm-protecting traits the past couple of years.

Insect pressure tends to ebb and flow. Fewer acres of traited corn and springs where larvae survival is more successful will likely result in a resurgence in beetle numbers. This may take some time, and field-scouting to monitor beetle pressure will be important. No one needs a year like 1995, when first-year corn rootworm caused significant yield losses due to unanticipated pressure.

European corn borer is a different story. Fields planted without Bt traits for ECB should be scouted diligently. Every year we observe fields planted without the Bt trait that experience yield loss from ECB. It doesn’t occur in every field, but it’s never a good day when it’s your field and the problem is found too late to control. If you don’t plant Bt for ECB, understand the life cycle, scout and make a foliar insecticide application when needed. A 4% to 6% yield loss can occur on whorl-stage corn with just one larva per plant.