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Why you don't want to find rootworm larvae in the corn field!

Why you don't want to find rootworm larvae in the corn field!
Make sure the risk is worth the reward before skimping on rootworm protection.

You’re wondering if you can skimp on corn rootworm control to save money. If you do,

setting the stage for what comes next are Jesse Grogan, LG Seeds agronomist, Lafayette; Tom Stein, Ceres Solutions, branch manager at Boswell/Templeton; and Greg Kneubuhler, owner of G & K Concepts, Harlan. All three are Indiana Certified Crops Advisers.

Question: I left off the GMO rootworm trait to save money. I’m not applying a soil insecticide either. I use conventional tillage and plant after soybeans. How should I watch for rootworms? When? Remedies?

WOULD YOU STILL DO THIS Greg Bossaer washed roots looking for rootworm larvae when he was an ag educator in White County. He now works for Purdue on-campus. (Photo courtesy John Obermeyer, Purdue University Extension entomologist)

Grogan: One usually learns of potential rootworm pressure by looking for adults in the previous crop year. This should be done each year at silking stages in corn, and in late July through August for soybeans. There are no ‘good’ in-season rescue treatments.

Stein: Scouting for corn rootworm larvae should begin when the larvae are hatched and actively feeding on corn. Generally speaking, 50% larval hatch occurs when soils have accumulated about 700 growing degree day heat units. The warmer the soil is, the earlier the hatch will be. Dig 10 or more roots from different areas of the field.

Grogan: Yes, one can dig plants in June according to hatch date to look for young larvae in suspect areas. But who is going to do that?  

Related: Bt resistant rootworm: Now what?

Stein: If you dig, there are a couple of different methods to determine infestation levels. One is placing roots in a bucket of water and washing away soil. Larvae float to the surface. The other is hand sorting by placing the root ball on a piece of black plastic and breaking away the soil. Count the average larvae per plant. If you find eight or more by washing roots or two or more by hand sorting, consider a rescue treatment.

Kneubuhler: If two or more larvae are found per plant, treatment can be warranted, but efficacy is poor. An insecticide applied at the base of the plant is the only option.

Grogan: Yes. A lay-by insecticide treatment is possible, but few are set up for it. Timing of insecticide application is critical. It needs to be within a seven to 10 day window when larvae are in the juvenile stage. Rescue operations are very limited.

Related: Options to protect from corn rootworm

Stein: There are several insecticides labeled for rescue treatments, but none are very effective. They require either a lay-by cultivator application applied to both sides of the row ahead of the cultivator, assuming you have a cultivator, and assuming field conditions allow it. You can also do a chemigation application if you have irrigation. Either method requires enough moisture to wet the root zone to the depth of the corn rootworm larvae where control is needed.

TAGS: Soybeans
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