corn rootworm larva on fingertip John Obermeyer, Purdue University
UNWELCOME SIGHT: Here is what you don’t want to see. This young corn rootworm larva was unearthed during root digging and washing.

Consider rootworm scouting options carefully

Corn Pest Beat: Assuming corn rootworms are history could be a costly mistake.

We used GMO protection for corn borer but not rootworm this year. My agronomist is bugging me to let his scouts check for rootworm larvae in each field. This is the first time we haven’t used GMO rootworm protection in several years. He wants $3 per acre to do it. Should I let him?

This month’s panel of Indiana certified crop advisers includes: Don Burgess, agronomist, A&L Great Lakes Labs, Fort Wayne; Jesse Grogan, corn product development manager for AgReliant Genetics, Lafayette; and Bryan Overstreet, Jasper County Extension ag educator.  

Burgess: Very few options are available for controlling rootworm larvae in an existing cornfield, and the effectiveness of those products can be quite variable. An individual grower’s ability and willingness to make an in-season application should be considered before deciding if it makes economic sense to hire a consultant to scout for rootworm larvae in a field in order to make an insecticide application.

However, this may provide an opportunity to evaluate the need and effectiveness of the GMO package used previously. If high rootworm populations exist in the nonprotected crop, it suggests that a protected crop should be grown in that location, and that the package used in prior crops has been effective at reducing rootworm pressures.

Scouting for adults in late summer is generally more effective, and a less labor-intensive process. By scouting for adults in fields where corn will be planted to corn the following spring, hybrids can be selected that provide rootworm protection, or insecticide applications can be made at planting to protect the crop from a potential larvae infestation.

Grogan: A scouting program should be consistent and repeated on an annual basis until a corn rootworm control method is utilized in areas where CRW has been a problem in past years. CRW pressure has been low in Indiana for several years. While this is a good thing, it will likely not be true when we least expect it. One cannot assume that they have disappeared. Heavy populations and damage occur in states to the west like northern and central Illinois, northern Iowa, southern Minnesota and Nebraska.

This insect has amazing survival instincts and dispersal habits. The rate of $3 per acre for scouting is a good rate compared to a $50-per-acre CRW control cost for chemicals or biotech traits. Coarse, sandy soils, mucks, and tight, poorly drained clays are usually suppressive to CRW. Well-drained silty clay loams and loams are among the favored soils. CRW will always be a threat, and we should be vigilant to protect against it.

Overstreet: Last year rootworm numbers were down from the past due to several reasons. including weather from the past few years featuring floods and drought. That said, I still think it would be a wise investment to have the agronomist scout the field if you’re going without any protection. Depending on what part of the state you’re located in, rootworm pressure could be traditionally much higher. I’m located in northwest Indiana, and most years in the past, some type of control was needed on corn after corn. With the new variant we have seen in the past 20-plus years, it was needed even after soybeans. 

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