Rating roots for corn rootworm injury

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Corn rootworm resistance to Bt proteins and hybrids is a growing concern for Ontario corn producers.

In this video report from Ontario Diagnostic Days 2021, University of Guelph research scientist Jocelyn Smith helps growers understand corn rootworm’s life cycle and how to rate corn roots for rootworm injury. She also discusses how growers can work with researchers to identify and assess resistance to Bt hybrids in their fields.

In southwestern Ontario, rootworms start hatching around June 10. They emerge from over-wintering eggs laid in the soil the previous year by adult beetles. These larvae hatch out in the field and start feeding on tiny roots and root hairs. As the corn develops so do the larvae, which grow to more than one centimetre in length as they feed on longer roots, notes Smith.

After three weeks, the larvae pupate and beetles begin to emerge around the end of July and feed on pollen, silks, and leaves. The beetles then mate and lay eggs in the field. “That’s why crop rotation is the number one way to control corn rootworm because they have to have corn roots to survive,” says Smith. “If you rotate to a non-host crop like soybeans, wheat, or alfalfa the rootworms will die once they hatch the following year.”

“If you can’t rotate and you have corn on corn you may end up with root feeding in the second year,” Smith adds. (Story continues after the video.)

In the video, Smith shares tips on how growers can assess corn roots for rootworm injury using the node injury scale developed at Iowa State University. The 0-to-3 scale rates the level of root pruning with 3 being the highest level of injury.

Smith recommends that growers conduct root ratings around the end of July. She also describes how growers can look for resistance to Bt events. In fields where pyramided Bt seed is planted (hybids with more than one rootworm trait) the injury threshold is 0.5. For hybrids with a single trait the injury threshold would be 1 on the node injury scale.

When growers see feeding on hybrids with rootworm protection traits, Smith recommends they contact her, OMAFRA field crops entomologist Tracey Baute, or their seed dealer to investigate the damage.

Click here to view the complete 2021 Ontario Diagnostic Days series.

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