Corn School: Managing Bt resistant corn rootworm

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Resistance to Bt corn rootworm hybrids has developed in Ontario and growers are asking: what’s the best strategy to control the yield-robbing pest if in-seed technology fails?

Resistance has been confirmed in several fields in Huron, Perth and Durham Counties in Ontario and the problem is likely even more widespread, says Tracey Baute, Ontario Ministry of Agriculture, Food and Rural Affairs field crop entomologist.

On this episode of the Corn School, Baute notes that early Ontario evidence suggests there is resistance to three of the four proteins used in commercial Bt rootworm hybrids and all four will likely lose their ability to effectively control the pest. She notes that this same scenario has played out in U.S. corn growing areas in recent years due to long-term use of Bt rootworm hybrids.

With the possibility of cross resistance to all available Bt rootworm proteins, simply rotating to another rootworm hybrid will not control the Bt resistant pest. Unfortunately, no new Bt proteins are expected to come to market in the future so growers will have to look to other control measures. (Story continues after the video.)

Baute notes that fields with continuous corn production — three years or more  — and that have been repeatedly planted with Bt rootworm hybrids are most at risk. Corn rootworm yield loss is linked to root feeding that occurs before fields exhibit secondary symptoms that typically include goosenecking or lodging.

In the video, Baute outlines the best strategies to mitigate resistance. Crop rotation tops the list. She notes that corn rootworm larvae must feed on corn roots to survive and that rotating to another crop and maintaining diverse rotations is the best choice.

Other control measures include ‘land swapping’ with local producers who do not grow continuous corn. If a field was not planted to corn the previous year there is little to no risk of corn rootworm injury.

If crop rotation is not possible, Baute’s best recommendation is to plant a non-rootworm Bt hybrid with seed treatment or soil insecticide protection.

Biocontrol may also be an option. Baute notes that Cornell University research has produced some impressive results indicating that soil-applied biocontrol nematodes kill corn rootworm larvae and can mitigate Bt resistant populations.

Click here for more Corn School episodes.

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