ear of corn with hole caused by corn borer larva
PLAN AHEAD: Whether or not to use Bt corn borer protection in 2018 may be an economic decision. If you opt not to use it, do you have a backup plan? This hole was caused by a corn borer larva.

Which method of insect control will you choose?

Corn Illustrated: If you elect to go without certain GMO traits next year, you need a plan.

Usually you have a choice. You can choose hybrids with Bt protection for one or more insects, or you can pass on that protection to save on seed costs. How much you save may depend on which insect protection you’re talking about, and how much of a threat the pest is in your area.

Some opted to skip Bt protection for corn rootworm and belowground pests in 2017.  Depending on the pressure where they farm, they might have not seen much impact from corn rootworms. Others chose to even forego corn borer protection by not using a Bt trait that controls European corn borer, an aboveground pest. Dave Nanda, a crops consultant based in Indianapolis, notes that just because a hybrid doesn’t have Bt protection for aboveground or belowground insects, or both, doesn’t mean it’s a non-GMO hybrid. It could have resistance to glyphosate, for example, which still makes it a GMO hybrid.

Nanda scouted a field this year where the farmer planted a hybrid that was selected so late, it wasn’t available any longer with Bt traits. It did have glyphosate resistance. It was planted in an alternating pattern across the field with a hybrid from a different company that did have resistance for European corn borer.

Scout and see
“We definitely saw a difference,” Nanda says. It was evident early in the season, after the first-generation corn borer larvae hatched and fed on corn. Since they often feed on leaves while they’re still in the whorl, when the leaves unroll, you typically see a shot-hole pattern where the larvae ate their way through the leaves.

“We saw more than 20% of the plants with shot-hole feeding in the hybrid without protection versus only about 3% in the hybrid with the trait,” Nanda says.

Why would there be any feeding in the hybrid with the trait that protects against European corn borer? Nanda says there are a couple of possibilities. First, refuge was included in the bag with the Bt hybrid. That means in an effort to control insect resistance from developing, 5% of the plants didn’t have Bt protection. Even with plants with Bt protection, larvae must take a bite of a leaf to ingest the protein contained in Bt corn leaves that kills the insects.

Although corn borer showed up in this field without Bt protection, another report from a non-GMO grower indicates he didn’t have issues with corn borer this year. Entomologists have noted for years that corn borer can be a sporadic pest that shows up some years in some fields, and not in other years or not in all fields. Moths fly in the spring, and weather can impact moth flights.

However, even the non-GMO grower who didn’t see damage this year says he has a detailed scouting plan, and is ready to apply insecticides if necessary. 

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