In the battle between biotech corn and the western corn rootworm, the rootworm is starting to win.

Research by Iowa State University assistant professor of entomology Aaron Gassmann indicates that rootworm populations in Iowa, Nebraska, Illinois and Minnesota are exhibiting resistance to Bacillus thuringiensis. Bt, as it is commonly known, is a toxin-producing protein that is genetically engineered into corn seed. When rootworms consume parts of corn plants containing the Bt traits, they die.

Or, in a growing number of cases, they don’t.

Gassmann found that farmers in northeast Iowa who planted Bt corn began noticing rootworm damage to their corn crops as early as 2009. He identified Bt-resistant rootworms in four northeast Iowa fields that year, and in nine to 25 fields he visited from 2010 to 2013.

Adult western corn rootworm beetles

The Iowa State researcher said it’s difficult to determine the extent of the rootworm resistance problem but urged growers not to accelerate resistance by overusing Bt corn seed.

Bt corn is “a convenient and safe technology that should remain part of an integrated pest management system,” Gassmann said. “But the future of the technology depends on how farmers use it.”

Gassmann recommended farmers employ the following practices if they plant Bt corn:

• Rotate crops. Planting Bt corn in the same fields year after year speeds up pest resistance. Planting soybeans, non-Bt corn or another crop in those fields every other season or so interrupts the lifecycle of rootworms and makes it harder for the pests to develop immunity to Bt proteins.
• Plant non-Bt “refuge” corn in sections of fields planted to Bt seed. Refuges keep alive resistance-prone rootworms, so that they can continue to mate with rootworms more susceptible to Bt proteins and produce offspring less able to withstand the toxic corn plants.
• Switch to corn with different Bt proteins than Bt seed that has performed poorly in the past.
• Use corn seed containing multiple Bt traits – a method called “pyramiding.”
• Apply insecticides for a growing season or two as a remediation measure if crop rotation and pyramiding are not options.

Establishing a long-term integrated plan for controlling rootworm is important, Gassmann said. An integrated approach should include field scouting, crop rotation, rotation of Bt proteins and applying soil insecticides at planting with non-Bt seed, he said.

Gassmann’s rootworm resistance research was published in a recent issue of the Proceedings of the National Academy of Science .

By Steve Leer

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