Cornfield pests are becoming resistant to transgenic corn. The deliberately modified genes that were once able to protect corn from being consumed by insects are no longer effective in protecting corn.

Aaron Gassmann, an ISU assistant professor of entomology, has been researching the Western corn rootworm that has been found to be resistant to current Bt toxins.

A Bt toxin is a protein that is toxic to chewing insects in the soil.

“The Western corn rootworm is the number one pest of corn in Iowa and much of the Midwest,” Gassmann said.

More than 75 percent of the corn in Iowa contains at least one Bt toxin, Gassmann said.

The Western corn rootworm feeds on the roots of corn, killing the corn and ultimately reducing the yield of corn. Gassmann said it makes corn plants more prone to falling over.

Erin Hodgson, an associate professor of entomology, said that Gassmann has confirmed resistance to two kinds of Bt toxins, mCryBb1 and mCry3A.

Hodgson said rootworms have developed cross-resistance to these toxins, meaning if they have resistance to one, it is likely they are resistant to the other.

Hodgson said the difference between the two Bt toxins is a small difference between the proteins produced by the toxins. Both kinds have proteins that bind to the mid-gut of the rootworm, causing it to explode. The bacteria that were naturally found in the mid-gut would end up in the body cavity causing a fatal infection.

Hodgson said that the Western corn rootworm could reduce the yield of a cornfield by up to 40 percent. This would be the case where there are large populations of Western corn rootworm.

“The field will be flattened,” Gassmann said.

The mCryBb1 was introduced to the market in 2003, and by 2009 the Western corn rootworm had developed resistance.

Gassmann said every year there is one generation of rootworm. He discovered a population of Western corn rootworm had developed resistance within three generations. This means that in three years, the rootworm population had developed resistance to a Bt toxin meant to kill it.

Gassmann said, “I think in some ways this is not surprising.”

There had been previous predictions and computer simulations that predicted resistance within six years, Gassmann said.

Through his research, Gassmann determined the resistance to Bt toxins was non-recessive, meaning that there only needs to be one resistance gene present in order for the rootworm to be resistant to the toxin.

Gassmann said are a couple of ways that farmers can prevent the Western corn rootworm from acquiring resistance.

The number one way to prevent Western corn rootworm from taking over corn crops is to rotate out of corn production, Gassman said.

Corn roots are Western corn rootworms’ main source of food, so without the corn, they will die.

Hodgson said if resistant populations are unchecked, they will impact the future of the crop. If a solution is made, the problem will only intensify.

If farmers only farm corn, Gassmann said that they could use Bt corn in conjunction with rotation or use insecticides with non-Bt corn.

While Gassmann found resistance in primarily Northeast and Northwest Iowa, the resistant Western corn rootworm populations seem to be scattered across neighboring states.

Hodgson said it would be interesting to see how the cold winter affected the eggs of the Western corn rootworm to see this year’s impact.