From success to failure
Researchers are already finding failures of Bt corn against the western corn rootworm in Iowa, Illinois, Minnesota and Nebraska, for example. In a study published in 2011 in the journal PLoS ONE, Aaron Gassmann and his colleagues at Iowa State University demonstrated that the offspring of western corn rootworms from rootworm-damaged cornfields in Iowa were less susceptible to a particular Bt toxin (Cry3Bb1, the type of Bt in use in the damaged fields) than the control population.
Gray has seen the same kind of severe rootworm damage in Illinois fields planted with the Cry3Bb1 variety of Bt corn, and he is collaborating with Gassmann on a new study of the Bt failures in Illinois. As yet, the researchers have not confirmed the cause of these Bt failures and are working to determine the mechanism.
“Like the fields Gassmann studied in Iowa, the failures here in Illinois are in fields planted in continuous corn for many years with no crop rotation, using that same Cry3Bb1 Bt protein over and over again,” Gray says.
Growers’ confidence in Bt corn probably stems from its great success against the European corn borer, a pest accidentally brought to the U.S. in the early 1900s. The insect arrived in Illinois in the late 1930s.
“It was a devastating pest,” he says. “It was a major yield robber.”
In a 2010 paper in Science, Gray and colleagues from several Midwestern universities reported that the introduction of Bt corn in 1996 led to a profound and lasting reduction of the European corn borer across the upper Midwest.
This success was in large part due to two factors, Gray says. First, it was relatively easy to get high levels of the Bt proteins in above-ground corn tissues, on which corn borers feed. And second, regulators required farmers who used Bt corn to provide large buffer zones, called refuges, planted with non-Bt corn. The refuges would supply a lot of Bt-susceptible insects to mate with those that survived exposure to Bt and were thus resistant to the toxin, to reduce the development of Bt-resistant strains.
Western corn rootworm larvae do the most damage to roots, however, and expression of Bt toxins in roots is much lower than in above-ground tissues. This means that the rootworms are getting a much lower dose of specific Cry toxins than the corn borers do, Gray said. Add to that the fact that some growers are ignoring the buffer requirement while also planting Bt corn year after year, he says, and you’ve got a recipe for resistance.
“We might not be having this conversation if corn was $3 a bushel and growers were asking us where to put their limited resources,” he said. “But with these high commodity prices the notion of adding inputs to squeeze out a couple of more bushels seems like cheap insurance.”
A paper describing corn producers’ failure to adopt integrated pest management practices appeared in the Sept. 27, 2010, Journal of Agricultural Food Chemistry. Gray will present his latest findings at the International Integrated Pest Management Symposium in Memphis, TN, this month.