August 3, 2009

Rootworms may resist herbicide, study says

Volunteer corn -- unwanted plants from a previous year's harvest -- can provide a safe harbor for pests from insecticides, U.S. researchers reported Monday.

Less insecticide is squeezed out by the volunteer plants than from new plants, basically exposing western corn rootworm larvae to non-threatening doses, Purdue University researchers said in a news release.

Now they're exposed to a sub-lethal source of (insecticide) Bt that didn't exist before, Christian Krupke, an assistant professor of entomology, said. That becomes problematic.

The low-level amount of Bt the rootworm received helped the pest build up a tolerance, the researchers found.

In field tests, Krupke and Bill Johnson, a weed science professor, found more than half of the volunteer plants delivered some amount of the insecticide Bt and some had severe rootworm damage. The tests raised the concern that rootworms might build a tolerance that, if passed to offspring, could allow the pests to survive a full dose of the insecticide in commercial corn hybrids.

A grower has to add a new herbicide to control a volunteer crop, Johnson said. They use (a popular herbicide) to kill a dozen weeds, but this adds a big expense for a grower to control just one. It's essentially developing a new weed problem.

The results were published in a recent edition of Agronomy Journal.