EAST PEORIA, Ill. — Farmers attending the inaugural Illinois Central College (ICC) Crops Conference were greeted with a conundrum by the first speaker, retired Western Illinois University agronomist Gordon Roskamp.

His “How to Get Pesticide Resistance” presentation actually served the purpose of advising producers how to avoid resistance in common weeds that invade crop fields. “How do you get (herbicide) resistance?” Roskamp asked the 100-plus attendees. “Just do what you’ve been doing. If you keep doing what you’ve been doing without changing, you are headed for resistance.”

Roskamp spent 36 years studying and teaching weed science at WIU before retiring to the Peoria area. Recently he was hired as an adjunct professor by ICC.

“Mother Nature is smart. Weeds are smart. But so are insects and so are diseases,” said Roskamp, lately awarded Fellow status by the North Central Weed Science Society.

He cited a recent University of Illinois at Urbana-Champaign study led by weed scientist Aaron Hager, which examined waterhemp collected from a field plot near Mt. Sterling. Though the plot’s soil was treated with both atrazine and post-emergence herbicides, the weeds’ resistance survived four different modes of action.

“It was what we call a multiple-resistant site, in that it had leaves survive all four herbicides,” Roskamp said. “Waterhemp is dioecious; it has both male and female plants. It cross-pollinates and creates bigger and better waterhemp from year to year.”

The Mt. Sterling plot is – so far – an abnormality compared to most crop fields in Illinois. Despite what occurred, Roskamp advised farmers there is no better way to control resistance outbreaks than by applying different products, or modes of action, to their fields before and during plant growth.

“One of the best ways to avoid weed resistance is to use different herbicides with different modes of action,” he said. “Part of the problem is that we have decided that Roundup is the greatest thing since sliced bread.

“Monsanto didn’t help the situation early on by saying not to use anything other than Roundup. Of course, now they have a rewards program, and Syngenta has a similar reward program, where they will help you buy a soil-applied herbicide with a different mode of action.”

Along with waterhemp, weeds in Illinois showing resistance to glyphosate-based sprays and other products include horseweed (mare’s tail) and, to a lesser extent, Palmer amaranth. Roskamp cited another recent study of fields in Illinois, Kentucky and Ohio that confirmed multiple resistances in 122 plants.

Fifty-nine percent of the plants proved resistant to ALS, while 13 percent were also resistant to PPOs and-or 66 percent to glyphosate. Seven percent of the plants were resistant to all three modes of action. On a field basis, 29 percent of the fields involved in the multistate study featured plants resistant to all three modes of action.

“Do we have glyphosate resistance in the state of Illinois? The answer is yes,” said Roskamp. “You can assume if there is agriculture ... there is glyphosate resistance.”

Nationally, 5,000 farmers reported glyphosate-resistant weeds in their fields during 2010, reported Roskamp. He recommended farmers visit the U of I’s integrated pest management website at www.ipm.illinois .edu/education/action.htm to stay informed about discoveries in plant resistance.