By TIM ALEXANDER
Illinois Correspondent

URBANA, Ill. — What to do about Illinois waterhemp that has evolved resistance to HPPD-inhibiting herbicides is the subject of a Syngenta-funded study being conducted by University of Illinois crop researchers.

The researchers, led by professor Aaron Hager, UoI extension weed specialist, also published an article in Pest Management Science on Jan. 26 confirming that waterhemp is the first weed to develop such a resistance to glyphosate-based herbicides and virtually any other kind of herbicide on the domestic market.

“Based on some of the survey work that we’re doing, we’ve been able to identify that glyphosate resistance in waterhemp is not anything that is isolated on one or two fields; it’s becoming more and more common each year that we sample these populations,” said Hager.

The UoI research team determined that because herbicides are so effective, they exert tremendous selection pressures that, over time, result in resistant weed populations as natural outcomes of the evolutionary process. They also found waterhemp can not be adequately managed with just one or two herbicides.

Perhaps most troubling, Hager said, researchers have already discovered one waterhemp biotype that is resistant to five different herbicide families.
“We’re very concerned not only with just resistance to glyphosate, which by itself is certainly a problem, but we’re actually very concerned that many of these waterhemp populations and biotypes actually have stack resistance in them already,” he explained.

“By that we mean resistance to more than just glyphosate. When you start stacking these resistances, what happens very quickly is you basically can run out of post-emergence herbicide options to try and control these biotypes in either glyphosate-resistant soybean varieties or conventional varieties.”
The pressure is now on industry to develop new options for growers and on farmers to change their practices in controlling the weed spectrum, according to UoI researchers, who will be studying resistant waterhemp populations in Illinois for the next two to three years.

“Farmers need to diversify somehow,” Hager said. “These resistant biotypes are becoming more frequent, so we have to rethink how to best manage these things in either corn or soybean systems.”

Hager, Dr. Patrick Tranel and Dean Riechers, a UoI associate professor of herbicide physiology, are collaborating with Syngenta scientists to seek answers to the genetic, inherited and mechanical resistances to HPPD inhibitors.

“We’re trying to better understand how this resistance mechanism is occurring,” said Hager. “What is it that is unique about these particular biotypes that allows them to survive following an application of (herbicide)?”

While at least two seed genetics companies are developing crop varieties with built-in resistance to HPPD inhibitors, it will be two or three years before those products will be introduced. That time will allow researchers and scientists to learn about the type of resistance offered by the varieties before they hit the market.

“If these crops are commercialized, we could have the recommendations in place from the onset to slow the evolution of this resistance,” Hager stated. “It’s a problem that is getting worse and is not going to go away. This is the fifth herbicide family to which waterhemp has evolved resistance. Couple that with the fact that we really don’t see much evidence that the industry is bringing out herbicides that are going to (provide) a solution to this resistance problem.”

Waterhemp resistance to HPPD-inhibiting herbicides mostly affects farmers in central and southern Illinois, Iowa and some areas of Indiana, Hager explained, though nuisance plants of other varieties have developed similar resistance to pesticides throughout other growing regions. Horseweed and giant ragweed are showing signs of resistance in Indiana, while south of Interstate 70, Midwest farmers are contending with glyphosate-resistant mares tail, for example.

However, very few annual weed species, if any, in the United States have evolved multiple resistances to common pesticides as has Illinois waterhemp.
“Waterhemp will continue to be the major problem with resistance of virtually any species that we have,” said Hager.