There it is. A waterhemp plant alive and well next to a very dead weed. You know you applied your postemergence herbicide program at the right time. You know you had the right rate. You may have even sprayed two post programs.

And yet, there it is.

Daren Bohannan, technical representative for Bayer CropScience, says he answered several calls and walked numerous fields in central Illinois this summer that had the same challenge: herbicide-resistant waterhemp.

ON THE RISE: “When you start finding that type of segregation in a population of weeds that were clearly sprayed at the same time, and they weren’t hidden underneath the canopy, you begin to realize it’s not just a weather-related event; it’s a genetic difference between biotypes,” says Daren Bohannan, technical representative for Bayer CropScience, referring to recent herbicide resistance found in waterhemp.

“When you start finding that type of segregation in a population of weeds that were clearly sprayed at the same time, and they weren’t hidden underneath the canopy, you begin to realize it’s not just a weather-related event; it’s a genetic difference between biotypes,” he says. “And this year, there was an awful lot of that.”

What’s happening out in the field is reflected in lab results. Diane Plewa, plant diagnostic outreach specialist at the University of Illinois Plant Clinic, says this is the second year for herbicide-resistant waterhemp testing at the clinic. So far, out of 333 fields tested in Illinois, 74% contained glyphosate-resistant weeds, and almost 70% contained waterhemp resistant to PPO inhibitors. 

Based on the samples they’ve seen, Plewa believes PPO-inhibitor-resistant waterhemp is on the rise.

And as glyphosate- and PPO-inhibitor-resistant waterhemp continue to spread, Angie Peltier, University of Illinois Extension educator, recommends changing the way we think and talk about herbicide-resistant weeds. Weed scientists advise paying close attention to the herbicide site of action, not just the herbicide. 

Peltier explains the “site of action” describes the molecular site the herbicide binds to in weeds. The term “mode of action” is the biological process that the herbicide interferes with. For example, Group 14 herbicides, PPO inhibitors, disrupt the weed’s cell membranes, causing them to burst. The leaves and stems of the weed die, ultimately destroying the plant.

“There can be several different ‘sites of action’ within the older term ‘mode of action,’” Peltier says. “Small genetic changes that result from random mutations result in changes to the site of action that the herbicide binds to and can lead to herbicide resistance.”

Resistance to multiple sites of action plus tissue testing indicating higher percentages of affected weeds adds up to one nasty problem.

To understand your enemy even more, check out the infographic below.