Dicamba drift: New use of an old herbicide disrupts pollinators

(Press-News.org) March 4, 2025

Contact: Morgan Sherburne, 734-647-1844, morganls@umich.edu  

 

Images of pollinators and plants 

  ANN ARBOR—An herbicide may "drift" from the agricultural fields where it's sprayed and harm weeds that grow at the edge of the fields, impacting pollinators.

A University of Michigan study examined the effects of the herbicide, called dicamba, and found that plants exposed to dicamba drift had a lowered abundance of pollinators, and that pollinator visits to flowers were reduced for some weeds, but not others. The study, led by U-M professor of ecology and evolutionary biology Regina Baucom, is published in the journal New Phytologist.

Dicamba was developed in the late 1950s and first registered for use in 1962. But the herbicide is moderately toxic to humans and wildlife, Baucom said. Its use fell out of favor, and farmers began using RoundUp on crops that had been genetically modified to tolerate glyphosate, the active ingredient in RoundUp. Glyphosate has generally been considered environmentally friendly.

But weeds have evolved resistance to the intense use of RoundUp. The agricultural company Monsanto, now owned by Bayer, turned to dicamba, and developed crops that were resistant to it. In 2018, the Trump administration reapproved its use. Soon after farmers began using the herbicide, other farmers began reporting that their crops were impacted, Baucom said.

"One of the reasons dicamba went out of favor is because it's volatile. A farmer will spray it, and it will go up into the air column or re-volatize, then drift and expose unintended areas," Baucom said. 

Farmers who weren't yet using dicamba reported that their crops such as soybeans, which are very sensitive to dicamba, weren't producing their typical yield, she said.

"Then we realized that not only is dicamba drift going to be impacting crops, but it's also going to be impacting plants or weeds that are in the waste areas around fields that serve as important reservoirs for insects," Baucom said. "Given that we are dealing with insect decline on a large scale and we're really concerned about pollinator health, I thought this might be an important thing to start investigating."

Baucom and colleagues wanted to see if weeds, and ultimately, pollinators were affected by this drift. They found that weeds exposed to dicamba drift had fewer pollinators near them, and that pollinator visits to flowers were reduced for some weeds, but not others.

Dicamba damages plants by mimicking the hormone auxin. Auxin is present at all times of plant development. It's in many different plant structures, including reproductive structures, and is also responsible for how plants tilt toward light. Baucom said that when a plant like soybean gets hit with dicamba drift, its leaves show a cup-like behavior. 

"We were wondering, does this exposure to dicamba drift cause changes to the production of flowers? Do we see changes to the timing of flowering? We were interested in anything that would potentially impact a pollinator’s interest in visiting a group of plants," she said. 

The group built plots in which they grew replicates of 11 different weed species. Five plots were exposed to dicamba a single time, early in the lives of the plants, at drift levels, or about 1% of a typical application. Each species was also replicated in multiple plots which served as a control and were not exposed to dicamba. 

The researchers then observed how the plants were growing, their growth size, signs of damage and the timing and abundance of flowering. They then screened for pollinator abundance and pollinator visits.

"In the control plots, we found significantly more pollinating insects than we found in the drift environment. And remember, these constructed weed communities were exposed to dicamba drift just once, early in their life," Baucom said. "So this one time exposure of a low dose of dicamba led to a lower abundance of pollinating insects in those constructed communities."

Baucom suspects that pollinator abundance and visits decreased because of the damage dicamba caused to the plants. Dicamba impacted the size of some plants, some plants produced fewer flowers, and some plants flowered later than they normally would. The researchers modeled this damage, which suggested that dicamba drift could impact plant traits. The damage could then impact the number of pollinator approaches and visits.

"In the control environment, there's a positive relationship between the number of flowers and the number of pollinator visits. The more flowers you have, the more often you're visited by a pollinator," Baucom said. "In the dicamba environment, that relationship goes away. There's no relationship between the number of flowers and the number of visits.

"What I thought was most interesting is how this one auxinic herbicide disrupts this weed community, and disrupts a long-standing relationship between plant signals and pollinator behavior."

Next, Baucom and U-M colleagues won a grant through the College of Literature, Science, and the Arts' Meet the Moment Research Initiative to investigate how dicamba drift may impact prairie strips, which are strips of native plants grown alongside farm fields to both catch agricultural runoff and provide crucial habitat for pollinators.

Study: Off-target drift of the herbicide dicamba disrupts plant–pollinator interactions via novel pathways (DOI: doi.org/10.1111/nph.20438)

END