Monarch Exposure Jeopardizes Dicamba Use

If dicamba’s in-season use decreases milkweed population further, monarch activists might demand further changes to the label or try to completely eliminate the herbicide’s use in-season. The butterfly’s population has steadily decreased since the introduction of Roundup Ready crops, and some fear dicamba might be what sends the species over the edge to become endangered.

“We don’t need to just single out dicamba—with any crop protection product we use we need to be mindful of their effects on susceptible plants and habitat,” says Wayne Fredericks, who farms corn, soybeans and cover crops in Osage, Iowa. He, like many farmers, wants to see activists and agriculturalists find middle ground on this increasingly contentious issue.

The problem, according to dicamba critics, is the herbicide kills not only what’s in the field but can also drift to nearby pollinator habitats. This results in lost food sources and potential for lost nesting ground.

“Dicamba in small doses doesn’t outright kill milkweeds, but it does reduce growth and other things that impact animals that feed off those plants,” says Nathan Donley, a senior scientist at the Center for Biological Diversity. He authored a recent report that suggests dicamba could have devastating effects on monarch populations.

Since there were more than 3.5 million acres of estimated off-target movement from dicamba last year, Donley brings into question what that means for pollinator habitat since the number of total acres planted to Roundup Ready 2 Xtend this year could double to 40 million. Even though monarchs don’t mind laying eggs in herbicide-injured milkweed, he is concerned about how food quality and quantity will be affected with damage.

“Research on other caterpillars, not monarchs, shows that when they feed on dicamba damaged weeds they become less functionally able adults,” Donley says. “Since it’s shown in other butterflies, it could be an issue for monarchs.”

“We’re not calling for a complete and total ban on dicamba—I don’t want it to seem like we are—we just don’t believe this can be used safely in post-emergent applications or at this volume,” Donley adds. “Many people say they want to see more research before making this kind of decision, but that takes time and could lead to further damage to monarchs in the interim.”

Experts say these claims against dicamba are exaggerated.

"Research shows that there are many factors that impact monarch populations like climate change, land use pattern changes, loss of nectar sources and the availability of milkweed," BASF provided in a statement. "We believe that technologies to battle yield impacting weeds can co-exist alongside habitats for the iconic monarch butterfly."

University experts, while they don't excuse off-target movement, agree that dicamba likely isn't as harmful to butterflies as some suggest.

“They’re suggesting that the dicamba products will further reduce the amount of milkweed,” says Bob Hartzler, Iowa State University Extension weed scientist and member of the Iowa Monarch Conservation Consortium. “Most farmers still use glyphosate and that’s more effective on milkweed than dicamba.”

Asking farmers to manage weeds in fields while allowing milkweed to survive isn’t realistic in Hartzler’s opinion. While Hartlzer recognizes the extent of off-target dicamba movement and injury in 2017 was “unacceptable,” he says they can’t assume all monarch habitat in these areas was destroyed.

The Center for Biological Diversity’s study extrapolated data in a way that isn’t accurate, Hartzler says. By using the number of soybean acres that suffered from off-target movement in 2017, they’re not taking into account the inherent sensitivity of soybeans. Milkweed, and most other plants, is less sensitive to dicamba than soybeans.

“Milkweed plants that are adjacent to fields sprayed with dicamba are much less likely to be injured than soybeans,” Hartzler says. “But if some do get damaged, the research we performed showed it didn’t affect monarch’s use of the plants.

“There are valid concerns, but their study takes the worst case scenario in every situation and the likelihood of that occurring is low,” Hartzler adds. “Hopefully, training and new label restrictions will reduce the extent of off-target injury in the future.”

According to Scott Partridge, vice president of global strategy for Monsanto, if applicators follow all product labeling and any local requirements before spraying the product won't cause damage to monarch habitat.

For Fredericks, Roundup Ready 2 Xtend soybeans aren’t right for his farm this year but he still wants to help protect other farmers’ access to the technology—so long as they use it correctly.

“Under the new dicamba label you have setbacks and buffers for susceptible crops, which includes road ditches,” he says. “Theoretically if everyone follows the new dicamba label I’m not concerned for pollinators at all.”


Track Monarch Flights Compared to Popular Spraying Dates: Click Here for Interactive

Monarch/dicamba overlap from Journey North
Monarch flight overlaid with cotton and soybean spraying. C: Journey North



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