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Dicamba is hurting monarch butterflies, scientists say

A monarch butterfly feeding on a milkweed plant.
Tom Koerner | U.S. Fish and Wildlife Service
A study from the Center for Biological Diversity is raising the alarm that dicamba-based herbicides could be damaging monarch butterfly habitats.

Scientists are concerned that monarch butterflies could be facing a new threat: pesticides that contain dicamba. 

A report released last week from the Center for Biological Diversity showed that monarch butterflies migrate through areas of the southern and Midwestern United States where dicamba is heavily used. The chemical can interrupt the growth of milkweed and other plants that the species feeds on. Monarch populations are critical pollinators of wildflowers and other plants, but the species has declined more than 80 percent in the last two decades

Since the 1960s, dicamba has been used to kill weeds, namely the notoriously tough pigweed. Its use expanded in the last couple years, after farmers began planting dicamba-tolerant soybean and cotton from Monsanto. In 2012, farmers used about 600,000 pounds of dicamba on crops.  Researchers estimate that nearly 60 million pounds could be used this year, based on Monsanto's projections. 

However, the herbicide harmed about 3.6 million acres of soybeans in the United States during the 2017 growing season, according to research at the University of Missouri-Columbia. In hot weather, dicamba can volitilize, or become a gas, and drift for miles, causing damage to sensitive crops. 

"We're not calling for an outright ban on dicamba, but we don't believe that it can be used safely on Monsanto's genetically engineered crops," said Nathan Donley, senior scientist at the Center for Biological Diversity. 

There has been major initiatives to save monarch butterflies. Former President Barack Obama issued a memorandum to save honey bees, monarch butterflies and other pollinators in 2014. Monsanto has also launched projects to conserve the species and distribute milkweed plants to promote habitat. However, such efforts to plant milkweeds in agricultural areas could be undermined by dicamba use, Donley said. 

"I think we really need to make sure we maintain hospitable conditions for milkweed, and that means limiting pesticide use," he said. 

A map that displays where butterflies migrate and where dicamba is projected to be used in June.
Credit The Center for Biological Diversity
The Center for Biological Diversity's study notes that monarch butterflies migrate in parts of the United States where dicamba-tolerant soybean and cotton are planted and during portions of the growing season when growers will use the herbicide.

Missouri and a few other states of issued restrictions and cut-off dates for using dicamba products during this year's growing season. Arkansas banned the use of the herbicide altogether.

Monsanto has maintained that damage to other plants could be controlled with proper pesticide application training. The company has conducted "1,200 studies" that show that the chemical does not move when used correctly, said Scott Partridge, Monsanto's senior vice president of global affairs.

"If growers and applicators are successful in doing that, the product isn't going to move off target and cause any damage to the habitat of monarch butterflies," Partridge said. 

This isn't the first time that a weedkiller made by Monsanto has been blamed for the decline of monarch butterflies. Scientists have also blamed the population drop in part to Roundup Ready, which contains glyphosate. But the effects of dicamba are much more immediate on monarch habitat, Donley said.

"With most pesticides, the damage is delayed and it's harder for people to make the connection between pesticide use and harm," Donley said. "But with dicamba, you can actually see the damage in real time. I hope this is a wake-up call to the EPA and the farming community." 

In November 2016, the EPA approved the use of dicamba-based products such as DuPont's FeXapan and Monsanto's XtendiMax on genetically modified soybean and cotton. The Center for Biological Diversity wants the agency to let the two-year registration for dicamba to expire later this year. 

Follow Eli on Twitter: @StoriesByEli

Eli is the science and environment reporter at St. Louis Public Radio.

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