Controversial weedkiller could spell big trouble for monarch butterflies: Report

Controversial weedkiller could spell big trouble for monarch butterflies: Report

Environmental group reports over the next year more than 60 million acres of the monarch's US migratory habitat will be sprayed with dicamba

By 2019, a weed killing chemical—designed to be used in tandem with genetically modified cotton and soybean seeds—is projected to be sprayed on more than 60 million acres of monarch butterfly U.S. migratory habitat, according to a report released today by the Center for Biological Diversity.


Citing this potential devastation to monarch populations, which have already decreased an estimated 80 percent over the past two decades, the report calls on the U.S. Environmental Protection Agency not to renew the registration of the weed killer, called dicamba, when it expires at the end of this year.

The concern is the chemical could cause more habitat loss and decreased milkweed, which is the only food plant used by monarch caterpillars. Monarchs winter in Mexico and some warm areas of Southern California and they return to areas throughout the U.S. in the spring.

"America's monarchs are already in serious trouble, and this will push them into absolute crisis," said report author Nathan Donley, a senior scientist at the Center, in a statement.

Donley and colleagues looked at monarch habitat in the U.S. and estimated how much dicamba will be sprayed. In addition to the estimated 60 million acres to be sprayed, an additional 9 million acres could be threatened by the chemical drifting.

The weed killer gained notoriety this year as farmers planted more than 25 million acres with new soybean and cotton seeds genetically modified to be resistant to dicamba. Monsanto, BASF SE and DowDuPont all make dicamba-based herbicides.

In many areas the weed killer drifted onto nearby fields and killed crops, spurring lawsuits in Arkansas, Missouri, Kansas and Illinois. Arkansas banned dicamba; North Dakota, Missouri and Minnesota put restrictions in place.

"In 2017 there were reports of at least 3.6 million acres of off-target, herbicide-induced damage to agricultural crops and an unknown amount of damage to native plants and habitats, including forests," according to the Center's report.

Dicamba is a threat to monarchs because it can destroy flowering plants that provide nectar for adult butterflies as they travel south for the winter and by harming milkweed, which is the "only food source of the monarch caterpillar" and "provides an essential resource for reproduction," stated the report.

"When dicamba's use on [genetically engineered] cotton and soybeans comes up for reapproval later this year, the only responsible thing for the EPA to do is allow that approval to expire," Donley said.

We've reached out to Monsanto to comment on the report.

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