Reuters — An environmental group sued the U.S. government on Friday, accusing regulators of discounting dangers a widely used herbicide is alleged to pose to the declining monarch butterfly population.

The Natural Resources Defense Council (NRDC) filed suit against the U.S. Environmental Protection Agency (EPA) in U.S. District Court in New York. The suit claimed the agency has failed to heed warnings about the dangers to monarchs posed by glyphosate, the key ingredient in a widely used herbicide. Glyphosate is used in Monsanto’s Roundup and other herbicides.

Federal law requires EPA to ensure that pesticides it approves will not cause “unreasonable adverse effects on the environment, including wildlife,” the lawsuit states. “However, the agency has never considered glyphosate’s impacts on monarchs.”

The NRDC and other environmental organizations have asked the EPA to review what they say is a large body of scientific evidence demonstrating glyphosate’s “devastating” effects on monarchs, but the EPA has not acted, the suit claims.

In response, the EPA issued a statement saying it is taking several steps to protect the butterflies but the science on the issue “is still evolving.”

“There are multiple factors that may be effecting monarchs including loss of habitat, weather and pesticides,” the EPA said.

The orange-and-black spotted monarchs, renowned for migrating thousands of miles over many generations from Mexico, across the U.S. to Canada, and then back again, have seen their numbers fall dramatically in recent years, according to the Center for Biological Diversity.

The lawsuit states that the monarch population was tallied at one billion in 1997 and this winter was down to 56.5 million butterflies, the second-lowest number ever measured.

The lawsuit states that a key cause of the monarch demise is the application of glyphosate to farm fields, and the resulting destruction of the milkweed habitat that migrating monarchs rely on.

Glyphosate is popular around the world because of its effectiveness in killing weeds that choke off gardens, yards and farm fields. Its use by farmers has risen over the last 20 years with the development of crops genetically engineered to tolerate being sprayed directly with Roundup.

The suit seeks a court order forcing EPA to evaluate glyphosate’s effects on monarchs and impose measures to mitigate harm to the butterflies.

This month, the U.S. Fish and Wildlife Service said it was helping launch a US$3.2 million campaign aimed at saving the butterfly’s habitat.

— Carey Gillam is a Reuters correspondent covering agribusiness from Kansas City.