The National Resources Defense Council (NRDC), a non-governmental environmental group, has filed suit against the Environmental Protection Agency (EPA) in an effort to force the agency’s hand on protection of the Monarch butterfly.

According to a statement by the group, “EPA’s failure to respond to an urgent petition that sought to limit the use of a pesticide that’s been destroying monarch habitat, filed more than a year ago by NRDC, has resulted in significant ongoing harm to the vulnerable butterfly population.”

Monarch butterflies migrate annually, across a 2,500 mile area from Canada, across the United States and into the forested mountains of central Mexico. According to the NRDC, an estimated 56.5 million of the butterflies were counted at the Mexican refuge last month, which is the second lowest estimate recorded to date.

The World Wildlife Fund (WWF) agrees that it was the second lowest total, but reports that it was an increase over the previous year. Rather than post estimated numbers of the insects, the WWF reports coverage in square acres. In 1993, according to the group, monarchs covered 27.48 acres of the Mexican refuge, compared to only 2.79 acres in 2014 a 69 percent increase over the record low of 2013.

According to the NRDC “experts say the primary cause for the population collapse is the skyrocketing use of the herbicide glyphosate (originally marketed as Roundup) on genetically modified corn and soybeans, which has wiped out much of the milkweed.”

Critics however have point out that while it is clear that milkweed has declined in recent decades and the use of glyphosate has gone up, no one has directly linked the two.

Andrew Kniss, an assistant professor in weed and plant sciences at the University of Wyoming, said in a blog post “the same highly publicized study that documented the decline in milkweed numbers suggested that herbicide use in agricultural fields was the culprit. But the study didn’t control for herbicide use, and therefore, had no way of knowing whether it was actually the herbicide or something else associated with crop production that caused the decline in milkweed.”

In February of 2014, the NRDC filed a petition with the EPA asking for an immediate review of glyphosate and for measures to “limit the harm the chemical has caused to the species.” However, according to the environmental group, the EPA has not responded or given an explanation for its silence.

For its part, the EPA claims that they are currently looking at a number of factors.

“EPA is taking a number of measures to protect the monarch butterfly and other pollinators. With regard to pesticide exposure, EPA is looking holistically at all herbicides, not only glyphosate, to determine the effects on monarchs and resources critical to butterfly populations,” the agency said in a statement to NBC News.

The NRDC warns that, at their current population levels, a severe weather event could destroy the remaining population. “In 2002, a major snowstorm in Mexico killed more than 80 million (out of approximately 100 million) monarchs wintering in Mexico, a number greater than the current total population,” they write.

In recent years the NRDC, the WWF and other groups have begun operating and promoting programs to plant milkweed by school groups and gardeners, in backyards, along highways and on other public and private lands. Additionally, earlier this month the US Fish and Wildlife Service launched a new 3.2 million dollar program aimed at preserving monarch butterflies.

While private efforts to plant and preserve milkweed are underway, it may take some time for such programs to become widespread. In some areas milkweed is seen as an invasive species or a “noxious weed”. Until last year farmers were required to eradicate the plant on agricultural lands in Ontario.

It may take even more work, or possibly other incentives, to convince farmers to stop clearing milkweed. “Milkweed is after all a weed, and since weeds decrease yield, farmers always have and always will fight them. Most of the world is fed via conventional farming whose aim is to maximize efficiency and output,” said Sarah Fecht of the Genetic Literacy Project.