Monarch Butterfly on MilkweedMonarch butterfly populations have been declining since the late ‘90s, but guerilla gardeners may have found a way to save them.

Every year, monarch butterflies make a nearly 5,000 mile journey from Mexico to Canada and back. As they fly north, they breed through four whole generations, then fly south as a single “supergeneration.”

In 1997, there were as many as 1 billion monarch butterflies wintering in Mexico’s Sierra Madre Mountains, blanketing a total of 45 acres of forests. In January 2014, that number was reduced to 33.5 million, on a mere 1.65 acres.

Butterfly populations rise and fall, but 2014 marks the ninth consecutive year that monarch numbers have measured below the long-term average of 350 million. But what’s to blame?

Scientists have pointed to several possible causes for monarch population decline. First, is deforestation. As more trees are felled in the fir forests of the Sierra Madres, cooler air seeps into the land and kills off the wintering insects. Second, is climate change. More extreme weather events have wreaked havoc, both on the butterflies’ health and their ability to follow chemical signals through their seasonal migration routes. Third, are the agricultural pesticides used in the U.S. and Canada, specifically Monsanto’s popular “Roundup” herbicide. Roundup contains glyphosate, which has been linked to fatal kidney disease in humans and has proved devastating to milkweed plants.

The loss of milkweed plants is cited as the main reason behind monarch butterfly declines in a new study featured in the Journal of Animal Ecology. Milkweed, says Tyler Flockhart, the study’s lead author and conservation biologist at the University of Guelph, is “the only plant monarchs lay their eggs on. And it’s the only plant they’ll feed on as larva before they develop into butterflies.”

Milkweed actually refers to a group of plants, with 73 species native to North America alone. They’re being wiped out by urban development and poisoned by “Roundup-ready” herbicides.

As Jason Bittel writes, “Put very simply: As the milkweed goes, so do the monarchs.”

Rebecca Riley, an attorney with the Natural Resources Defense Council, has suggested that the EPA should restrict glyphosate use and set aside land for milkweed cultivation.

While the government debates over whether to create glyphosate “buffer zones,” some renegade gardeners have taken matters into their own hands. Literally, in this case.

These guerillas are throwing “milkweed seed bombs” wherever milkweed might have a chance to grow. First they make a mud pie out of dirt and milkweed seed (preferably with milkweed seeds native to the area and that have been untreated with pesticide), then they shape them into a nice round shape and let them partially dry in the sun. Finally, the bombs are dropped wherever milkweed is likely to thrive. OnEarth recommends highway medians, ditches and gravel patches near railroad tracks. (Areas where milkweeds are likely to be cut down, such as private office parks and other manicured facilities are to be avoided.)

Can this guerilla action help? “The whole basis of the monarch’s occurrence in North America is based on the migration, and the fact that there’s supposed to be milkweed all over the place,” says Andy Warren, a lepidopterist and collections manager at the McGuire Center for Lepidoptera & Biodiversity. “So, really, anything anybody can do to put more milkweed out there is good.”

By July’s end, Jenny Kendler, an NRDC artist-in-residence, plans to hand out handfuls of milkweed seed bombs near the St. Louis headquarters of Monsanto.

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