As many as 35 million monarch butterflies take flight between Mexico, the United States, and Canada each year in a multigenerational migration that covers more than 2,000 miles. The numbers seem impressive, but the monarch population has fallen 90 percent over the past two decades.
“It’s scary,” said Tierra Curry, a senior scientist at the Center for Biological Diversity. “If these were people, it would be like losing everyone in the U.S. except for those living in Ohio and Florida.”
The size of the monarch population reached an all-time low in December 2013. A survey of the forested areas in Mexico frequented by the butterflies over the winter months showed that they occupied two-thirds of a hectare, a 44 percent drop from the same time the previous year, according to WWF–Telcel Alliance and Mexico’s National Commission for Protected Areas. The size of the monarch’s footprint has been shrinking since surveys began in 1993.
Urbanization, deforestation, and forest degradation in the monarch’s wintering grounds; extreme weather conditions; and decreases in the abundance of milkweed have all been implicated in the dramatic decline of the iconic orange, black, and white butterfly.
But a recent study pins most of the blame on the ongoing disappearance of milkweed in the monarch’s breeding grounds on the Midwestern U.S. Monarchs lay their eggs on milkweed, and it is the only plant the monarch caterpillars feed on before transforming into butterflies.
“There’s a huge loss of milkweed in the Midwest—that’s what’s driving the monarch population down,” said Tyler Flockhart, a conservation biologist at the University of Guelph, in Ontario.
Flockhart and his colleagues teased apart the factors that have contributed to the drop in migratory monarchs over the last 20 years. The mathematical model revealed a loss of 1.5 billion stems of milkweed, a 21 percent decline, in eastern North America between 1993 and 2005.
The culprit? Industrial agriculture. The most rapid loss was seen where genetically modified, herbicide-resistant corn and soybean crops have taken root.
Farmers plant the herbicide-resistant crops so that they can use glyphosate and other herbicides to keep their fields weed-free. The U.S. Department of Agriculture says 93 percent of soybean and 85 percent of corn crops planted in 2013 were genetically engineered to tolerate herbicides. The herbicides kill the otherwise hardy milkweed.
Flockhart’s model shows that monarch populations will further decline by 14 percent from current levels over the century if land-use change and climate change continue as expected.
Curry would like to see the monarch protected under the Endangered Species Act. “In general, species can be protected if they are declining and their habitat is threatened, and the monarch certainly is both,” she said. “But butterflies generally get listed when there are only a couple thousand of them left.”
By then, it’s often too late to save the species.
For monarchs, the fix now centers on milkweed. Cutting the loss of milkweed in the monarch’s breeding grounds should be a priority, said Flockhart.
This past spring, the Ontario Ministry of Agriculture, Food, and Rural Affairs removed milkweed from the province’s noxious weed list, which previously would have required its destruction on croplands. In June, the White House ordered the creation of a Pollinator Health Task Force to find ways to protect bees, birds, bats, and butterflies.
People can plant native milkweed in their gardens or along roadsides. (Avoid those that have been treated with pesticides, advised Curry.) The Xerces Society for Invertebrate Conservation, Journey North, and Monarch Watch carry information on monarch conservation and purchasing milkweed.
A recent study found that more than half of U.S. households were not aware that the monarch population was in decline but that they would donate to monarch conservation programs and purchase plant milkweed. The survey of 2,290 randomly selected respondents indicated they would be willing to spend as much as $6.7 billion on monarch conservation, putting the value of the butterfly on par with such animals as elephants, giant pandas, gray whales, and bald eagles.
“A lot of environmental issues are so huge that it can be hard to feel like you can do anything,” said Curry. “But if you can plant milkweed native to the region, you can help the monarch.”