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Monarch butterflies losing their food source, numbers declining sharply

Jan 8, 2015

Monarch butterfly populations have decreased by 90% in 20 years.
Credit Collette Adkins Giese / Center for Biological Diversity

Monarch butterflies are declining.

The U.S. Fish and Wildlife Service is considering whether to add the monarch butterfly as a threatened species under the Endangered Species Act. The agency just launched a one-year review of the butterfly’s status.

Tierra Curry is a senior scientist with the Center for Biological Diversity. It’s one of the groups that filed a petition to launch that review by the federal government.

She says the butterflies are found in the summer in every U.S. state except Alaska.

"But now they've declined by more than 90%," Curry says. That decline has happened in less than 20 years.

Monarchs need milkweed

Monarch butterflies are famous for migrating thousands of miles each year from Mexico to the U.S. and Canada and back. Curry explains that most monarchs from east of the Rockies converge in the mountains of central Mexico in the winter, where they form tight clusters on just a few acres of oyamel fir trees. Most monarchs west of the Rockies spend the winter in California.

Curry says in the mid-1990s, there were about a billion monarchs overwintering in Mexico. Now, she says that overwintering population is down to about 35 million. 35 million butterflies might sound like a lot, but Curry says monarchs need a very large population size to be resilient to other threats, like harsh weather.

"And the reason for that is largely the decline of milkweed in the Midwestern United States, because milkweed is the only plant that monarch caterpillars can eat," says Curry.

With the invention of Roundup Ready (glyphosate-resistant) crops, farmers were able to increase their use of the herbicide and spray it directly on their corn and soybean crops. But Curry says the herbicide kills milkweed.

"That has just caused milkweed to decline; before that, monarch butterflies successfully bred in corn and soybean fields," Curry says. "The other thing that has driven that was the increasing price of corn for the push for biofuels. So with the big push for ethanol, farmers started planting corn on conservation reserve lands and in other areas."

The difference between 'threatened' and 'endangered'

Curry says her group petitioned the U.S. Fish and Wildlife Service to list the monarch butterfly as threatened rather than endangered.

"We want people to still be able to handle them and interact with them. Everyone loves monarchs and we want that love to continue. We think that love will help save them."

"And the reason we petitioned for a threatened listing is because kids forever have brought monarch caterpillars inside to watch them metamorphose," says Curry. "We want people to still be able to handle them and interact with them. Everyone loves monarchs and we want that love to continue. We think that love will help save them."

She says if the Fish and Wildlife Service decides to list the monarch as threatened, it's likely the agency would develop conservation measures that would set aside land for milkweed and monarchs and make sure farmers can still do their jobs.

Backyard butterfly gardens

Since milkweed is the only thing that the monarch larvae eat, conservationists are encouraging people to plant it.

"The only complicated part of it is, there are a lot of different kinds of milkweed. So if people do want to plant milkweed at their school or park or their home, they need to find a variety of milkweed that is native to the region where they live," says Curry.

She says you should also make sure to avoid any that have been treated with systemic insecticides such as neonicotinoids which can kill butterflies as well as bees. "It's important to ask when you go buy the plants that you are going to plant to attract butterflies if they have been treated with systemic insecticides," says Curry.

Monarch Watch, a nonprofit based in Kansas, has created a map of ecoregions that show which milkweed species should be grown in which areas. The southern region of Michigan falls within ecoregion 222 and northern Michigan falls within ecoregion 212. Monarch Watch says in our region, monarchs rely mainly on the milkweed species Asclepias syriaca (Common milkweed). Other species used by monarchs in Michigan include A. incarnata (swamp milkweed), A. tuberosa (butterflyweed), and A. verticillata (whorled milkweed).