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What's driving the drop in monarch butterfly numbers?

May 19, 2015

Monarch butterflies are not around in the numbers they used to be — not by a long shot. By some estimates, monarch populations have dropped by 90% over the past twenty years. 

But why has that happened to these iconic butterflies? 

A male monarch butterfly perched on an aster.
Credit Monarch Watch

On a butterfly survey 

I took a walk with Jerry Wiedman to count butterflies. He’s a retired chemist. Wiedman brought a clipboard, and a butterfly net with a pole almost as tall as he is. We walked around a park that overlooks Lake Erie in northeast Ohio.

This isn’t just a casual walk. This path through meadows and woods is what’s called a butterfly transect. Wiedman and others set it up.

“And we count butterflies within 7.5 feet on either side of us," he said.

As we walked, he jotted down how many butterflies and what species we saw.

“There’s your red admiral, see him?”

Wiedman has been fascinated with butterflies since he was a kid.

A mystery of nature

For many young people, monarch butterflies are the first to get their attention. Their large orange and black wings are easy to spot. And their migration story is a mystery of nature.

Each spring, these tiny, fluttering creatures fly thousands of miles. They leave their winter homes in forests high in the mountains of Mexico, and the females lay eggs on milkweed plants in the southern U.S.

Most monarchs live only a few weeks. So the new butterflies continue the journey started by their parents. It can take five generations to complete one migration to Canada.

Some years, Wiedman has personally counted hundreds of monarchs migrating through Ohio.

But that’s changed.  

“We don’t get the the numbers we did in the late 90s or early 2000," he said.

Chip Taylor watches as streaming monarchs head for a watering site. He is the director of Monarch Watch at the University of Kansas.
Credit Monarch Watch

A troubling downward trend

His counts show fluctuations in the monarch population, but it’s a downward trend. It’s the same story around the country.

Chip Taylor is the director of Monarch Watch at the University of Kansas and he says monarch numbers started to plummet in the early 2000s. That’s when new farming technology —  known as Roundup Ready (glyphosate-resistant) seeds — became popular. With Roundup Ready seeds, farmers could spray entire fields with herbicide. The weeds would die, but the crops were fine. 

Taylor says it also killed off the milkweed, and that’s the only food monarch caterpillars can eat.

“You know, I’ve got really nice pictures of milkweed growing in corn and soybean fields around the year 2000. Then around 2006 you couldn’t find milkweed growing in corn and soybean fields anywhere. I mean it was just about impossible,” he says.

Around 2007, President Bush made a push for huge increases in corn-based ethanol and other homegrown sources of energy. With the help of Roundup Ready technology, many farmers planted corn and soy on land that had been fallow, or in conservation. This killed off even more milkweed.  

“And the estimate is that we’ve lost 100 million acres of habitat just to the adoption of this particular technology,” says Taylor.

Should the monarch be listed as threatened?

A display at the Ohio Lepidopterists workshop in Toledo, March 2015.
Credit Julie Grant

According to estimates by the Center for Biological Diversity, monarch populations have dropped by about 970 million since the mid 1990s. They’ve petitioned the U.S. Fish and Wildlife Service to protect the monarch as threatened under the Endangered Species Act.

Taylor argues against that.

“I don’t think they should be listed as threatened. I think that is counterproductive. The value of this petition is that it’s caught everybody’s attention,” he says.

He says regulation could set up conflicts between farmers and the federal government. 

Still, he sees a need to restore tens of thousands of acres of habitat for the butterflies. He says that’s already starting to happen, with many people planting milkweed in their home gardens and teaching children about it in school.