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Does Lack of Milkweed Adequately Explain a Decline in Monarch Butterflies?

Joanna Gilkeson
USFWS / Creative Commons
A monarch butterfly egg on a milkweed leaf. Monarch caterpillars feed exclusively on milkweed plants, but a new study suggests milkweed availability might not be sufficient to explain migratory population declines.

A recent paper noted dramatic declines in migratory monarch populations over the last decade.

Every year, monarch butterflies make an amazing journey from Canada and New England all the way down to Mexico.

It’s an ecological wonder celebrated by all three countries -- but recently, scientists have noticed a dip in the number of butterflies making the trip. Climate change and severe weather could be threatening the migration -- what’s been seen as the primary driver is a lack of milkweed plants, which are vital food for the insects.

A controversial new paper is now questioning that theory. 

Before we get to the debate over monarch and milkweed, imagine this scene: each year, all of the monarchs from Eastern North America travel thousands of miles, coming together on 12 mountain tops in Mexico.

"The air is thin. You're at 10,000 feet of elevation. You're a little bit huffing and puffing," said AnuragAgrawal, a professor of ecology and evolutionary biology at Cornell University. He's visited those mountains, and said as you hike into the forest, at first, it's rather quiet. "And then you notice them. There are trees that are literally weighed down -- their branches are weighed down by butterflies," he said. "Upwards of 5,000 butterflies on a single tree -- and when the sun hits them in the morning, some of them will take to flight."

That can mean hundreds, if not tens of thousands of butterflies swirling around you.

In early spring, these butterflies mate and head north to the Gulf states. There, they lay eggs and die --- and the migration continues when the next generation matures and flies north to spots like New England.

"There, every summer, there are two to three more generations of monarch butterflies -- from egg to caterpillar to chrysalis to adult," Agrawal said. "For many of us in eastern North America, that's the very familiar site of summer with monarch butterflies -- fluttering around, drinking nectar."

Credit Katie Steiger-Meister / USFWS/Creative Commons
USFWS/Creative Commons
A monarch butterfly caterpillar.

Then, at the end of the summer, the incredible happens -- a butterfly weighing less than a dollar bill flies up to 3,000 miles back to Mexico.

But a recent paper published in the journal Nature noted dramatic declines in migratory populations over the last decade. A dip so drastic, it’s caught the attention of President Barack Obama, who issued a memorandum on the issue in 2014.

A lot of the blame has been placed on milkweed, a flowering plant known for its milky sap -- that’s the only food eaten by monarch caterpillars.

Karen Oberhauser, a professor at the University of Minnesota, said as farmers use more genetically modified crops and the herbicides that go with them, milkweed has declined in the U.S.

While she agrees climate change and weather could be impacting monarchs, "in science, we use something called Occam's Razor," Oberhauser said. "That means -- if there's a simple explanation that's usually the best explanation. We know that monarchs need milkweed. We know that milkweed availability has declined -- and we know that decline in milkweed availability correlates very tightly with the size of the winter population," she said. "So that's the simple, most elegant, and I would say, probably the right explanation for what's driven monarch numbers."

Credit Greg Kramos / USFWS/Creative Commons
USFWS/Creative Commons

But Agrawal thinks the ecology surrounding monarch migrations might be more complex. So his team modeled 22 years of monarch population numbers collected at points along the insect's epic journey.

"Our findings demonstrate that the decline happens after the period that caterpillars are using and growing on milkweed plants," Agrawal said.

When monarchs are adults and they’re flying back to Mexico, they’re feeding on nectar and water -- not milkweed.

"What that suggests to us is it’s something in between the beginning of flight and establishment in Mexico that’s the problem," Agrawal said.

One possible cause, he continued, is a lack of nectar sources as monarchs go South. Or it could be a host of other issues -- illegal logging in Mexico, or severe weather. There are also possible issues with parasites, predators, and pesticides.

Credit Ashley Spratt / USFWS Pacific Southwest Region/Creative Commons
USFWS Pacific Southwest Region/Creative Commons
At overwintering sites in Mexico, monarch butterflies cluster on trees -- bursting into flight every morning when temperatures rise.

"I think the important conclusion to take away from Anurag’s paper is not that milkweeds are unimportant, but that we should may entertain other hypothesis to explain the decline," said Matt Forister, a biologist at the University of Nevada, Reno. 

"Perhaps we've stirred the pot a little," said Agrawal, "I think all of the scientists that study monarchs are on the same side. We care about the species, it's charismatic, it’s been an important biological subject for us to study. And there are currently differences in opinion about what to do to help this declining population."

Agrawal said he hopes his research fires up scientists to get out there and do more work on monarchs, before it's too late.

Patrick Skahill is a reporter and digital editor at Connecticut Public. Prior to becoming a reporter, he was the founding producer of Connecticut Public Radio's The Colin McEnroe Show, which began in 2009. Patrick's reporting has appeared on NPR's Morning Edition, Here & Now, and All Things Considered. He has also reported for the Marketplace Morning Report. He can be reached at pskahill@ctpublic.org.

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