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The eastern monarch butterfly population is declining. Researchers at MSU say they know why

Monarch Butterfly
Posted at 5:25 PM, Aug 06, 2021

EAST LANSING, Mich. — The eastern monarch butterfly is hard to miss. Its beautiful orange wings with heavy black lines make them stand out from the rest.

But the population is declining and researchers at Michigan State University say they now know why.

The monarch butterfly migrates across the United States every year. They spend their winters in Mexico and head up north to Texas to lay eggs when it gets warm.

Those monarchs die. But the monarchs that develop from those eggs continue their migration north and end up in Michigan for the summer.

"There, they produce another two to three generations, so they spend quite a while in the Midwest. And then later in the summer, kind of starting about now in August and September, the last generation of monarchs, then they shut down their reproductive systems. And they migrate all the way back down to central Mexico," said Erin Zylstra, a research associate at MSU.

Monarch butterfly resting on a child's hand
Monarch butterfly resting on a child

Measurements of the population done in Mexico over the past 20 years have shown that "those aggregations have gotten smaller generally," she said.

Programs in the Midwest where volunteers go out and do systematic surveys are also recording a drop in numbers

"We've seen that the number of monarchs they count on these surveys has generally gone down. It'll bounce around, you know, obviously, because some days are better than others, Some summers are better than others. But generally, there has been a decline in the number of monarchs that people are counting on their surveys," Zylstra said.

All of this has raised alarms.

"People are wondering what's happening with monarchs? Why are they declining? And because they traverse such a large area and things it's complicated to try and figure out. Is it something that's happening in Michigan in the summer? Is it something that's happening in Texas. We really don't know. And so that is what spurred the research" Zylstra said.

They started looking at all the collected data from the winter when the monarchs were in Mexico and all the data that was available from the Midwest from those systematic surveys. Five different monitoring programs shared monarch data with them.

They also collected information about what the conditions are like for monarchs along their migratory path and data on herbicide use.

A closer look at a Monarch Butterfly
A closer look at a Monarch Butterfly

"When they stop and breed in Texas and lay eggs, what did the conditions what had those conditions look like for monarchs? And each of the years we have data? So what were the temperatures like? What was the precipitation like? And we did the same thing in the Midwest," Zylstra said.

They put that information it into a statistical model to try and explain the ups and downs in the monarch population.

"We're trying to explain like, when it's big one year, how does that relate to what the precipitation and temperature look like and herbicide use? When it's small, how does that relate to those factors? And basically, what we do is, we create a whole bunch of different models that account for those things," Zylstra said. "What we found is basically since about 2004, the primary driver that's determining whether monarchs have a good year or a bad year and has been involved in their decline since that time has been climate."

If it's too hot, exceptionally dry, or wet the monarch's population tends to be smaller. Zylstra says they like moderate mild conditions where it's warm, but it's not too hot, it's wet, but it's not too wet. She also says the climate in the summer breeding grounds matter. They've found that, if it was really dry, the monarchs didn't do so well.

"That's probably because their milkweed plants weren't as productive," Zylstra said.

She also says they believe herbicide use plays a role in it because it kills off milkweed, which is a food source for monarch butterflies.

Here's what you can do to help save them.

Zylstra says the most important thing you can do is invest in resources to try to reduce carbon emissions to limit climate change.

"We can then we can help monarchs by planting milkweed. So there, there are multiple factors in there. And generally keeping places wild and having more flowers and nectar available for monarchs are good," Zylstra said.

For more information on how you can help click here.

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