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Study Confirms Reason For Monarch Decline

A Monarch caterpillar feeding off a milkweed plant in a residential Des Moines neighborhood.
Michael Leland / IPR
A monarch caterpillar feeding off a milkweed plant in a residential Des Moines neighborhood.

An Iowa conservation group wants to plant more than 800,000 acres of monarch habitat in the next 18 years.

With the end of summer, monarch butterflies are beginning to migrate to Mexico. Over the past 20 years, the population has been in steep decline. Scientists speculate it due to a lack of milkweed plants and last month a science journal published a study concluding the loss of those plants is the key reason.

It’s a theory Iowa State University researcher John Pleasants has supported for years.

“The milkweeds in agricultural fields were really preferred by monarchs,” said Pleasants. “Once those milkweeds were gone it was clear there was gonna be some consequence to this and that’s ‘smoking gun’ if you will for the decline of the monarch population.”

Monarch caterpillars can only survive on milkweed but it takes several patches for the female butterfly to lay enough eggs.

“One of the things that these monarchs do is they'll come to a patch of milkweed, they'll lay a couple of eggs, and then move on to the next patch. They're constantly on the lookout for the next patch and when you strip the landscape of lots and lots and lots of patches, it's harder for them to find the next patch,” Pleasants said.

In response to the study the Iowa Monarch Conservation Consortium is leading an effort to plant 830,000 acres of monarch habitat by 2038.

The study also disproved another popular theory that more butterflies were dying during their southern migration. Pleasants cited data stating the number of tagged butterflies has not decreased enough to support the migration theory.

Environment Environment
John Pemble is a reporter for IPR