Environment & Science

Western monarch butterflies are still in trouble

Monarchs cluster at Pismo State Beach in San Luis Obispo County, CA in December 2014.
Monarchs cluster at Pismo State Beach in San Luis Obispo County, CA in December 2014.
Flickr user Sandy Harris

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Results for the annual count of monarch butterflies along the California coast are out, and the news is not encouraging: numbers of the iconic insect continue to drop.

Volunteers for the Xerces Society’s Thanksgiving Count tallied just under 300,000 butterflies hunkering down in California for the winter, down from 1.2 million in the late 1990s.

At the historic butterfly watching spot at Pismo State Beach, interpreter Danielle Patterson said just under 20,000 were counted. While that may sound like a lot, it pales in comparison to the numbers the park’s grove of eucalyptus trees used to attract.

“When I was a little girl, I attended our park, and we had over 200,000,” she recalled. “That was in the early 90s.”

There are a number of factors contributing to the decline. The primary food source and breeding habitat for monarch caterpillars is milkweed, and the plant has been declining as farmers increase their use of glyphospate-based herbicides like Roundup in farm fields. The product kill weeds but spares herbicide-resistant crops.

According to a 2013 University of Minnesota study, as milkweed in the Midwest disappeared, so did monarchs: Between 1999 and 2012, milkweed dropped 58 percent while the number of monarchs declined 81 percent.

In addition, insecticides like neonicotinoids are toxic to monarchs, and they are more common than they used to be, according to a recent Xerces study. 

Bad weather can also wipe out an entire cohort of monarchs, especially since thousands of them cluster together all in one place. This winter the Pismo State Beach site, for example, was home to almost 10 percent of the entire Western monarch population.

Development can also threaten the sites that the butterflies return to year after year. According to Sarina Jepson, director of endangered species at the Xerces Society, there’s little protection for winter monarch habitat in California.

But Jepson said the decline isn’t irreversible because so many of the factors threatening monarchs are human-caused. To help, people living in coastal California can plant native flower gardens, which are nice to look at and provide food for butterflies, or volunteer with the winter count.

Still, she finds it alarming how fast things have gone downhill for monarchs.

“This is not a butterfly you’d expect to go extinct,” she said. “This is a butterfly that can migrate thousands of miles and is pretty resilient.”