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Western Monarch butterfly population has declined 74 percent in past 20 years

Monarch butterflies cluster together while spending the winter at Pismo Beach in San Luis Obispo County, CA.

The monarch butterflies along the California coast are dying: Their population is down 74 percent from 20 years ago.

But because Western Monarchs aren’t nearly as well-studied as their Eastern counterparts, scientists aren’t really sure what’s killing them. A new report from the insect conservation group Xerces Society has some theories.


In the upper Midwest, where many Eastern Monarch butterflies breed, there has been a proliferation of herbicide-resistant corn and soybeans. That has led to increased sprayings of glyphosate-based herbicides like Roundup, which kill weeds but leave the crops intact. As more farmers switched to herbicide-tolerant crops, weeds like milkweed – the only plant monarch larvae will eat – began to disappear. According to a 2013 University of Minnesota study, as milkweed populations in the Midwest decreased, so did monarchs: Between 1999 and 2012, there were 58 percent fewer milkweed and 81 percent fewer monarchs in the Midwest.  

Because there are fewer acres of corn and soybean in the Western U.S., it’s not clear what role the herbicide-resistant crops play in milkweed –and therefore monarch – decline in California.


Monarchs are also affected by insecticides like neonicotinoids, which are used everywhere from farms to suburbia. And they’re much more common than they used to be, according to Xerces.

These chemicals are toxic to both butterflies and bees, and researchers have been studying whether there is a connection between their uses and honeybee die-offs.

Bad weather

Monarch butterflies are susceptible to cold and freezing temperatures, which is why they choose to spend their winters in temperate places like Mexico and the California coast. They huddle together, clinging to trees in microclimates with dappled sunlight, no wind and high humidity. Because so many of them cluster together at once, they are uniquely vulnerable to a natural disaster, like an unusual winter storm in Mexico in 2002 that killed more than 220 million of them. In California, one popular overwintering spot in San Luis Obispo county hosts 11 percent of the Western monarch population. 

Threats to winter habitat

Many of the places along the California coast that monarch butterflies return to year after year are threatened. Most winter habitat sites are groves of trees, and many are single-species stands of eucalyptus that are now aging and beginning to die. Other groves are at risk of being destroyed by housing or commercial development, and still others are dying due to drought or disease.

“There’s not a lot of protection that monarch overwintering sites have in California,” said Sarina Jepsen, Director of Endangered Species and Aquatic Programs at the Xerces Society. She would like to see the state protect winter monarch habitat year-round, plus create buffer zones around the sites to protect them from development.

The formerly ubiquitous butterfly is now under consideration for protection from the federal Endangered Species Act.