California's Monarch Butterfly Population Has Declined by 99% Since the 1980s

The western population of monarch butterflies had particularly bad years in 2018 and 2019. Bill Kennedy/Shutterstock

Each year since 1997, the Xerces Society for Invertebrate Conservation has conducted the Western Monarch Thanksgiving Count, an annual event in which citizen scientists count the monarch butterflies overwintering in California.

The most recent results — collected in November 2019 and released this month — are not good. Volunteers reported 29,418 monarchs, according to the Xerces Society, which is just barely above last year's all-time low of 28,429. Like last year, it's also less than 1% of the millions of monarchs that wintered in California as recently as the 1980s, indicating the western monarch butterfly population "remains at a critical level."

The 2018 record low not only represented a 99% drop since the '80s, but also an 86% decline in just a single year, after the 2017 Thanksgiving Count found more than 192,000 monarchs at 263 sites. That suggested the butterflies' long-term decline had sped up significantly, although conservationists hoped it would turn out to be an outlier, followed by at least some improvement in 2019. And while the numbers at least didn't fall any further, the new results are not encouraging.

A dramatic decline

Multiple sites around California serve as overwintering grounds for the monarchs, and each year, the Xerces Society counts the butterflies that stay in the Golden State. Historically, millions of monarchs have flocked to California's coast to ride out the winter together, gathering on eucalyptus branches in large groups. As a result, sites like Muir Beach and Pismo Beach are normally awash in orange-and-black winged creatures as the butterflies leave the northern U.S. and Canada. They stay in California or continue to Mexico.

Around Thanksgiving, Xerces Society volunteers visit the most popular sites, roughly 100 or so, to collect a preliminary count. Another count is conducted in late December and into early January. The overall number of sites changes each year, with 263 sites monitored for the 2017 count, 213 in 2018 and 240 in 2019.

Butterflies fly around a tree
The number of observed monarch butterflies fluctuates every year due to a variety of factors. Samuel [CC BY 2.0]/Wikimedia Commons

In 1997, volunteers counted more than 1.2 million butterflies, and experts suggest as many as 4.5 million monarchs stopped over in California during the 1980s. Since then, however, numbers have dropped, or even plummeted, depending on the year. No year since '97 has even come close to 1 million butterflies, with most counts in the hundreds of thousands since 1998. Now, those numbers have dipped into the low tens of thousands for two years in a row.

These yearly totals tend to fluctuate from year to year, sometimes even by double-digit percentages, as Xerces Society conservation biologist Emma Pelton acknowledged in a 2018 blog post. But that didn't happen between 2018 and 2019, and these totals are particularly concerning because they suggest fewer than 30,000 monarchs have wintered in California for the past two years. That number may be a key threshold: According to a 2017 study published in the journal Biological Conservation, 30,000 western monarchs is the bare minimum needed to sustain this migratory population.

Unclear causes for concern

Monarch buttterflies cluster together along Pismo Beach
Monarch butterflies stick together during the winter. Elliott Lee [CC BY 3.0]/Wikimedia Commons

The count for the 2018 overwintering population was not expected to be a good one, according to Pelton. The breeding and migratory seasons were "rough." Migratory populations were late arriving to breeding sites in 2018, and they were much harder to find than they had been in previous years. This was despite a decent crop of milkweed, a particular favorite of monarchs. Long-term monarch counting projects in the state indicated the numbers were low even from the start of the breeding season in March and April, and it seems the population simply never recovered.

The details behind these declines aren't entirely clear, but myriad events have come into play. In 2018, for example, a late rainy season may have harmed the butterflies during a particularly vulnerable phase in their life cycle; a severe and prolonged wildfire season in California contributed to increased smoke levels and bad air; and California's various ecosystems are still recovering from drought.

Pelton discounts the notion that a delayed migration is occurring, that the butterflies are simply elsewhere right now.

"Monarchs are not being reported in large numbers elsewhere in their range, and Thanksgiving counts are not generally increasing despite repeated visits by volunteers eager to see an uptick," she wrote in 2018. "In addition, two years of the New Year's counts have suggested that monarchs do not generally arrive later than the Thanksgiving count period at the coast, at least in large numbers. Over the past two years, we have seen that New Year’s counts in early January are 40–50 percent lower than Thanksgiving counts in November."

While the last two years have been especially rough, they are part of a long-term decline for western monarchs, Pelton points out. And when these butterflies have a bad year or two, their ability to bounce back may be dwindling due to "the cumulative impact of all the stresses the population has been facing for years and years."

Those stresses include habitat loss for breeding and migration, pesticides and climate change.

How you can help

A migrating monarch butterfly perches on a plant branch
Monarch butterflies just leave when it gets cold. Neil Aronson/Shutterstock

On an individual level, there are some actions you can take to help the western monarch butterfly population.

1. Become a citizen monitor. If you're in interested in helping the Xerces Society's annual count, you can receive training to become a volunteer monarch monitor. Their website has the rundown on how to do this.

2. Plant nectar plants. Sources of nectar that bloom throughout the year, but particularly during the fall and spring periods, will help the monarchs feed and continue their life cycle. Xerces has a nectar guide to help you get started.

3. Plant milkweed. Milkweed occurs naturally in some areas, but planting it where appropriate, especially in California, can help the butterflies immensely. They use milkweed plants to play their eggs, and, once hatched, the soon-to-be butterflies eat the milkweed.

4. Stop using pesticides and insecticides. These chemicals harm not only butterflies but other insects. There are other ways to protect your plants that won't damage insect populations, and Xerces' guide to pesticides in your garden can help you with that.