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.

While the final numbers won't be in until late January, the preliminary numbers are "disturbingly low," according to a Xerces statement.

"In 2017, [the preliminary count] sites accounted for 77 percent of the total monarch overwintering population, hosting approximately 148,000 monarchs," Emma Pelton, a Xerces Society endangered species conservation biologist, wrote in a statement. "In 2018, the same sites have only 20,456 monarchs. This represents an 86 percent decline since last year."

A steady 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, around 90 to 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 more than 250 sites monitored for the 2017 count.

Butterflies fly around a tree The number of observed monarch butterflies fluctuates every year due to a variety of factors. (Photo: Samuel/Wikimedia Commons)

In the year's first count, volunteers counted more than 1.2 million butterflies, and experts suggest that 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 may become tens of thousands.

Pelton acknowledges that these numbers fluctuate, sometimes even by double digit percentages, but the drop observed in this year's preliminary numbers means California may see fewer than 30,000 butterflies this winter. Such a low number is particularly troubling since, based on a 2017 study published in the journal Biological Conservation, 30,000 butterflies is the bare minimum needed to sustain the species' migratory population.

The Xerces Society will release a full count of butterflies in late January.

Unclear causes for concern

Monarch buttterflies cluster together along Pismo Beach Monarch butterflies stick together during the winter. (Photo: Elliott Lee/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 that the numbers were low even from the start of the breeding season in March and April, and it seems the population simply never recovered.

As for why there was such a steep decline, the causes are unknown but myriad events have come into play. A late rainy season may have harmed the butterflies in March, 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 writes. "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."

For Pelton, these reasons may only help explain this year's low numbers. A longer view, Pelton argues, is necessary to really help the butterflies. This low count comes as part of a long-term decline. "So while western monarchs appear to be suffering from some particular bad luck this year, the real concern is that the population may not be able to bounce back from this very quickly given 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.

The real test, Pelton says, will be the 2019 count, and whether or not the western population of monarch butterflies can survive. The eastern population in Mexico seems to be holding steady, despite experiencing an estimated 80 percent decline since the mid-1990s.

How you can help

A monarch butterfly perches on a plant branch Monarch-friendly plants will help them during migration and breeding seasons. (Photo: 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.

California's monarch butterfly population drops 86% in single year
Xerces Society's Western Monarch Thanksgiving Count isn't done yet, but it found that the butterfly population dropped 'down an order of magnitude' in 2018.