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The EFM Notebook

A Commentary on What’s New and Newsworthy

by Susan Holloway | Bio

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Western Monarchs: Bouncing Back?

9 Feb 2022 2:16 PM | Gayle Marsh (Administrator)

The western monarch butterfly has been disappearing so fast that extinction has become a real possibility. But recently we have had good news of a resurgence of monarchs overwintering on California’s coast!

Thanks to the Western Monarch Thanksgiving Count, we know how many western monarchs were overwintering in coastal sites in November of 2021. The Thanksgiving count is a truly great example of community science, with over a hundred volunteers observing in 283 sites from Mendocino to the Mexican border. And we can compare the 2021 numbers to similar counts that have occurred every year since 1997. 

The Life and Loves of the Western Monarch

Before getting into the numbers, let’s look at the amazing life of western monarchs. During the winter months they typically find a resting site within 1.5 miles of the ocean or the San Francisco Bay. They seek out dappled sunlight, high humidity, and access to fresh water. No dummies, they don’t like freezing temperatures or high winds. They cluster in dense groups on the branches and leaves of eucalyptus, oak, redwood and other trees, doing relatively little besides sunning and sipping nectar and water.

Monarch on a eucalyptus tree.
Source: San Francisco Forest Alliance

Monarch emergence. Photo credit: Becky Hansis O’Neill

When the weather warms up in February and March, they head up toward Oregon or east into Nevada and Idaho.

While overwintering monarchs live for months in somnolence, the migrating monarchs don’t have too much time on the planet, just 2-6 weeks. After mating, the female lays her eggs on a milkweed leaf. After hatching, the caterpillars remain on the milkweed plants, devouring the leaves for nutrition. They eventually form a chrysalis and emerge as an adult butterfly.

During the rest of spring and summer, successive generations of butterflies continue their eastward dispersal. In the fall, they head back to the west coast, with the final generation arriving at an overwintering site in September or early October. 

Snatched from the Headlines

The Good News! Compared to last year, the number of monarchs observed in overwintering sites in California increased 100-fold! This year’s count of 247,237 butterflies has caused considerable excitement among monarch lovers.

  Happy days! But let’s not get too excited

As usual, the majority of this year’s monarchs was found along the central and southern coast from Santa Cruz to Los Angeles counties. The highest count was in Santa Barbara county, where over 95,000 were reported.

Fewer monarchs typically overwinter in the Bay Area, and this year was even more skewed to the south than usual. In all, there were fewer than 600 butterflies counted from Mendocino to San Mateo county. 

This chart summarizing data collected since 1997 shows a pattern of sharp and sudden decline between 1997 and 1998, plunging from over 1.2 million to just under 600,000. Between 1999 and 2017, the number remained low but relatively stable, dropping again in 2018 to a mere handful until this past year when the population returned to the place it had been throughout most of the past two decades.

Zooming in on Marin 

But what about Marin county in particular? Thanks to the Xerces Society, who made the data public, I was able to see that only 75 monarchs were observed across the Marin county sites. 

I graphed the counts obtained at the two most popular overwintering sites in Marin, Chapman Ravine in Stinson Beach and Purple Gate in Bolinas. Due to the limitations of my graphing skill, counts under 100 are not actually visible as a bar. Please use your imagination – there have typically been at least a handful of butterflies at these Marin sites, even in the lean years.

The Marin graph allows us to see “bounciness” as the counts at both sites bumped up and down across the years. For instance, 1999 was quite low compared to 1997, but then the numbers jumped all the way back up in 2000. Counts in 1998, 2004 and 2015 were moderately high, with intervening periods where the monarchs were approaching the “extinction vortex.”  

Count of monarchs in two overwintering sites in Marin

Some Reasons for the Rebound

What is going on? Botanists have identified several possible reasons for this partial recovery, but we don’t yet know for sure what is going on. It’s hard to account for the bounciness but here are some candidates:

Monarchs are fertility goddesses. Monarchs in the West produce multiple generations a year and a female monarch can produce 12 adult daughters in ideal conditions, with four or so being typical under normal constraints!

Small in number, rich in resources? When animal populations are small, they are more vulnerable to random fluctuations in the environment such as a bad winter. On the other hand, these reduced populations experience less competition for resources like food and may rebound quickly.  

Weather patterns. Monarchs generally prefer mid-range temperatures and it is possible that the warm dry summer and relatively cool winter we experienced in 2021 hit a sweet spot.  

Winter breeding monarchs. Native milkweed dies back in the colder months, prompting the monarchs to take off for their overwintering sites. In recent years, urban gardeners have planted large quantities of non-native tropical milkweed, which is evergreen. The abundance of year-round milkweed may have enticed some monarchs to stay longer in the Bay Area, engaging in “extra” winter breeding. (But bear in mind that consuming tropical milkweed also makes monarchs more susceptible to parasites.)

Influx from eastern monarchs. Another proposal is that eastern monarch butterflies that overwintered in Mexico are coming to the western states in spring rather that going back to their original territory.

On the Other Hand…Reasons that Overall Numbers Remain Low

Before we break out the champagne, we should remember that the monarch population remains more than 98% below its size in the 1980s. What are some of the ongoing threats?

Parasite poisoning and predators. During the summer of 2021, local organizations and residents noted that monarch eggs were being laid and larvae are hatching, but then not surviving more than a few days. Predators such as the paper wasp and yellow jacket are among the suspects.

Another theory is that caterpillars were ingesting spores from a common parasite called Ophryocystis elektroschirra (OE). OE spores can fall from an infected adult butterfly onto a milkweed leaf and take up residence in the caterpillar’s gut when the leaf is eaten. 

Caterpillars with a particularly high dose of OE parasites are likely to die before reaching the pupa state. An important but little understood part of the puzzle is that monarchs that have not migrated because they have access to non-native tropical milkweed are more susceptible to OE infections than those who migrate to overwintering sites. 

Pesticide poisoning. A persistent risk to monarchs is posed by neonicotonoids (AKA neonics), a class of insecticides applied to mature plants or to seeds. Neonics can live in the environment for months or even years after being applied to plants. They also leach into subsurface water and they contaminate soil which can then be dispersed by wind. When absorbed by plants, neonicotinoids can be present in the leaves that the caterpillars eat as well as on the pollen and nectar ingested by the butterflies.

Neonics are widespread and persistent

This year’s [western monarch] total is a step in the right direction, but still indicates a severe population decline. Now more than ever, we have an opportunity to double-down on our conservation efforts. Acting quickly to harness the momentum of this upswing is our best chance at preventing western monarchs and other at-risk butterflies from being lost forever.

Isis Howard, Endangered Species Conservation Biologist, Xerces Society


To Learn More…

…about the reasons for the 2021 surge, check out this report from KQED or this one from Nova

…about the life and loves of the Western Monarch, read this interesting article or check out these Notebook posts: The Disappearing Monarch and Midsummer Monarch Update.

…about neonics and their effects on insects, read this excellent thesis by Kendra Mann.

To Be Completely Amazed…

Watch some videos of monarchs. In this one, witness how a newly emerged monarch inflates its wings with liquid from its abdomen. 

To Do More…


Don’t plant tropical milkweed!


Get involved in data collection. Community scientists provide important information about monarch breeding and survival. You don’t have to be an expert to make a big contribution. Check out this Western Monarch Count website to learn how to get involved. 

Plant native milkweed and nectar plants that are free of pesticides. Plant native milkweed and be sure not to get the tropical kind! For ideas on nectar plants, check out the Calscape website. You can get a list of native plants specific to any address in California, with a detailed description of each plant’s characteristics and needs. I particularly love the fact that there is a search term for identifying plants that are “very easy” to grow.

Here’s a good site to learn more about ecologically sound pest management. 

Make a spot to splash around. Monarchs can’t land on water or drink from a dripping fountain, but they do need moisture and they obtain important minerals from mud. Channel your inner child and create a little monarch play area in a shallow dish. Just add rocks and a little bit of soil, along with just enough water to keep everything wet. 

Keep your eyes peeled. The Western Monarch Mystery Challenge is about to get underway. Running from February 14 (Valentines Day) to April 22 (Earth Day), the challenge is a call to action to report a monarch if you see one. You can even win a prize! Read about it here.

Make some noise! Contact your elected officials and ask them to support the MONARCH Act which would provide critical funding for habitat restoration for the western monarch. Introduced in the House of Representatives in March of 2021 by Jimmy Panetta, it remains in the first stage of the legislative process.

Insects can be amazingly resilient if we give them a chance. Everyone has a role to play, whether that’s adding pollinator plants and avoiding pesticides in your home garden or advocating for monarch-friendly policies within our neighborhoods, public lands and plant nursery and agriculture providers.

Xerces Director Scott Hoffman Black.

That’s it for this installment of the Notebook! Special thanks to Mia Monroe, co-founder and coordinator of the Western Monarch Thanksgiving Count, for her advice on this post as well as her valuable work on behalf of the monarchs!

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