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The EFM Notebook

A Commentary on What’s New and Newsworthy

by Susan Holloway | Bio

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Midsummer Monarch Update: How Are They Faring?

21 Jul 2021 8:22 PM | Gayle Marsh (Administrator)
  A flutter of Monarchs

Monarch enjoying a mud puddle

Pretty much everyone knows what a Monarch butterfly looks like (i.e., gorgeous) but here are some other cool things you may not know:
  • A group of Monarch butterflies is called a flutter. 
  • A Monarch flaps its wings five to 12 times per second, approximately 720 times per minute. 
  • After eating milkweed leaves for up to two weeks, Monarch caterpillars weigh 3,000 times what they did as an egg.
  • Monarchs cannot land on water to drink; rather, they sip liquid from muddy soil, behavior known as “puddling.” They obtain not only moisture from this brew, but also salts and other dissolved minerals. 

Bad News, Good News about Monarch Butterflies

The bad news is that Monarchs have been disappearing from the West Coast at an alarming rate over the last 40 years. Reasons for their decline include loss of breeding and overwintering habitat, climate change, pesticides, and natural predators.

The Monarch population in California and Baja California has dropped from 4.5 million to 2,000 in 40 years.

The good news is that many individuals and groups have rallied to the cause of saving the Western Monarch from extinction. Two great organizations focused on education, advocacy, and research are the Xerces Society and Monarch Joint Venture. Locally, the Pollinator Posse has been working effectively to educate the public and create habitat for butterflies, as has the Environmental Action Committee of West Marin. 

The movement to recover Western Monarchs is multi-pronged. In Marin, there has been a significant push to encourage residents to plant native milkweed, essential to the survival on Monarch caterpillars, and to stop using insecticide to control garden pests. Read more about these efforts in an earlier Notebook post

Habitat Improvement in Agricultural Lands

  Pollinator habitat on McCracken Family Farm in the Central Valley (source: Monarch Joint Venture website)
Let’s focus briefly on recent attempts to improve habitats other than the home garden. For example, the Monarch Joint Venture recently initiated a free seed program to enhance the habitat for pollinators on working lands in California. Farmers and ranchers can apply for technical assistance in the planting of native wildflower seed mixes and milkweed plugs provided by the program. This year’s participants are preparing their sites this summer and will plant the seeds and plants in the fall. While drought conditions may pose challenges, the program offers a compelling blueprint for creating pollinator habitat on agricultural lands.

The Mysterious Disappearance of the Monarch Caterpillar

What is happening to the caterpillars?  
In contrast to these positive developments, other Monarch news from the Bay Area is not so positive. This summer, local organizations and residents have noted that the number of Monarch caterpillars is lower than expected, especially given all the effort to improve local habitat with milkweed and nectar plants. It appears that eggs are being laid and larvae are hatching, but then not surviving more than a few days. 


Risk to Monarch Caterpillars from Predators

Paper wasp consuming a Monarch caterpillar

Two of the caterpillar’s most deadly predators are the paper wasp and the yellow jacket. Most of us are familiar with yellow jackets, a predator wasp with a nasty stinger. If you’ve ever been to a barbecue, you know that they love meat. Ordinarily, if they don’t have access to a hamburger, they eat insects. This summer, with hot days and no rain, yellow jackets are having a hard time finding water and food. Our green backyards have become having a particularly attractive alternative to the natural spaces they ordinarily depend on for sustenance, particularly if they are full of milkweed-munching caterpillars.

European paper wasps may also be a factor in the loss of Monarch caterpillars this summer. First reported in North America in the 1970s, these insects are now widespread in urban areas. They have a strong proclivity to nest in sheltered places around buildings. Research suggests that urban gardens with lots of milkweed and pollinator plants may unintentionally create an “ecological trap” in which the congregated butterflies are vulnerable to predation by paper wasps who already inhabit the area. This may be at work in Marin, although we need more evidence to substantiate this hypothesis.

  Small Milkweed Bug
The Small Milkweed Bug is a common visitor to milkweed plants in Marin. They are mostly herbaceous, extracting nectar from flowers and feeding on milkweed seeds. However, they sometimes feed on honeybees and monarch caterpillars when other forms of food are scarce. Perhaps the current drought, and associated lack of vegetation, has pushed them to carnivorous behavior. 

What can you do to mitigate the danger these predators pose to Monarch caterpillars? Yellow jackets are carnivorous, so don’t leave pet food outside and be sure garbage is contained in tightly sealed cans. If you find a wasp or yellow jacket nest, you can try to remove it yourself, or call the Mosquito & Vector Control Association of California at (916) 440-0826 for assistance. For other ideas, take a look at these suggestions from the UC Integrated Pest Management Program

Risk from Parasites and Parasitoids

Monarchs exposed to OE parasites often have crumpled wings  
Parasites are smaller organisms that live and multiply inside their hosts, taking nutrients and resources from them. 

Perhaps the most-studied parasite of Monarchs is Ophryocystis elektroschirra (OE). OE infections occur when a caterpillar ingests OE spores that have fallen from an infected adult butterfly onto a milkweed leaf. The spores then take up residence in the caterpillar’s gut.

Caterpillars with a particularly high dose of OE parasites are likely to die before reaching the pupa state. Surviving infected adults often have difficulty emerging from their pupal cases and expanding their wings. They are generally smaller and shorter-lived than uninfected adults.

One third of Western Monarchs are heavily infected with OE. Generally, non-migrating Monarchs are much more heavily infected than those that migrate to an overwintering site. Access to non-native tropical milkweed, which flowers throughout the year, reduces the need to migrate, suggesting that it is not a good choice for Bay Area gardens. 

Monarch on native milkweed
(photo source: Xerces Society)

Tachinid fly..a face only a mother could love
Parasitoids are specialized insects that lay eggs on or inside another insect, which then develop by consuming the host. It’s a sweet deal for the parasitoid. It has a nice, protected place to grow and develop, as well as a reliable source of food. 

One parasitoid that loves Monarch caterpillars is the Tachinid fly, which resembles a large house fly. Female Tachinids lay eggs on Monarch caterpillars. One study of a common Tachinid parasitoid found it in about 13% of wild Monarch caterpillars

It’s hard to appreciate predators, parasites, and parasitoids when we suspect them of scarfing down Monarch caterpillars. However, these “natural enemies” offer a sustainable way of controlling garden and agricultural pests compared to using pesticides. It is just with respect to the Monarch caterpillar that these creatures are on the wrong side of justice.


Risks from Pesticides

This label is better than nothing, but doesn’t warn the consumer that neonics are harmful

The most persistent risk to Monarchs comes from neonics, a class of insecticides widely used in agriculture and landscaping. Neonics are applied to mature plants or to seeds, which absorb the pesticide. The concentration of neonics in products sold for residential use is approximately 30 times the allowance permitted in the agricultural sector.

Neonics were initially marketed as being less harmful than other insecticides, but we now know their devastating impacts on pollinators and beneficial insects. Experimental studies as well as those conducted in agricultural settings focusing specifically on Monarch caterpillars show that their growth and survival is adversely affected by exposure to neonics.

Pressure from consumers and conservation organizations has led some large retailers to label plants treated with neonics. But the hope is that they will cease selling this type of plant altogether.  

More Ideas for Action

Become a Monarch Parent? Given the high mortality of Monarch caterpillars, it is tempting to gather eggs and bring them inside where you can nurture them throughout their development. Indeed, Monarchs reared in this way are much more likely to survive than those in the wild. However, there are serious drawbacks to captive rearing, and it is not recommended by Xerces or the Monarch Joint Venture. Here’s a good summary of the issue. While they do not endorse large-scale attempts at captive rearing, MJV gives the OK to people interested in rearing them “for enjoyment, education, or community science.”

Hands-on learning!
Western Monarch Count regional coordinator Mia Monroe and volunteers (Photo: Carole Fitzgerald) Don't plant tropical milkweed!

Teach others. If you work with (or have) elementary-aged children take a look at this great toolkit from the World Wildlife Fund. It is well-written and full of ideas for activities. Journey North also has some great resources for kids. 

Get involved in data collection. Citizen scientists can provide important information about monarch breeding phenology. You don’t have to be an expert to make a big contribution. You can report your monarch adult, caterpillar, egg, and milkweed sightings to an easy-to-use site managed by one of the following organizations: Western Monarch Milkweed Mapper, iNaturalist, Journey North, or the Pollinator Posse.
 A puddling area for Monarchs

Plant native milkweed. Ask your local nursery to start supplying native milkweed and pollinator plants that are free of insecticides. Organize a group to collect and propagate milkweed seed. Plant native milkweed and be sure not to get the tropical kind!

Get into puddling. Monarchs obtain moisture and important minerals from mud. Create a puddling area for Monarchs by digging a wide, shallow depression in the ground and lining it with plastic weed barrier or pond liner. Or just use a shallow dish. Then add a 1-2 inch layer of landscape sand mixed with soil, along with just enough water to keep it wet. You can also put in a few rocks to serve as landing and basking places. 

Get help! The Pollinator Habitat Help Desk line offers anyone in the United States personal recommendations and answers to your pollinator questions. Give them a call at 833-MILKWEE (833-645-5933) between 9 and 5 PM Central Time, or send an email to

That’s it for this installment of the EFM Notebook! Thanks very much to Mia Monroe, Wynter Vaughn, and Alice Cason for their helpful suggestions on this post. And thanks to Rob Badger and Nita Winter for sharing Rob’s beautiful image for the Notebook banner. Check out their award-winning book or visit their website to see more!

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