Got milkweed for monarch caterpillars? Check. Nectar plants that feed the adults all season long? Check. Dead and dried plants that contain pyrrolizidine alkaloids? … Say what?

Monarchs need milkweed and an abundance of nectar plants: we all know that, right? But did you know they likely benefit from substances they take up from certain dead and decaying plants too?

If I hadn’t seen it with my own eyes, I wouldn’t have either. But for the past two summers, monarchs visiting our habitat have introduced me to a whole new world of mysterious behaviors. What started as aimless meanderings around the garden led me on a much broader journey and an overseas collaboration with entomologists Michael Boppré in Germany and Dick Vane-Wright in the UK. The result is a paper in Ecological Entomology, “The puzzle of monarch butterflies (Danaus plexippus) and their association with plants containing pyrrolizidine alkaloids,” and the creation of a community science project, Monarch Rx.

The First Sighting

It all began one warm June evening in 2019, when I was wandering around the front patch and noticed a monarch sitting on a leaf of late boneset (Eupatorium serotinum). Assuming he was resting, I lazily lifted my camera. That’s when the zoom lens showed me this was no perching butterfly: The monarch was busy sinking his proboscis into the leaf, rubbing it with one of his feet, and flying up when a car passed before quickly returning to his task.

A butterfly probing a leaf instead of a nectar-filled flower? What could he possibly hope to gain from that? I posed the question on a pollinator Facebook group and received a quick reply from lepidopterist Don Harvey, who shared a link to a 1983 paper by Michael Boppré that explained it: Many clearwing and milkweed butterflies gather pyrrolizidine alkaloids (PAs) from dry or injured leaves for use in male courtship, pheromone production and defense. They do this by first applying a fluid from their proboscises that will dissolve the PAs into a liquid they can imbibe.

But the more papers and books I read, the more perplexing my observation seemed. If I hadn’t obtained photographic evidence, I would have begun to doubt what I’d observed. Though PA-gathering is common among a number of species, the literature is full of statements declaring that monarchs don’t engage in it. A passage in one recent book states that though virtually all studied butterfly species in the milkweed butterfly tribe, Danaini, sequester PAs, monarchs are the outlier. Authors of a study published last year declared that butterflies in the Danaus genus in the Americas, which includes monarchs, “are not attracted to vegetative PA sources.”

If that were really the case, the reasons seem sensical enough: Monarchs are thought to already have all the chemicals they need from milkweed. They ingest and sequester protective cardenolides from the plants they eat as caterpillars, and they aren’t known to use pheromones in courtship the way so many of their relatives do.

When I first read about PA-gathering in Boppré’s papers, most striking to me were the contextual similarities, even down to the role of flea beetles. On leaves of Heliotropium species in East Africa, he explained, holes made by flea beetles had released the chemical cues necessary for attraction and made the PAs accessible to several butterfly species. When I re-examined my videos, sure enough, a flea beetle had been walking the same hole-filled leaf that the monarch was busily probing. (See the tiny beetle make his appearance at the two-second mark in the clip above.)

Further reading revealed that past sightings of monarchs gathering PAs from non-flowering plant parts do exist, and the phenomenon is likely common. But it’s rarely reported, confined to a handful of observations worldwide, including only two in the U.S.: one in Florida at a bait trap made of certain dry plants in the 1970s and one in Missouri on the roots of an upturned nonnative pond plant about a decade ago.

Intrigued, I wrote a blurb about it, added it to my presentations as yet another illustration of the many unknown and mysterious roles plants and animals play, and put it aside for awhile when I got busy with other projects.

Another Year, Another Sighting!

But the monarchs weren’t going to put the matter aside. It turns out that during that summer of 2019, they were just gearing up for the real show. In August 2020, on a walk to the mailbox, I came across a monarch on another boneset—a completely dead one I’d been meaning to cut down—at the corner of the driveway and the road. This time I knew enough about PA-gathering to want to document as much as I could. And to my surprise, the monarchs were again breaking barriers of scientific knowledge. This one, it turned out, was a female!

Everything I had read suggested that when a species does gather PAs from leaves for defense,  the behavior is usually confined to male butterflies. What was a female doing gathering PAs?

More than half an hour went by as I snapped photos and recorded video of her intently probing both the leaves and the stem. When cars passed, she took flight only a few feet away and then returned quickly to the task at hand.

Plants in a number of genera are known to contain pyrrolizidine alkaloids; in the U.S. they include Artemisia, Amsinckia, Crotalaria, Eupatorium, Hackelia, Heliotropium, and Senecio. PAs are secondary plant metabolites, chemicals produced to aid in survival, giving plants a competitive edge or protecting them from being eaten. Often studied for their toxicity to livestock and humans, PAs also make plants unappealing to deer, rabbits and other mammalian herbivores. But many insects have found ways to use these and other secondary plant metabolites to their advantage, often ingesting them while eating leaves or nectar.

What distinguishes the leaf-probing behaviors I witnessed in my garden from other forms of PA intake, though, is that the monarchs were not actually eating while collecting the chemical compounds. As I would later learn from Boppré, this is called pharmacophagy, a term referring to the gathering of beneficial substances for reasons other than nutrition.

The monarch near the mailbox was also near milkweed, but I didn’t see her go to its flowers—or any other flowers—during the half-hour I watched her. Her focus was singleminded and intense as she made her way around the dead plant. I was so glad I hadn’t cut it down.

And Then There Were Two

Nine days later I meandered toward the back one afternoon and couldn’t believe my eyes. On another half-broken boneset I’d been considering trimming back—it marred the view of the garden, after all—both a male and female monarch were working away at the drying, crinkly leaves. That was more than enough evidence to convince me that this behavior was no anomaly. There had to be more monarchs on more PA-containing plants in more habitats than just our two suburban acres.

But why hadn’t anyone else ever reported this phenomenon in the United States, except once at a bait site and once on roots of an exotic plant? And what were the monarchs doing it for, since they had already sequestered powerful defense chemicals during their days as caterpillars? Did it have something to do with the type or quality of the milkweed they’d eaten back then? Was the monarchs’ collection of PAs a protective response to the presence of new predators or parasitoids  in the environment? Had this been happening for years, but we just hadn’t noticed?

I wanted to pose these and many more questions to monarch scientists whose work I had read, but, to my surprise, my outreach received a lukewarm reception. One said she’d never heard of it before, though she thought my photos were convincing. Another said it was likely a vestigial behavior, reflexive but not important. That explanation didn’t sit right with me given how long the monarchs had spent on this activity. Insects don’t have time or energy to waste; why would they spend at least half on hour on something irrelevant to their survival?

When I contacted Bob Robbins, curator of Lepidoptera at the Smithsonian, he welcomed a conversation and offered a helpful primer on the state of knowledge about PA-gathering by butterflies. He encouraged me to publish my observations and also mentioned more of Boppré’s papers.

Just at the point when I thought I may have followed the PA scent trail as far as it would go, I received an email back from the source himself, Michael Boppré. He was at least as excited as I was, explaining that he was in the process of writing a review of the science on insect PA-gathering and was surprised by the lack of observations in the U.S. He’d looked at my Dropbox folder full of photos and videos and knew there was more to the story. He suggested that we write a paper, sent me a draft for input, and recruited the help of Vane-Wright in England. Many emails, phone calls and revisions later, we had a fascinating paper in Ecological Entomology that provides scientific context for the phenomenon and poses exciting questions, many of which need to be answered by scientific natural history studies.

You Too Can Make a Discovery!

And that’s where you come in. The Monarch Rx community science project aims to capture as much information as possible about monarchs and PAs: when and where monarchs are seen gathering them from leaves, what plants they are using, weather conditions and more. The more details we obtain, the more we can understand about monarch needs. To encourage participation, we’ve detailed a series of intriguing questions in a one-page primer that summarizes the paper and the goals of Monarch Rx. We also explain the concept of “baiting,” which involves using already dried PA-plants as a way to more closely monitor and understand the conditions under which monarchs are attracted to them.

You may have even seen this phenomenon before and not known what it is; in that case, we’d like to hear about those observations too so we can understand more about which plants the monarchs are using. I don’t know why this behavior has gone so unnoticed until now, but I have my suspicions. Even if people do come across monarchs on leaves, they might assume, as I initially did, that they’re just perching. They’re more likely to direct their cameras toward caterpillars, females laying eggs, and adults nectaring on beautiful wildflowers. And PA sources might be limited in the first place, at least in managed landscapes, where anything decaying and brown is usually cut down.

By letting go of our need for so much tidiness and reshifting our line of sight, maybe we can channel our inner monarchs—and start to notice and nurture more of what we’ve been missing. Who knows what else we might discover?

Check out the Monarch Rx project on CitSci.org, where you’ll find links to the paper, a one-page primer about the topic, and a place to put on record the many observations we hope you’ll make. Happy observing!

Quick Links to More Info:

Read the paper in Ecological Entomology: The puzzle of monarch butterflies (Danaus plexippus) and their association with plants containing pyrrolizidine alkaloids

Read the paper in News of the Lepidopterists’ Society: Monarch butterflies gather pyrrolizidine alkaloids from dead and injured plants: a call for citizen science contributions

Read the article in Entomology Today: New Citizen-Science Project Explores Little-Known Behavior in Monarch Butterflies

Read the article in the Wild Ones Journal: Monarch Rx: Butterflies Obtain “Drugs” from Withering Plants

Watch the webinar hosted by the Natural History Society of Maryland: Monarch Rx: Exploring a Little-Known Behavior of a Beloved Butterfly

Listen to the podcast with Joe Lamp’l of Joe Gardener: Monarch Rx: The Prescription for Healthier Butterflies

Read the primer: Summary and Fact Sheet about Monarch and PAs

Record your observations: Monarch Rx

Photos and videos by Nancy Lawson/The Humane Gardener