As much as we love watching monarch butterflies fluttering in our gardens and landing on flowers to sip nectar, there is much about monarchs that remains a mystery to both gardeners and scientists. My guest this week, Nancy Lawson, is part of a new citizen science project named Monarch Rx, and she’s here to explain an unexpected way that gardeners can support monarch populations.
Nancy lives in Maryland, where she runs a blog, HumaneGardener.com, and she writes a column for the Humane Society’s magazine, All Animals, on ways that we can be friendlier stewards of land. She also has a book, “The Humane Gardener: Nurturing a Backyard Habitat for Wildlife.” As a naturalist with a background in journalism and an interest in science, Nancy explores how we can work with plants to change the world.
Requirements for Monarch Fitness & Health
Monarch butterflies need nectar plants, especially fall nectar plants like goldenrods and asters, Nancy says. They also require milkweed, the only type of plant that their larvae eat. As monarchs face habitat loss, gardeners can help them by planting more of these types of flowers. Monarch Rx is also uncovering a number of other plants that are important to monarch fitness and health. These plants help the butterflies to live longer and to have greater defenses from predators. As Nancy learned, these plants contain a chemical that is toxic to many butterfly parasites and predators.
Unexpected Monarch Behavior
Nancy recalls in June 2019 taking a photo of a monarch butterfly on a white boneset (Eupatorium serotinum). It appeared to Nancy that the butterfly had just landed there to perch, but then through her zoom lens she noticed that the insect was extending its proboscis onto a leaf. Whenever the butterfly was disturbed by a car going by, it would fly around but always come back to the boneset, ignoring the milkweed and other plants.
After 10 minutes of watching and recording the butterfly, Nancy downloaded the videos and took a closer look at what the butterfly was up to. It was using its feet to scratch a leaf. Though most butterflies have chemoreceptors on their feet and antennae, it was thought that monarchs don’t practice leaf scratching behavior.
After sharing the video to a butterfly group on Facebook, she received a response from Dr. Don Harvey, a retired lepidopterist (someone who collects or studies butterflies and moths) at the Smithsonian Institution’s Museum of Natural History. Don shared a 1983 scientific article on leaf scratching by Michael Boppré, a since-retired professor in Germany who observed that butterflies scratch outward from holes created by flea beetles. Sure enough, when Nancy inspected again she found a flea beetle in the video and holes in the leaves.
The butterfly was engaging in pharmacophagy — the consumption of plants to acquire beneficial chemicals rather than nutrition. Think of it as going to the pharmacy rather than going to the grocery store, Nancy says. Specifically, the butterfly was after pyrrolizidine alkaloids, known as PAs, in the leaves. The toxin accumulates in the bodies of the butterflies, making them unpalatable to many predators.
Many milkweed butterflies (Daniadae) use PAs as a chemical defense against predators and also synthesize PAs into pheromones for courtship, but it was thought that monarchs were an exception. Monarch courtship behavior includes males chasing the females and pinning them down, Nancy says, though she adds there may be other aspects of courtship behavior that we don’t witness.
Nancy dove in. She downloaded as many papers by Michael Boppré as she could and learned all there was to know about PA pharmacophagy. She learned that pharmacophagy often involves dried leaves, withered plants and dead plants. Scientists have actually successfully baited butterflies and moths using dried leaves or vials of concentrated pyrrolizidine alkaloids. Some grasshoppers, beetles and flies are likewise attracted to leaves containing PA.
Butterflies and other insects can detect PAs in a range of wildflowers, such as some species in the dogbanes, borage, aster and legume families.
The Most Valuable Flowers for PA Pharmacophagy
It’s well known among gardeners that monarch caterpillars can only eat plants in the Asclepias genus, known as milkweeds. And by eating milkweed, the caterpillars accumulate cardiac glycosides that make them unattractive to predators. Cornell University Professor Anurag Agrawal wrote at length about this relationship in his book “Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution,” which he discussed with me on the podcast last year.
But less is known about what flowers monarchs rely on to gather pyrrolizidine alkaloids. Michael Boppré is trying to find out which flower species are the most important to the monarchs when it comes to PAs.
Once Monarch Rx was established to help answer that question, the first respondent was Brice Claypoole, a 13-year-old in Florida who runs a nature and conservation blog, The Kid Who Cares. Brice told Nancy that he observed a monarch scratching dead Joy Pye weed.
The reports kept coming: monarchs on Virginia bluebells (Mertensia virginica) in Michigan and on marble seed (Onosmodium) in Iowa.
In all cases, the plants were dead or dying. This makes sense, as Nancy points out that PAs are not detectable in plants until the cells start to break up, because the PAs are concealed within the cell walls before then.
The Possible Link Between PAs and Monarch Eggs
One theory is that monarchs gather PA as a defense against Ophryocystis elektroscirrha, or OE, a parasite that can cause monarchs to be deformed and weakened. The OE spores are most likely to infect caterpillars in their early stages of life.
Nancy has observed both male and female monarchs scratching for PAs. It was thought that only male milkweed butterflies gather PAs, but females were gathering too, perhaps to pass the PAs to eggs. The first-stage larvae may get protection by eating their own PA-containing eggshells.
What to Report to Monarch Rx
When citizen scientists report observing monarchs gathering PAs, there are a number of questions they are asked: What were the weather conditions? Did it just rain? What’s the condition of the plant? What plants are growing around it? How many monarchs are males and how many are females?
By better understanding what plants monarchs rely on, we can make better plant choices to benefit wildlife. It will be some time before studies can confirm the best plants for PA pharmacophagy for monarchs, but a lesson that we can put to use right now is that tidying up our gardens is not what’s best for insects. Beneficial insects — not just butterflies — use spent plants for food, habitat and (now we know) pharmacophagy.
I hope you found my conversation with Nancy Lawson interesting and enlightening. If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.
Do you see monarchs in your garden? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
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joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges.
The Humane Gardener – Nancy’s blog
“The Humane Gardener: Nurturing a Backyard Habitat for Wildlife” by Nancy Lawson
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.