New Citizen-Science Project Explores Little-Known Behavior in Monarch Butterflies
By Leslie Mertz, Ph.D.
Monarchs may be the most widely known butterflies in the United States, but one of their behaviors—although exhibited in plain sight—has gone nearly unnoticed. That changed in 2019 when Maryland gardener and nature writer Nancy Lawson spotted one of the large, orange-and-black butterflies doing something odd. Rather than engaging in the predictable feeding on a milkweed flower, this male monarch (Danaus plexippus) perched on a boneset plant (Eupatorium serotinum), scratching at a withered leaf and then extending its proboscis onto the scratched area. “It just didn’t follow the pattern,” she says.
After posting a video of the behavior in an online pollinator group, Lawson heard about a more-than-30-year-old research paper that reported similar behavior in other milkweed butterflies. She filed the information away, but, when she again spied monarchs scratching and sipping at boneset leaves in her garden in 2020, she decided to track down the paper’s author, ecologist Michael Boppré, Ph.D., at the Albert-Ludwigs-University of Freiburg in Germany.
Boppré was intrigued by the observation, especially since little is known of this peculiar behavior in monarch butterflies. “I am just very surprised that Americans have almost completely neglected or forgotten about this intrinsic part of the lives of monarchs,” he says. In answer, he and Lawson not only collaborated on a newly published article that both reviews the context of the behavior and raises questions, but they have also just launched “Monarch Rx,” a citizen-science project to gather information about this facet of monarch natural history.
So, why do milkweed butterflies scratch on and sip at leaves of non-milkweed plants? They are searching for pyrrolizidine alkaloids (PAs), compounds known as secondary metabolites that are present in the cells of certain plants, Boppré says. Such plants include boneset and other wildflowers in the aster family and various additional plants notably in the borage and legume families. To access the PAs, the butterflies either regurgitate saliva on the dry plant tissue, which extracts PAs from the cells, and finally suck up the liquid, or they take up juice from injured plant parts.
While the PAs provide no nutrition to the butterflies, they do have benefits. In most milkweed butterflies (grouped together in the tribe Danaini), it is the males that almost exclusively engage in PA gathering, and they use the PAs to synthesize a pheromone to entice females during their rather elaborate courtship routines. But this is not the case with monarchs, Boppré says. In monarchs, the males essentially bypass any sexual prelude and instead chase down females for mating, so they have no need for PA enhancement of pheromones.
A general benefit of PAs is defense against attackers, he says. Just as milkweed plants contain chemicals (called cardenolides) that make monarch caterpillars and adults unpalatable to birds and other predators, boneset and other plants contain bitter-tasting PAs, and PA-sequestration protects Danaini (and various other insects, too) against predation. In addition, studies of the butterflies Danaus gilippus of the southeastern United States have shown that the male transfers PAs in his spermatophore (a sperm-containing packet) to the female during mating and thus ultimately delivers PAs and their predator-fighting attributes not only to the female but also to her eggs, Boppré says.
“With the monarch, however, there is such limited scientific literature about PAs that we don’t even know which plants are involved, whether both sexes visit PA plants equally, and if PAs are gathered regularly,” he says. “It is because of questions like these that we wrote the article and why we have now started this citizen-science project.”
The Monarch Rx citizen-science project kicked off in June 2021 and encourages the public to submit observations of PA-gathering behavior in monarchs, along with photos and pertinent details (e.g., date and time, type of plant, sex of monarch). “This natural-history information will hopefully give us an overview of which plants are involved, how often monarchs gather PAs, and whether they do it regularly or only facultatively in certain circumstances, so these are observations that need to be done,” Boppré says.
Simple monitoring studies that capture monarchs in traps baited with a PA-containing plant would also provide valuable information, he says, adding, “We need data both from the field and from the lab on this intrinsic aspect of monarch’s lives that potentially increases their fitness and longevity but has so surprisingly been overlooked.” He also hopes the article and the Monarch Rx effort will spur monarch researchers to consider the butterfly’s association with PA plants in their studies, noting that PAs might also work like a drug in monarchs to curtail infection from the microscopic protozoan parasite Ophryocystis elektroscirrha.
For Lawson, the connection and collaboration with Boppré have made her an even more resolute supporter of native-plant gardening and of giving the native plants—like boneset—space to flourish. She says, “One of the things that this has reinforced for me is that we have to let things develop naturally sometimes, because you never know who’s going to join the party in your garden.”
She adds, “I have to say I’ve been very surprised that here’s this iconic monarch butterfly that everyone thinks they know everything about, and there’s this whole new world that really hasn’t been explored at all. I am fascinated by it.”
Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.