Beautiful swallowtail butterflies like this eastern tiger swallowtail seem to be less common this year in parts of the DMV. Photo credit Michael Raupp
Five years ago in an episode entitled “Missing bugs of the week: Swallowtail butterflies, Papilionidae” we explored some of the reasons pollinators in general and butterflies in particular are imperiled in the DMV and around the world. Last month a concerned nature enthusiast remarked “normally by this time of the year our meadow would be full of butterflies… even a monarch or two…. This year there’s virtually none and I hear it’s that way throughout the county.” In a sample size of one, in my own flower garden in Columbia, MD, thus far this season it has been a rotten one for large butterflies like swallowtails and monarchs that usually are plentiful by now. My barometer for gauging butterfly abundance is a large native composite known as cup plant (Silphium perfoliatum). In years past during the last weeks of July and first weeks of August, I have seen more than a dozen swallowtails simultaneously enjoying nectar treats on my cup plant. To date only a single tiger swallowtail has visited my cup plant. How disappointing is that?
Over the last few weeks, folks expressed concern over the absence of swallowtail butterflies in their gardens and meadows. I’ve had the same impression. This video from 2019 shows dozens of tiger swallowtails visiting my cup plant in late July. Fast forward to the same time this year when only a single tiger swallowtail has stopped by. Is this just annual variation in weather conditions or signs of larger issues related to climate change, development, and other sources of butterfly decline? Maybe missing butterflies is just a local suburban thing. Earlier this year on the banks of the Potomac, zebra swallowtails, tiger swallowtails, and other swallowtail species were common gathering minerals from the mud. And in my gardens skippers and cabbage butterflies showed up in force right on time. I’m not really sure what’s up with the swallowtails but I hope a second generation of swallowtails and maybe some monarchs make an appearance soon.
How are butterflies fairing around the country? A recent article in the Proceedings of the National Academy of Sciences provides some clues as to why butterflies and other pollinators are becoming harder to see and why they may be declining in the United States and Canada. Tara Cornelisse and 14 colleagues found that roughly 26% of vertebrate and invertebrate pollinators are at risk of extinction in North America north of Mexico. When it comes to butterflies, of the 632 butterfly species studied, almost 20% are at risk of extinction. Of these approximately 8% of charismatic swallowtails are at risk. Yikes!
While scarce at my home in Columbia, several species of swallowtails frequented the muddy banks of the Potomac River to gain nutrients. Photo credit Michael Raupp
What are the underlying ecological threats to pollinators in general and butterflies specifically? We have known for decades that changes in land use patterns associated with urbanization around the world are responsible for dramatic losses of several insect species, including butterflies. In addition to urban development, the study by Cornelisse et al. ranked climate change, invasive species, changes in the natural patterns of water movement and storage, and changes in wildfire regimes as key risk factors for butterflies and moths. On a national level pollution also imperiled all pollinators in several eastern states.
Findings of this study send a warning regarding the overall fate of pollinators in our rapidly changing world. However, declines in the abundance of butterflies from one year to the next are often linked to more immediate ecological events such as annual variation in weather. Elevated temperatures are known to reduce the survival of caterpillars. Drought is known to reduce the abundance and nutritional quality of some plants. One butterfly expert suggested that last summer’s extended heat and drought may have negatively affected the survival of butterfly larvae, thereby reducing the populations of butterflies in 2025. Perhaps, our unusually soggy spring weather favored pathogens lethal to butterfly larvae or pupae.
Pretty silver spotted skippers showed up right on schedule in my garden. Photo credit Michael Raupp
Should we fear that the apparent scarcity of swallowtails in 2025 in my garden in Columbia portends a pending butterfly apocalypse? Maybe not. Beyond swallowtails, other species of butterflies like silver spotted skippers and their kin arrived in my garden right on schedule in fine numbers, as did cabbage butterflies. Recent adventures along the C & O Canal this spring and summer revealed several species of butterflies including tiger and zebra swallowtails in good numbers. Rick Borchelt’s butterfly surveys in Allegany and Kent Counties in Maryland revealed solid numbers of dozens of butterfly species. But as I finish writing this episode and look out the window to the cup plant, swallowtails are still absent. However, hope springs eternal in the hearts of bug geeks and maybe an upcoming brood of swallowtails and some late-to-arrive monarchs will stop by garden my before the summer ends.
Acknowledgements
Bug of the Week thanks Paul Bade for his interest in swallowtails that provided the inspiration for this episode. Science writer, butterfly guru, and keeper of the LepLog Rick Borchelt provided great insights and observations of butterflies throughout the region. Many thanks to colleagues in the Department of Entomology, especially Karin Burghardt and Leo Shapiro for providing references and helping clarify several points discussed in this episode. The following fascinating papers were consulted: “Live fast, die young? Day- and night-warming affect the growth, survivorship, and behavior of caterpillars in the field” by Louie H. Yang, Elizabeth G. Postema, Heran Arefaine, Fernanda Y. Cohoon, Emma A. Deen, Yvonne L. Durand, Gwendolyn I. Erdosh, Hailey Ma, Courtney N. Mausling, Sarah Solís, and Madeline R. Wilson, “Western Monarch Population Plummets: Status, Probable Causes, and Recommended Conservation Actions” by Emma M. Pelton, Cheryl B. Schultz, Sarina J. Jepsen, Scott Hoffman Black and Elizabeth E. Crone; “Multiscale seasonal factors drive the size of winter monarch colonies” by Sarah P. Saunders, Leslie Ries, Naresh Neupane, M. Isabel Ramírez, Eligio García-Serrano, Eduardo Rendón-Salinas, and Elise F. Zipkina; “Declines and Resilience of Communities of Leaf Chewing Insects on Missouri Oaks Following Spring Frost and Summer Drought” by Robert J. Marquis, John T. Lill, Rebecca E. Forkner, Josiane Le Corff, John M. Landosky and James B. Whitfield; and “Elevated extinction risk in over one-fifth of native North American pollinators” by Tara Cornelisse, David W. Inouye, Rebecca E. Irwin, Sarina Jepsen, Jonathan R. Mawdsley, Margaret Ormes, Jaret Daniels, Diane M. Debinski, Terry Griswold, John Klymko , Michael C. Orr, Leif Richardson, Nicole Sears, Dale Schweitzer, and Bruce E. Young.