Pollinator of the Week: Western Tiger Swallowtail

This entry is from Lucas Costner, an undergraduate horticulture student at Oregon State University. It highlights a common Oregon pollinator.

A common feature of the urban and suburban landscape, a street lined with large London plane trees (Platanus x acerifolia), complete with lumpy trunks and exfoliating bark, can be a delight to behold. The tree was the favorite species of New York City planner Robert Moses, and became one popular choice for replacing elms affected by the infamous Dutch elm disease during the previous century (1). Chosen for its long life span, interesting growth habit, and ability to withstand the challenges of street life, the London plane has become a ubiquitous member of our cultivated environment. By happy accident, the tree also plays host to the larvae of the familiar and striking western tiger swallowtail (Papilio rutulus) (3).

This large yellow and black butterfly is found throughout the western continental United States and into southern British Columbia. Swallowtails tend to be specialists (2), so the addition of known host trees like the London plane and other members of the Platanus genus, along with willows (Salix spp.), poplars (Populus spp.), and alders (Alnus spp.) is of benefit not only to our parks, streets, and yards, but to these incredible pollinators as well (3). Adult females lay single eggs on the undersides of the leaves of host trees, which then provide food for the developing larvae (4). These larvae eventually form chrysalids and overwinter, emerging the following season as, well, beautiful butterflies (4). While the larvae of the western tiger swallowtail have somewhat specific tastes, the adult butterflies will feast on nectar from many flowering species. These pollinators typically take flight from June through July, but in Pacific coastal areas may be found throughout much of the year (3). They are noted as preferring wetter areas generally, and even as being avid visitors to mud-puddles (5).

Butterflies are of course important to us as gardeners because of the pollination services they provide, but throughout their lifespan they also play an important role as food for other organisms. Douglas Tallamy, from the University of Delaware, writes “if we were forced to care for only one group of insects in our restored suburban ecosystem, we would do well to choose the Lepidoptera” (the order consisting of moths and butterflies) (2). Fortunately for the western tiger swallowtail, and for those of us who enjoy its beauty, our cultivated landscapes should provide ample habitat. With that being said, global declines in flying insects are being documented (see: Insects are In Serious Trouble), and our attention needs to remain fixed as much on what we are planting as on how we are managing these landscapes.

Personally, I wasn’t fortunate enough to see many species of Lepidoptera this past season. I did manage to catch sight of a western tiger swallowtail early in the year, however, at the North Willamette Research and Extension Center foraging on a patch of showy milkweed (pictured above) neighboring the native plants study plot. Being a recent transplant from east of the Rockies, this was my first time ever seeing the butterfly and I was truly blown away. I’m hopeful that despite the challenges faced by these and other pollinators, we will all be able to enjoy them well into the future.

Sources:

1. Jonnes, Jill. Urban Forests: A Natural History of Trees in the American Cityscape. Penguin, 2017.
2. Tallamy, Douglas W. Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens. Timber Press, 2007.
3. Haggard, Peter, and Judy Haggard. Insects of the Pacific Northwest. Timber Press, 2006.
4. “Western Tiger Swallowtail.” Butterflies and Moths of North America, 21 June 2016, www.butterfliesandmoths.org/species/Papilio-rutulus.
5. Layberry, Ross, et al. “Western Tiger Swallowtail.” Butterflies of Canada, Canadian Biodiversity Information Facility, 9 July 2014, www.cbif.gc.ca/eng/species-bank/butterflies-of-canada/western-tiger-swallowtail/?id=1370403265809.