Baseline Pollinator of the Month – Two-tailed Swallowtail butterfly

June 2, 2022

Written by Amy Yarger, Butterfly Pavilion Horticulture Director

There are certain garden stories that I hear so often, I can recite them from memory. This year, I predict that at least one person will tell me about the monarch butterfly that visits their garden every day at the same time. I will nod with interest until they conclude with, “But a yellow monarch, not one of the orange ones.” This is my cue to share the wonder of Colorado butterflies – we have more than monarchs. In fact, Colorado boasts over 230 species of butterfly, and our largest (and one of our most reliable) is the two-tailed swallowtail (Papilio multicaudata).

I can see why people confuse this swallowtail with the fabulous monarch – both are extremely showy. However, the two-tailed swallowtail is larger, with a wingspan of up to 6 inches, and has yellow wings with black tiger stripes and 2 “tails”. A closer lookalike butterfly is the western tiger (Papilio rutulus), but the western tiger only has 1 tail per wing and tends to fly at higher altitudes. The two-tailed swallowtail often flies in neighborhoods, parks, streams and other developed areas at lower elevations in western North America – anywhere where their favorite larval food plants, ash trees or chokecherries, grow.

One reason these butterflies are so near and dear to people’s hearts is their habit of “patrolling” in late spring, when males follow a repeated route on their search for available females. You can almost set your watch by them when they are on patrol. Once mated, females lay their eggs one at a time to limit competition among siblings. The young caterpillars look like glossy, shiny bird poop at first, and to protect themselves from predators and weather, they fold the leaves of their host plant and tie them together with silk they make themselves. There they can eat in comfort as they grow and morph into big green caterpillars with eyespots behind their heads. These eyespots give these harmless caterpillars a snakelike appearance. If the scary eyespots don’t fool the birds and wasps, the threatened caterpillar will exude an unpleasant fragrance through horn-like organs called osmateria. Just imagine if your breakfast suddenly sprouted stinky horns- that would make me drop my toast fast!

Late in the summer, you may see these caterpillars, traveling across sidewalks or garden paths, looking for a safe place to spend the winter. Once they find a sheltered area, often at the base of a tree, they transform into a brown pupa that is held upright by a slender silken tether. If you didn’t know any better, you’d think it was just an extra piece of bark. There they will lie dormant until the following spring.

Unlike the migrating monarch, two-tailed swallowtails spend the winters here and are adept at surviving our sudden weather changes. They will find shrubs and trees to hide in on cold or blustery days, then hydrate on hot days by visiting lots of flowers. Many people express concern about butterflies in May and wonder if bringing them inside is the right thing to do. For the most part, these wild winged creatures have behaviors that will keep them safe and it’s better to let them fly free.

If you are as enchanted with two-tailed swallowtails as I am, you can help them by planting their favorite late-spring blooming flowers. Some of their spring favorites include showy milkweed (Asclepias speciosa), Rocky Mountain penstemon (Penstemon strictus) and lilacs (Syringa spp.). You can also help them during their quiet season by leaving leaf litter and other debris in place through the winter while they are pupating. Finally, just as with other butterflies, avoiding pesticides will protect these sensitive flower visitors. With the spread of the emerald ash borer (EAB) in Colorado, there has been more concern about the safety of the swallowtail’s ash hosts. The use of systemic pesticides in ash trees can kill swallowtail caterpillars as well as the invasive beetle pest. I recommend removing ash trees whenever possible and replacing them with the other host plant species for this butterfly such as chokecherry (Prunus virginiana) and hoptree (Ptelea trifoliata).

During these past two years, when it’s been so difficult to get out and see our loved ones, there’s something reassuring about building community with our neighbors. The two-tailed swallowtail reminds us that some of our tiniest neighbors may look like bird poop or glide gracefully on golden wings, but they always manage to connect us to home.

Want to learn more about pollinators and how to help them? Contact Butterfly Pavilion’s horticulture department at!