Black Swallowtail


~ Profile of Pollinators and Backyard Wildlife ~

 

Black swallowtail caterpillars have been called parsley worms, parsnip worms, dill worms, etc. because they are often found on these plants in the garden. Younger caterpillars look like bird droppings, but older caterpillars take on a distinctive striped appearance. Photo credit: Judy Gallagher, cc-by 2.0 

The black swallowtail (Papilio polyxenes) can be found throughout most of the eastern U.S. and parts of the southwestern U.S. They tend to be slightly smaller than the more familiar tiger swallowtails. Black swallowtails are most often found in open areas such as fields, meadows, and yards. It is rarely found in more wooded areas, and is thus one of the many species that rely on open grasslands and prairies.

This is one of the rare butterflies where the caterpillars may be more familiar than the adult butterflies. Like most swallowtail caterpillars, younger black swallowtail caterpillars look like bird droppings. Older black swallowtail caterpillars are striped with alternating black bands containing 6 yellow dots and solid bands of green or white. The reason that the caterpillars are so well recognized is because their primary host plants are plants in the carrot family including many garden favorites such as carrots, dill, and parsley. Away from garden settings, the caterpillars may be found on native members of the carrot family like golden Alexander or introduced members such as Queen Anne’s lace, poison hemlock, or water hemlock.

Female black swallowtails are primarily black with iridescent blue near the bottom edges of the wings. They may or may not also have two relatively faint bands of yellow dots. Other common species of butterflies that they may be mistaken for include pipevine swallowtails, spicebush swallowtails, and the dark form of the female tiger swallowtails. Photo credit: Paul VanDerWerf, cc-by 2.0 

Adult black swallowtails are sexually dimorphic meaning that males and females look different, at least when their wings are open. The upper surface of the wings of a male black swallowtail is mostly black with two yellow bands of yellow – one relative broad band and one thinner band near the edge of the wings. Sometimes, males also have a little bit of blue between the yellow bands. The upper surface of the wings of a female black swallowtail is mostly black with a good-sized band of blue on the back wings. Some females have smaller yellow bands in the same location as the males, but they tend to be much lighter and not nearly as wide. Many females lack any yellow. Both sexes have a black body with a row of yellow dots running down each side right above where the wings join the body.

Black swallowtails, especially the females, mimic the coloration of pipevine swallowtails which are toxic to birds. When a black swallowtail closes its wings, both sexes look very similar to a pipevine swallowtail. However, when their wings are open, male black swallowtails lose the advantage of mimicry because pipevine swallowtails lack the yellow stripes. (The yellow stripes on the males appears to help establish their hierarchy among other male black swallowtails.) In addition to the pipevine swallowtail, female black swallowtails can also be mistaken for spicebush swallowtails and the dark form of the female tiger swallowtails, as well as a few other species that are less common in this region.

Male black swallowtails are primarily black with two yellow bands – one relatively broad and one thinner band. Sometimes they have some blue in between the bands, but it is relatively little compared to the amount seen on the females. Photo credit: Judy Gallagher, cc-by 2.0 

In Kentucky and all of the surrounding states, black swallowtails can have 3 generations per year. In each generation, the males tend to emerge first and set up territories where they will wait for potential mates to fly through. These territories seem to serve primarily as display locations and don’t necessarily contain any resources that the female may use. Both the males and the females may mate multiple times with multiple different individuals. After mating, the female will begin laying eggs. Each egg will be laid individually on a host plant, often near the ends of the leaves. Each individual female may lay 200-400 eggs, although they tend to spread those eggs out over a relatively large area. You aren’t likely to be competing with hundreds of black swallowtail caterpillars for the parsley growing in your garden.

The last generation of black swallowtails each year will overwinter as a chrysalis. During most of the summer, black swallowtail chrysalises are green to blend in with all of the green leaves and other vegetation. However, as the day length gets shorter and the last generation gets ready to overwinter, the chrysalises that the black swallowtails make will be brown instead of green in order to more easily blend in with the dead leaves and vegetation throughout the winter.

The fact that black swallowtails prefer open areas, means that this is a butterfly that can be attracted to our gardens and yards. If you want to attract black swallowtails, make sure you have plenty of “baby food” in the form of dill, parsley, carrots, etc. in the garden (or grow golden Alexander if you prefer a native plant). You’ll also want to have a wide variety of flowers available throughout the season because adult black swallowtails will drink nectar from many different types of flowers including milkweeds, clovers, phlox, ironweed, and members of the carrot family.

 

 



Shannon Trimboli is a beekeeper, farmer, wildlife biologist, and author. She owns Grassy Roads Farm and Busy Bee Nursery & Consulting. Busy Bee Nursery & Consulting specializes in plants and habitat consulting services for honey bees, native pollinators, and wildlife conservation. In 2018, her first book, Plants Honey Bees Use in the Ohio and Tennessee Valleys, was published. Shannon also writes a weekly blog called Kentucky Pollinators and Backyard Wildlife. The blog features profiles of pollinators and wildlife, tips for attracting pollinators and wildlife, highlights of different plants for pollinators and wildlife, and life on the farm and nursery. You can sign up to have her blog emailed to you.

 

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