Papilio glaucus

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Black Eastern Tiger Swallowtail

An eastern tiger swallowtail black form female nectars on nepeta ‘Walker’s low.’

It’s late spring with the entire summer ahead of us. Nature’s greens are deep and full. Tiger swallowtails (Papilio glaucous) are on the wing now, the first of two times during the Minnesota summer. The first flight is typically in May-June and the second in July-August. The offspring of the second flight overwinter in their chrysalides.

I watch a bright yellow male patrolling his territory, repeating the same route through the large silver maples across the alleyway, over our apple tree and under my neighbor’s birch tree. He is quick to give chase to other males that trespass.

Eastern tiger swallowtail males are always yellow with black stripes and lack the wash of blue on the hind wings.

On this sunny, breezy morning, a black form female nectars in a sea of blue nepeta in our front garden. Her iridescent blue-on-black wings flutter repeatedly from one end of the garden to the other. She is full of energy and free of wing tatters and tears.

The female’s underwings are marked with bright orange spots and light blue scales.

Most eastern tiger swallowtails in Minnesota are yellow with black stripes. However, the female is dimorphic, or appears in two forms: the familiar black-striped yellow and a rarer black form washed with shimmering blue across its hind wings. Faint black stripes are often visible on the dark female.

Just a tiny percentage of females appear in the black form, especially this far north, but are  common in the southern United States. Why? Scientists think that the black form is a mimic of the pipevine swallowtail, which tastes horrible because the caterpillars feed solely on pipevine plants. (Think of monarchs and viceroys, another example of mimicry. Birds hate the taste of monarchs because they eat milkweed. Viceroys closely resemble monarchs, so birds often avoid them.) Pipevine swallowtails occasionally come as far north as Minnesota.

Interested in attracting eastern tiger swallowtails to your yard or garden? Favorite caterpillar foods include: chokecherry, ash, poplar, maple, apple and mountain ash. Adults nectar on many flower species including phlox, milkweed, Joe-Pye weed, blazing star, bee balm/bergamot and red clover.

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Eastern Tiger Swallowtail Butterflies

A female eastern tiger swallowtail – black form (Papilio glaucus) nectars on an ironweed flower.

It was an unexpected gift on an August afternoon: A black eastern tiger swallowtail butterfly, its wings faded and worn, even tattered in places, showing its age.  It settled in for a long drink of nectar from an ironweed flower on the banks of the Snake River in Pine County, MN. We’re more likely to see the yellow form of the tiger swallowtail in the North. The black form occurs much less frequently and only in females. The butterfly’s name refers to the long projections on its tail, which resemble a barn swallow’s tail.

An eastern tiger swallowtail – yellow form nectars on a monarda blossom.

In the northern United States, eastern tiger swallowtails typically fly from mid-May to late August or September. They usually have two broods per summer, and the young of the second group overwinter as chrysalids to emerge the following May. The caterpillars are bright green with two blue eye spots. (They turn brown when ready to form a chrysalis.) They mainly feed on the leaves of trees such as wild cherry, chokecherry, basswood, cottonwood, ash, willow and maple. Adults nectar on many types of flowers, but prefer wild cherry and lilac in spring, Joe-Pye weed and milkweed during the summer.

The name swallowtail comes from the long tail projections that resemble a barn swallow’s tail.

The ironweed on which I spotted it belongs to the Composite family, which also includes coneflowers, sunflowers, daisies, goldenrod, Joe-Pye weed, asters and others. Ironweed is a native perennial, prefers full sun and often grows near marshes, along riverbanks, edges of moist fields, and wet prairies. It’s typically 4-to-6 feet tall and its violet-colored flower clusters are often visited by bumblebees. The black tiger swallowtails seem attracted to ironweed and I’ve seen them on the blossoms in past years, too. I enjoy seeing both forms of the butterfly.

Ironweed (Vernonia fasciculata) grows on the banks of the Snake River in Pine County, Minnesota.