Black Swallowtail Caterpillars in the Fall

Black swallowtail caterpillar (Papilio polyxenes) eating blossoms on fennel plant in backyard butterfly garden in Hampton, Virginia.

October 22, 2021

Black swallowtail caterpillars (Papilio poxyxenes) can be found in various settings such as gardens, fields, wetlands as well as many other areas. Most often everyone sees butterfly caterpillars in the spring. The black swallowtail caterpillars seen in the spring will emerge over the summer as black swallowtail butterflies.

Did you know that butterfly caterpillars overwinter? Usually caterpillars seen in the fall with the shorter days and cooler temps will overwinter. Overwintering black swallowtail caterpillars may be observed in the fall on a host plant.

Host Plants

Hosts plants are the plants butterfly caterpillars use as source food during the caterpillar stage. Black swallowtail caterpillars use the following as their hosts plants: fennel, dill, parsley, Queen Anne’s lace or carrot tops. They will eat the leaves and flowers of the host plant. These caterpillars will eat as much as they can to fuel for overwintering. They will not build a nest in the plant.

Black Swallowtail caterpillar feeding on fennel plant in the fall.
Adult black swallowtail caterpillar on fennel plant in the larval stage as it is prepares to enter into the pre-pupal stage. As the caterpillar transitions to a pre-pupal larvae, it will shed its larva skin and become immobile. During the pre-pupal stage, they will internally secrete enzymes which digest their entire innards into a soup.

5 Stages of the Black Swallowtail Caterpillar

A black swallowtail caterpillar will undergo five stages of various colors while maintaining its characteristic as a caterpillar. These five stages occur from after the laying of egg to the pupa. Each of these stages is called an instar. As the black swallowtail caterpillar molts, it progresses to the next instar or stage. When this caterpillar molts or transforms, its coloring dramatically changes. In the first instar the black swallowtail caterpillar is solid black with a white band. As a full grown caterpillar, it molts into striped bands of green, white, yellow and black. An adult caterpillar can be up to 2 inches long.

Black swallowtail caterpillar (Papilio polyxenes) feeding fennel flowers in backyard butterfly garden in Hampton, Virginia.
Full grown black swallowtail caterpillar in this stage will eat to provide the energy needed to transition to the nest stage of pre-pupa.

Overwintering in Chrysalis

Black swallowtail caterpillars will transform into a pupa inside a chrysalis to overwinter. Once this full grown black swallowtail caterpillar stops eating, it will wander a short distance from the host plant. It will then shed its skin and transform into a pupa. The overwintering pupa will then remain in their chrysalis during the winter. It will then emerge as a butterfly in the following spring. The overwintering black swallowtail chrysalis will always be a brown color to blend with fall and winter surroundings.

Fall Black Swallowtail Caterpillars in My Garden

Last week (October 8th), I noticed the fennel in my backyard butterfly garden had 12 caterpillars feeding on it. I went inside my house and grabbed my camera. Of course, I have tried to document all the activity of the caterpillars and butterflies in my new backyard butterfly garden. I was surprised to see such a large amount of black swallowtail caterpillars so late in the year.

Adult black swallowtail caterpillars in the fall feeding on fennel plant in Hampton, Virginia.

As a conservation photographer and storyteller, I always spend a great deal of time researching my subjects. So, after photographing these fall black swallowtail caterpillars in my butterfly garden, I started my research on overwintering caterpillars.
I thought the info was enlightening and wanted to share some of this knowledge.

At this posting the caterpillars are no longer on the fennel. I have searched my backyard for chrysalis. So far, I have not found any but will continue looking.

Thank you for reading my Field Notes blog, and I hope you will share this post with others.

Let’s protect our wildlife and nature!

All the very best,

Lori

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