Black Swallowtail Larvae, Last of the Summer Butterflies

Black Swallowtail larvae
Herd of Black Swallowtail larvae

I was surprised to see these caterpillars munching on a dill plant so late in the season. I first saw this small herd of black swallowtail butterfly larvae last week, the last week of September. Several of them have grown quite a bit seen then. Some of the larvae are actually smaller than others.  They must have hatched at different times. I wonder if they will have enough time to grow to the next stage before the cold weather sets in. Tonight is supposed to dip down to a chilly 46°F.


Black Swallowtails Larvae Feeding Before Winter Sets In

These brightly striped black, white, and yellow caterpillar is the larvae of the Black Swallowtail (Papilio polyxenes).  The key piece of evidence to identify this butterfly is that it is feeding on my dill plant. I don’t know of another butterfly larvae that feeds on dill.

Tiny Black Swallowtail butterfly larvae
Tiny Black Swallowtail butterfly larvae

This growing season I didn’t harvest any dill because each time I went to harvest leaves, I found Black Swallowtail larvae quietly munching away on the leaves. I have dill plants in another GreenStalk vertical planter and there are tiny Black Swallowtail larvae on those plants, too. I have to plant much more dill next year.


Travelling Black Swallowtail butterfly larvae
Travelling Black Swallowtail butterfly larvae

Black Swallowtail Larvae Getting Ready for Winter

I’m interested to see where these larvae pupate and hibernate. I’ll be repotting my herbs this fall in another planter. I’ll have to watch out for the brown pupae case in the soil. I read that this species coordinates the color of its pupae (metamorphosis/hibernating) case with its’ surroundings.

In short-day photoperiods (now in the fall with less daylight hours) the pupae cases tend to be brown. During long-day photoperiods (like summer) the pupae cases tends will be brown or green depending on the surface its on. The pupae tends to be brown on rough surfaces, such as rocks or soil. The pupae tends to be green on smooth surfaces like plant stems. This means the pupae will most likely be brown because this time of year has a short-day photoperiod.

Among parsley Black Swallowtail butterfly larvae
Among parsley Black Swallowtail butterfly larvae

These  Black Swallowtail Larvae Will Hibernate

There is no way these larvae will metamorphosis into adult butterflies before the cold weather sets in for good. There have been a few nights when the temperatures dipped into the forties. And the next morning the larvae were back on the dill stalks munching away.

These species overwinter as pupae (hard cases) encasing larva which will turn into adults in the spring. Several generations of Black Swallowtails have been born, lived, and turned into adults in my humble backyard. This makes me happy.

butterfly life cover color with border
Butterfly Life

The Butterfly Life Cycle

If you would like to learn more about the butterfly life cycle, I wrote The Butterfly Life to explain how butterflies are born, live and support the Earth’s Ecosystems.

More on Black Swallowtails

Black Swallowtails: How to Identify Them

The Swallowtail Butterfly Family

Early Autumn Butterflies in My Garden




  1. In my experience with black swallowtail cats, the chrysalis over winters suspended from a stick, not in the dirt.

    • Hi Mary, That is good to know. Your Swallowtail larva picked a plant to cling to. I bet the one you saw was during the summer season when green plants abound. The larvae currently on my dill plants, won’t have a green plant to pupate and hang from in a few weeks time as the frost date is October 30. The dill plant will be dead.
      The lepidopterist, James Scott says in his book (The Butterflies of North America), the Black Swallowtail pupates on the ground or on a green plant. We know to look in both places, on host plant stems and the ground probably in leaf litter near the hostplant. It depends on the time of year, summer or autumn.

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