Out My Backdoor: Best Bet for Black Swallowtails

Black Swallowtail Caterpillar
Black Swallowtail Caterpillar
Male Black Swallowtail
Male Black Swallowtail


By Terry W. Johnson

The black swallowtail is a large dark butterfly that can be seen in any backyard across the state. This is especially true if a homeowner plants one of the butterfly's favorite host plants—bronze fennel (Foeniculum vulgare).

With a wingspan that can measure as much as 4.2 inches across, the black swallowtail is one of the largest butterflies you are likely to see in your yard. (Giant swallowtails are larger, though in Georgia they are seen mostly in southern part of the state.) Male black swallowtails are easily identified by the broad yellow band that extends from wing tip to wing tip across the top if the butterfly’s black wings. While the female also has black wings, it lacks the male's showy yellow band. Instead, females have two narrow bands of yellow spots along the trailing edge of the upper surfaces of their wings.

Bronze fennel is a perennial herb that hails from the Mediterranean. The plant can reach a height of 6 feet. The feather-like foliage of the plant is truly striking. And when crushed, it gives off a pleasing aromatic licorice scent. Bronze fennel flowers are small and yellow. Although inconspicuous, they are visited by nectar-feeding insects varying from hoverflies and butterflies to bees. The seeds of bronze fennel are consumed by sparrows and other birds.

Fortunately, this plant is not on the Georgia Exotic Pest Plant Council's list of Non-native Invasive Plants of Georgia (https://www.gaeppc.org/list/).

Mankind has long prized fennel. The ancient Romans and Greeks used fennel for many purposes—to repel insects, treat sickness and eat as food. Warriors were even encouraged to drink fennel tea before they marched off to battle in the belief the drink would imbue them with courage. The Emperor Charlemagne mandated the herb be grown on the imperial farms.

In the New World, fennel was a mainstay in colonial gardens. The Puritans called it “the meeting seed,” often chewing the plant’s seeds during meetings.

American poet Henry Wadsworth Longfellow, best known for poems such as "Paul Revere's Ride” and “The Song of Hiawatha," alleged in his 1842 poem “The Goblet of Life” that fennel improved eyesight.

Bronze fennel grows best when planted in sites bathed in full sunlight. While it will grow in a variety of soil types, it prefers well-drained, fertile soils. Fortunately, fennel will grow in soils with a pH ranging from mildly alkaline to mildly acidic. As for the plant's water requirements, it does need regular watering. However, care should be taken not to overwater, which encourages root rot.

Fennel can be planted as an addition to a traditional flower garden. For this, since the plant grows quite tall, it is best to locate bronze fennel toward the back of a garden.

It can also be one of a variety of herbs grown in a culinary herb garden. In such settings, you can enjoy being able to harvest fresh herbs used in preparing mouthwatering dishes for your family, while providing handsome black swallowtails with much-needed food plants for their caterpillars.

Bronze fennel can be grown from both seeds and transplants. Again, since the plants can get large, space the plants 12 to 18 apart.

I have never planted fennel seed. Instead, I have set out small plants. Although I have always been successful in establishing a stand of fennel in this manner, it takes a couple of years for the plants to attain a decent size. Since I am at a stage in life where I do not want to wait that long, I now buy larger plants. They cost more, but I get to enjoy the butterflies fennel attracts much sooner.

By trial and error, I have also learned it is not a good idea to plant one or two fennel plants in a small area. It seems when you set out only a few plants, if a female black swallowtail lays eggs on them, before long a gang of hungry boldly marked green and black-banded caterpillars with orange-yellow spots will voraciously devour all of the plants' foliage, leaving you with the dilemma of figuring out how you can keep the caterpillars from starving.

(To try to avoid this, I have fed caterpillars fennel purchased at a grocery store, a strategy that works if the fennel was not exposed to pesticides. At other times, I have given black swallowtail caterpillars to friends that had plenty of fennel but no caterpillars. I have even placed caterpillars on an alternative host plant such as wild carrot in hopes they will continue to feed and develop.)

My recommendation, then, is that you create a fennel patch of at least six or eight plants planted well away from birdbaths, feeders and other features frequented by birds. Why? One year I hung a suet feeder near a fennel patch. Needless to say, it did not take the sharp-eyed mockingbird that regularly visited the feeder long to spot the juicy caterpillars feeding on the nearby fennel. In only a few days, the mockingbird ate all of the caterpillars.

Aside from creating a beautiful natural backdrop to a garden, bronze fennel provides homeowners an ideal opportunity to watch the life cycle of a butterfly without ever leaving their backyard. Each summer, I watch for female black swallowtails fluttering about my fennel. If one curls her abdomen in the shape of a comma and touches it to a fennel plant, I know she is laying eggs. After she leaves, I carefully examine the area visited by the butterfly, looking for its tiny pearl-like eggs. If there are eggs, it is not long before tiny caterpillars appear. Then it is fun to watch how rapidly they grow.

If all goes well, each caterpillar will eventually encase itself in an oddly shaped chrysalis. When it emerges, the once rotund, boldly striped caterpillar has been transformed into a magnificent butterfly.

When this occurs, you are handsomely rewarded for all of your efforts to provide the black swallowtail with a place to produce a new generation of these stunning butterflies.

Terry W. Johnson is a former Nongame program manager with the Wildlife Resources Division and executive director of The Environmental Resources Network, or TERN, friends group of the division’s Nongame Conservation Section. (Permission is required to reprint this column.) Learn more about TERN, see previous “Out My Backdoor” columns, read Terry’s Backyard Wildlife Connection blog and check out his latest book, A Journey of Discovery: Monroe County Outdoors.”