There is a love it or hate it attitude with this butterfly among gardeners. Most folks love the butterfly, but herb gardeners hate the caterpillar. The Black Swallowtail caterpillar feeds on plants that are members of the Apiaceae or Carrot Family which includes parsley, dill, fennel, celery, cilantro, anise, caraway and Queen Ann’s lace. The hungry caterpillars can devour a patch of parsley in no time.
The Black Swallowtail will over winter in our area as a pupa and is one of the first butterflies to show up in the spring. These are large butterflies that are mostly black with a double row of yellow spots along the wing margins. The underside of the lower wing has orange spots. If you look closely you can tell a male from a female. The yellow spots on the male are larger and sometimes run together to look like a band of yellow. Both males and females have an iridescent blue patch on the lower wing but on the female the patch is larger. So if it has small yellow spots and a big blue patch it is a female. If it has big yellow spots and a small blue patch it is a male. Both will also have an orange eye spot with a black center on the lower wing near the tail. Eye spots on butterflies and moths are common and are believed to confuse or scare predators.
Butterflies are cold blooded and rely on the sun warm them up so they can fly and feed. You may have noticed butterflies are rarely seen on cloudy days. In the morning they sit in a sunny place and warm up so they can fly. By the way, the best time to photograph butterflies is in the morning while they are sunning.
The black color allows the Black Swallowtail to warm up quickly. Black is a popular color for many of our local butterflies. The Palamedes Swallowtail, the Giant Swallowtail and the Morning Cloak are all predominately black.
The caterpillar of the Black Swallowtail starts out mostly black but when it gets big enough for you to see, it will be green with black stripes. Look closer and you will see yellow spots down the sides of the caterpillar. Of course, this caterpillar will be easy to identify because it is eating your favorite herbs.
If you want to have some fun, take along skinny stick and gently poke the caterpillar. If it feels threatened, a horn like gland called an osmeteria will pop out of its head and release a chemical with a stinky odor. Do not try this with your finger or you will be washing your hands for days trying to wash the stink off.
Ever wonder what they ate before Europeans brought culinary herbs for the caterpillars to dine on? There are some native “weeds” in the Apiaceae family which can be found in our gardens and along waysides. Roughfruit scaleseed or Spermolepis divaricate is a small dill-like plant (24”) with a very small umbrel of tiny light green flowers. It was first described in 1788 by local botanist Thomas Walter. When I first saw this plant in my garden, I thought my dill had re-seeded itself. The foliage has a carrot like scent. Roughfruit scaleseed is a common name that is more than a mouthful. I find it hard to remember, I think I will rename this plant Carolina Dill Weed.
Mock Bishopweed or Ptilimnium capillaceum looks like a shorter dill plant (12”) with white flowers that are showier than Carolina Dill Weed. Mock Bishopweed is often found in wet and brackish coastal areas. French botanist Andre Michaux first described this plant in our area in 1803. When these weeds pop up in my garden, I usually leave them alone and let the Black Swallowtail butterfly lay their eggs on them instead of my parsley. Maybe it will work someday.