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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Parsley and dill beware: Black swallowtail, Papilio polyxenes


A female black swallowtail butterfly lays her eggs on dill.


Because many herbs are easily cultivated in small flats or pots, even gardeners with limited growing space can enjoy fresh culinary delights throughout the summer. This is one reason growing herbs like parsley and dill in a backyard garden has become so popular. Unfortunately, humans are not the only creatures with a taste for savory herbs. Many insects find these summertime treats irresistible. Last week I received a phone call from my parsley planting neighbor who discovered several very impressive green, black, and yellow caterpillars joyously feeding on her parsley. These were the larvae of the black swallowtail butterfly.


A black swallowtail caterpillar carefully inspects a plant before taking a nibble.

Several months ago, the adult butterfly emerged from a chrysalis after surviving the chill of winter. Nectar and pollen from a variety of flowers sustain the butterfly in spring and summer. After mating, the female swallowtail searches for wild plants in the carrot family, such as Queen Anne’s lace, or cultivated delicacies including carrot, fennel, dill, and parsley.

Tiny yellow, beach ball-like eggs deposited by the female butterfly soon hatch into hungry caterpillars.

She lays a few eggs on a plant and in a matter of days these hatch into tiny caterpillars. At first, tiny black swallowtail caterpillars resemble bird droppings. As with other swallowtails we visited in previous episodes like the spicebush swallowtail, this scam may help them escape detection and death at the mouths of would-be predators like birds. Older black swallowtail caterpillars are banded with dazzling swatches of black and yellow on a field of green.


These creatures do not appear to attempt to blend in with their surroundings. Black swallowtail caterpillars have their own clever defense to ward off enemies intent on making them a meal. Just behind the head of the caterpillar is a specialized structure called the osmeterium. Usually, this forked, orange appendage is tucked beneath the skin out of sight. However, when the swallowtail larva is threatened, it extends the osmeterium in the direction of the disturbance. This glandular organ is coated with foul smelling chemicals reminiscent of rancid butter. The disturbing visual and olfactory display discourages hungry predators from wanting to dine on this beautiful caterpillar. In addition to the stinky fluid from the osmeterium, the caterpillar will often disgorge its last meal to help repel an attacker. Yuck!


If stinky fluids from the osmeterium don’t dissuade an attacker, regurgitating the last meal just might.

Sometimes the larvae of black swallowtails become abundant enough to cause damage to herbs. A few of these large caterpillars can wreak havoc on a small flat of parsley or dill on a patio or deck. Fortunately, larvae are easy to spot and can be moved out of harm’s way to Mother Nature’s garden like a nearby patch of Queen Anne’s lace.    


Tubular flowers like those of Vinca are irresistible to nectaring black swallowtails.


Bug of the Week thanks Anne Marie and Dennis for providing the black swallowtail larvae and inspiration for this episode. Thomas Eisner’s delightful book “For Love of Insects” was used as a reference for this episode.

To learn more about the black swallowtail, please visit the following web site: