Current Issue

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

Parsley poacher: Black swallowtail caterpillar, 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 cultivating herbs like parsley and dill in a backyard garden or patio containers has become so popular. However, humans are not the only creatures with a taste for savory herbs. Many insects find these fresh summertime delights irresistible. A week or so ago, while plucking parsley from the parsley patch I discovered a few tiny caterpillars doing their best to imitate bird droppings. In a matter of days these tiny larvae developed into several very impressive green, black, and yellow caterpillars joyously devouring my precious herb. And now, after a couple of weeks of this caterpillar calamity, it looks like I will be buying my parsley at the market until my poor parsley plant recuperates.

Tiny bird-dropping-mimic caterpillars take tiny bites, but just watch what happens to my parsley when an almost fully grown caterpillar goes to work! (Ok, it is at double speed).

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

This saga began several months ago, when the adult black swallowtail butterfly emerged from a chrysalis that had survived 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. 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 these tiny tasty treats escape detection and death at the beaks 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 attempt to blend in with their surroundings. Older stages of 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. While chillin’, this forked, orange appendage is tucked beneath the skin out of sight. But 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, isobutyric and methylbutyric acids, with a fragrance 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.

A quick fly-over with the camera tells me the parsley is in trouble. And when you grab one of these rascals, oh watch out. If stinky fluids from the osmeterium don’t dissuade an attacker, regurgitating the last meal just might.

Serious herb growers sometimes find black swallowtail caterpillars so abundant that crops are ruined. For me, well, a little extra parsley or dill planted for caterpillars ensures that I get to enjoy the adults moseying through my flower beds, and provides the opportunity to give a caterpillar a gentle squeeze to witness the crazy osmeterium.  If sharing your herbs with large gorgeous caterpillars is not exactly your cup of tea, larvae are easy to spot and can be moved out of herb garden 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 and spicebush 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 “Secret Weapons” was used as a reference for this episode.

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