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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.

Herbs beware! Black swallowtail, Papilio polyxenes


Dill is the perfect plant on which to lay eggs if you are a black swallowtail. 


Many herbs are easily cultivated in small flats or pots, so 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 a gardener who discovered several very impressive green, black, and yellow caterpillars joyously feeding on her parsley. These were the larvae of the black swallowtail butterfly. 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 and, after mating, the female swallowtail searches for wild plants in the carrot family, such as Queen Anne’s lace, or for 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.

Soon these tiny eggs will hatch into hungry caterpillars.

At first, tiny black swallowtail caterpillars resemble bird droppings! As with other swallowtail larvae, this scam may help them escape detection and death at the mouths of would-be predators like birds ("Bird-droppings, snake eyes, dead leaves – Swallowtails, Papilio sp."). As the little caterpillars develop and grow larger, they become banded with a dazzling display of black bands and yellow spots on a field of bright green. These older black swallowtail caterpillars, unlike the tiny newly hatched ones, seem to make little effort to blend in with their surroundings. However, they have their own clever defense to ward off enemies intent on making them into a meal. Just behind the head of the caterpillar is a specialized structure called the osmeterium. This forked, orange colored appendage is usually tucked beneath the skin out of sight. 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 likely discourages hungry predators from wanting to dine on this beautiful caterpillar. Sometimes the larvae of black swallowtails become abundant enough to cause damage to crops. A few of these large caterpillars can wreak havoc on a small flat of parsley or dill on a patio or deck. Fortunately, the larvae are easy to spot and can be moved to less delectable plants such as a nearby patch of Queen Anne’s lace, preserving both your herbs and one of our most beautiful butterflies.


Bug of the Week thanks Judy 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: