Very few butterfly caterpillars are pests. We certainly have plenty of caterpillars that are pests here in the South, but almost all of these are the larvae of moths, not butterflies. Eastern black swallowtails are one of the few exceptions, and the caterpillars are called parsleyworms in acknowledgment of their pest status.
Parsleyworms are common pests of herbs such as dill, fennel and parsley, as well as vegetables such as celery, parsnips and carrots. They especially like plants in the carrot family and occur on weeds such as Queen Ann’s lace and even on highly toxic plants such as spotted water hemlock. These caterpillars create an interesting division between gardeners. Butterfly gardeners view them as miniature wildlife and often plant dill, parsley and/or fennel in their butterfly gardens to help attract black swallowtail butterflies and to grow more, but herb gardeners consider them pests because it can be difficult to grow these herbs without controlling parsleyworms.
Destruction of a planting of herbs by parselyworms can be quite dramatic. One day those fennel plants look great, but a couple of days later there is nothing left but stems and a bunch of large, striped caterpillars that are eating those stems. This happens because the younger caterpillars, which do not eat very much and are camouflaged to resemble bird droppings, are easily overlooked. Like most caterpillars, parsleyworms do most of their eating in the last few days of their lives as caterpillars. Once they are fully grown (about 2 inches long), they usually crawl away from the host plant to form their chrysalis. This is a good survival strategy because the odor of their feces and the feeding injury to the host plant is attractive to parasites and predators.
Black swallowtail caterpillars are often confused with Monarch butterfly caterpillars because both species have black, white and yellow/green vertical stripes. But they have different hosts and Monarch caterpillars have a distinguishing pair of black, whip-like projections at each end of their bodies.
Parsleyworms have a special pair of projections near the front of their body, but these are usually kept out of sight. This is an evertible gland, known as an osmeterium (see that horizontal orange slit at the top of the three black spots just behind the head). When a caterpillar is startled, it will arch its back and evert this orange, forked gland, which is accompanied by the release of an unpleasant odor. The effect is startling to would-be predators or parasites and often discourages further attack. To see and smell this for yourself, use a small stick or plant stem to gently prod a resting caterpillar on its back. All swallowtail caterpillars have these defensive organs, but color and odor vary with species.
Control: Hand-picking is usually all that is needed to protect a few dill or parsley plants but check plants regularly because young caterpillars are difficult to spot, and once caterpillars grow large enough to really eat, a few days late is too late. For preventive control on larger plantings, zeta-cypermethrin (GardenTech product) is labeled for use on parsley and fennel with a one day preharvest interval. Organic gardeners can use spinosad (Bonide and GreenLight products); preharvest intervals vary depending on crop being treated.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only. Always read and follow current label directions. Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.
Mississippi State University is an equal opportunity institution.