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That 'parsley worm' is really a caterpillar

Jessica Walliser
| Saturday, Aug. 12, 2017, 9:00 p.m.
The Eastern black swallowtail
Jessica Walliser
The Eastern black swallowtail
Though often known as a “parsley worm,” this caterpillar is not a worm at all. Instead, it’s the larvae of a native butterfly called the Eastern black swallowtail.
Jessica Walliser
Though often known as a “parsley worm,” this caterpillar is not a worm at all. Instead, it’s the larvae of a native butterfly called the Eastern black swallowtail.

If you grow parsley, dill, fennel or other members of the same plant family, you may come across a distinctive caterpillar feasting on the foliage.

Though often known as a “parsley worm,” this caterpillar is not a worm at all. Instead, it's the larvae of a native butterfly called the Eastern black swallowtail.

The Eastern black swallowtail (Papilio polyxenes asterius) is one of the most common butterflies here in Western Pennsylvania. Their black wings spread 2 to 3 inches wide, and they're often found nectaring on zinnias, coneflowers, phlox and many other garden plants.

When their wings are open, the females have pale yellow spots on the edges of their black wings and light blue at the base of their hind wings. You'll also find a small, orange eye-spot at the base of the wings, just above the black “tail” that extends from the base of each hind wing. Females are typically a bit larger than males.

Male black swallowtails have a large yellow band across the middle of the wings and the blue coloration and eye-spot are less pronounced. They also have black “tails” extending from the base of each hind wing.

This butterfly is found across southern Canada and most of the eastern and mid-western U.S. all the way west to the Rocky Mountains.

The larval stage of the Eastern black swallowtail feeds exclusively on members of the carrot family (Apiaceae) such as those mentioned above.

Females lay eggs singly on host plants, and upon hatching the caterpillars are very tiny. They start out black with a white saddle and small orange spots. As they mature through several instars (life-stages) over the course of 15 to 30 days, the caterpillars change to a bright green with black and white stripes and rows of yellow dots. This is the stage at which most gardeners notice them munching on their plants. But, by the time the caterpillars reach this stage, they're almost done feeding and ready to pupate. If you find any on your plants, please don't disturb them.

When the caterpillars are ready to pupate, they often crawl off of host plants to look for a safe place to build their chrysalis. They position themselves along a plant stem, head up, and they produce two silken threads about a third of the way down their bodies. These threads hold them to the stem during pupation.

Eastern black swallowtail chrysalises are green with a tinge of yellow, and they're hard to spot in the garden. As the pupae mature, they may or may not turn brown. The generation that overwinters in the garden as pupae are almost always brown. There are two to three generations of swallowtails per year here in Pennsylvania.

If you'd like to encourage this beautiful butterfly to take up residence in your garden, plant lots of their favorite caterpillar host plants, including caraway, celery, dill, parsley, fennel, zizia, and even Queen Anne's lace. Also be sure to plant lots of nectar plants for the adults.

Another must-do for gardeners who want to encourage all butterflies, is allowing the garden to stand through the winter, rather than doing a fall cleanup. Because Eastern black swallowtails -- and many other butterflies ­— overwinter in our gardens (monarchs are one of only a select few species that migrate away for the winter), it's essential that we provide overwintering habitat for them in the form of standing perennial stems, ornamental grasses and leaf litter.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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