Question: Last summer, my parsley plants were inundated with caterpillars for about two weeks. Then they went away. The caterpillars were green and black striped with yellow dots. Were they monarch butterfly caterpillars? Or were they some kind of pest?

Answer: The insects you describe are often called parsley worms, though they are not worms at all; they're caterpillars. Parsley worms are the larval stage of the Eastern black swallowtail butterfly ( Papilio polyxenes asterius). These butterflies are quite common in our area, and their caterpillars feed on many different species of plants in the carrot family ( Apiaceae), including parsley, parsnips, caraway, fennel and dill.

The wings of adult butterflies are black with two rows of yellow spots and light blue markings, though the blue on the males is much less prominent than on the females. Both male and female Eastern black swallowtails have an extension, or “tail,” that projects out of the base of their wings. The adults feed on the nectar of many different flowering plants.

Female butterflies lay tiny, cream-colored eggs on host plants throughout the summer. When the caterpillars first hatch, they're very small and mostly black with a white saddle across their mid-section and tiny orange spots. As they mature, the caterpillars develop their distinct coloration. The caterpillar stage lasts between two weeks and a month, depending on the weather conditions and other factors.

Full-grown larvae are light green and black striped with yellow spots, and they're about as big as your pinky finger.

The caterpillars pupate in a green or brown chrysalis that's anchored to a plant stem or other structure with two white, silken threads. They spend one to two weeks in pupation before emerging as an adult butterfly. There are two generations throughout the summer, and the final generation spends the entire winter in pupation.

As an interesting project, you can carefully collect a full-grown parsley worm and rear it in a large, glass Mason jar with holes cut into the lid. Feed it leaves from the host plant on which you found it, and place a stick diagonally inside the jar. The caterpillar will enter pupation and form its chrysalis on the stick. Watch the jar very carefully because about two weeks later, the butterfly will emerge. After its wings have fully expanded soon after emergence, take the jar out into the garden and release the butterfly. It's a pretty amazing thing to watch.

Though parsley worms sometimes feed on desirable garden plants, they aren't generally considered to be pests. Their feeding seldom causes anything more than superficial damage to the plants, which quickly rebound when the caterpillars go into pupation. If you're worried about them consuming more than their fair share of parsley, dill and other host plants, just plant a few extra plants next year. That way, they'll be more than enough to go around.

If the caterpillars were those of a monarch, they would be found on milkweed plants, rather than on members of the carrot family. Monarch butterflies can only use species of milkweed as host plants for their young. In fact, the vast majority of butterflies are host-exclusive, feeding only on one particular group or species of plants as larvae. Monarch caterpillars also are striped, though their stripes are black, white and yellow.

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, 503 Martindale St., Third Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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