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A recent study tracking butterfly abundance in Ohio over the last 20 years has discovered a 33% decline. We can only assume that in Illinois we have similar patterns in decline in our butterfly populations. Scientists believe the decline may be attributed to climate change, habitat degradation, and agricultural practices. 

Swallowtail Larvae & Adult
(Larvae) Photo by Whitney Cranshaw, Colorado State University, Bugwood.org; (Adult) Photo by Deanna Frautschi

With this news on your mind, what if I told you there was a plant that could go in your garden tomorrow that would greatly contribute to the health of one of Illinois' most beloved butterflies, the black swallowtail. The plant is parsley, and it is 2021 International Herb of the Year and caterpillar food for black swallowtails. Not only will the caterpillars, also known as parsley worms, munch on this easy-to-grow garden plant, but it can also be used for your culinary adventures. The black swallowtail’s caterpillar and I have shared in the harvest without much overlap because parsley thrives when good growing conditions are provided.

Parsley is one of America’s most popular culinary herbs. It is easy to grow from seed, compliments a heap of dishes, and has added health benefits when used in larger amounts.

Parsley makes a great addition to your existing landscape and containers and can even be used as a ground cover. Parsley should be seeded about 1/8-inch-deep and plants should be spaced 10 inches apart, in full sun to part shade. They can be slow to germinate, so be patient and consistently watering to ensure they do not dry out. University of Illinois Extension suggests soaking seeds overnight before planting for better germination rates.  

Most gardeners grow flat-leafed or Italian parsley (Petroselinum crispum var. neapolitanum) for culinary purposes as it has more flavor than curly leaf type (Petroselinum crispum crispum). However, the curly type is more ornamental with its bright green curly leaves, remains more compact, and can be enjoyed in dishes when chopped up or dried.  

These black swallowtail caterpillars that eat parsley in your garden bring further intrigue when you investigate their defense mechanisms. Although they are light green with black stripes and yellow dots when they are larger, they start out dark colored with tiny spikes and white patches, resembling bird droppings. They defend themselves when poked with a horn-like, stinky yellow scent gland, called an osmeterium, that emerges from behind their head and is believed to fend off ants and other predators. Many times, I have poked a caterpillar and amazed kids in the garden with this fun investigation.