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Garden Q&A: On black swallowtail butterflies and boxwood blight

A black swallowtail is a good native butterfly. They are fond of plants in the carrot family, including fennel and dill.
A black swallowtail is a good native butterfly. They are fond of plants in the carrot family, including fennel and dill. (Ellen Nibali/For The Baltimore Sun)

I was excited to have Monarch caterpillars, even though they were chomping away on my fennel, but then this gorgeous number emerged from the chrysalis! Seems to be a black swallowtail. I’m a little confused.

This is, indeed, a black swallowtail, a good native butterfly. The caterpillars of swallowtails and monarchs are similar. Caterpillars with “drips” on the black and yellow stripes indicate swallowtails. If you squeeze one (gently!) they can stick out ‘horns’ and emit a sweet scent.

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Swallowtail caterpillars are fond of plants in the carrot family, including fennel and dill, but also citrus foliage, which includes rue, another herb. As you have found, it’s good to plant extra, to share with them because big caterpillers, which become big butterflies, also have big appetites!

Rue is a good contrasting foliage to fennel and would provide your swallowtails with a more varied diet. Monarchs, on the other hand, stick to milkweed family plants.

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It has been confirmed that one of my boxwood has the new disease, boxwood blight. I first noticed when it began to shed brown leaves at a fast clip, starting at the bottom. If I spray a fungicide will it stop the leaves from dying? Will the bare branches regrow? Will I have to spray every year?

A fungicide is only useful for protecting plants that do not have symptoms. Fungicides are not curative.

If your boxwood has black stem cankers and leaves are dropping off your plants, a fungicide cannot stop this. It will not return to health.

Search ‘Boxwood: Preventing and Managing Common Pests and Diseases’ on the Home and Garden Information Center website for the best available current information on managing this disease. It explains carefully digging up and disposing of the boxwood and even laundering clothes to avoiding spreading the disease to other plants.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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