Backyard habitat for black swallowtail caterpillars

Two weeks ago today I chanced to witness a black swallowtail butterfly, Papilio polyxenes, taking advantage of my backyard habitat. It was laying eggs on dill plants in my garden and the neighbour’s. The eggs were yellow and smaller than the head of a pin. Over the weekend they turned orange.

After four days tiny black caterpillars appeared. They looked like frass. That’s the technical word for insect shit. It’s probably a good look for avoiding attention when you’re small. Not so effective when you’re fat and juicy. The ones in this photo, taken today, are about as long as my thumbnail. They’re starting to show some colour. As they mature they’ll grow more vivid and poisonous looking.

So far the mob of robins and starlings on our street hasn’t noticed them. Or maybe they’ll leave these bugs alone. The caterpillars taste bad and can release a foul smell to deter predators. In any case, hopefully a few will survive to turn into chrysalises and metamorphose into their gorgeous adult form.

The caterpillar is known as parsley worm because it favours parsley. It will feed on other umbellifers (Apiaceae), the family that includes carrots, celery, fennel and various seedy herbs such as coriander. This particular butterfly passed over a nearby parsley plant in preference for dill.

I’ve seen this before, long ago in the herb garden I planted as a teenager. I didn’t notice until the nearly full-grown caterpillars appeared in dramatic colouring on the dill plants. I hadn’t seen the butterflies, but with adolescent persistence I figured out what they were.

I make no allowance for cabbage worms on the kohlrabi, or earwings and leaf miners in the chard. But swallowtail caterpillars are allowed to live. It’s interesting, isn’t it, how we judge creatures differently if they are beautiful? The world has no shortage of white cabbage butterflies, but black swallowtails are also widely distributed across North America, and one of the most familiar butterflies locally. Undoubtedly somebody somewhere regards these striking insects as pests. I don’t begrudge them one or two dill seedlings, which volunteer themselves abundantly each spring.

I count 10 caterpillars on the one little plant, which doesn’t appear to be suffering yet. That’s likely to change because these caterpillars are growing quickly. I might even move some of them to a new plant.

Just call me a swallowtail farmer. Or a cultivator of beauty.

If the worms began gnawing at the crown of my zucchini vines or boring into ripe Brandywine tomatoes, I might treat them differently. Honestly, I wouldn’t think twice. So much for aesthetics.

2 thoughts on “Backyard habitat for black swallowtail caterpillars

  1. I’ve only had one year where I actually saw the caterpillars, and when I realized what they were, I no longer minded that they ate my carrot tops right down to the ground…
    I expect they are fussy about their climate, as are Monarchs; too cold, too hot, too something and they flap away.

    These are wonderful photos, btw. Thank you.

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