These two Black Swallowtail larva caught my eye while I was planting and pruning in my garden.
This must be the first brood of the usual two or more broods that this species reproduces from June onward. The adult butterflies are seen from April to October.
The adult female butterfly lays just a few eggs on one plant, so the caterpillars can’t eat a great many plants. It rarely becomes a pest in the garden.
The host plants for this caterpillar are in the carrot family. They eat carrots, celery, dill, parsley and caraway. None of which are native to North America. This leads me to believe this is an introduced species. The young caterpillars eat both leaves and flowers. Later instars (larval stages) prefer to eat just the flowers.
Black Swallowtails are found most readily in open spaces. Such as fields, gardens, marshes, deserts and along the coasts. Seldom are they found in forest interiors. As former agricultural land returns to forest, the Black Swallowtail is becoming less common.
A fascinating behavior of Black Swallowtails (and all Swallowtail butterflies) is their curious and amusing defense mechanism.
All Swallowtails have an organ called osmeterium. This organ is attached immediately behind the head. When threatened (or poked gently with a stick, which I did), the caterpillar raises its osmeterium and waves it about. The osmeterium is laden with butyric acid and emits a foul-smelling odor. Some say it is similar to fresh vomit. I didn’t get that close so I can’t say what it smells like. I doubt I ever will.
This caterpillar’s orange osmeterium was quite small. Perhaps this is due to youth. Larger Black Swallowtails have organs that are quite impressive. They must terrify birds or some other unfortunate creature who dares to tangle with this caterpillar.
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