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.
Hostplants for Black Swallowtail Larva
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 Swallowtail larva (and all Swallowtail butterflies) is their curious and amusing defense mechanism.
The Orange Osmeterium
All Swallowtail larva 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 Swallowtail larva have organs that are quite impressive. They must terrify birds or some other unfortunate creature who dares to tangle with this caterpillar.