The Black Swallowtail butterfly (Papilio polyxenes) is a common species found across much of North America. Understanding its diet is central to supporting its life cycle, particularly for gardeners. Its nutritional needs change dramatically as it progresses from the larval (caterpillar) stage to the adult butterfly. The caterpillar focuses on massive growth, while the adult concentrates on reproduction and maintenance, a fundamental shift that allows the species to thrive.
The Essential Host Plants for Larvae
The Black Swallowtail caterpillar’s diet is highly specific, relying exclusively on members of the carrot family (Apiaceae) for necessary nutrients. The female butterfly seeks out these host plants to lay her eggs, ensuring the emerging larvae have an immediate and acceptable food source.
Common culinary herbs are frequently consumed hosts, including parsley, dill, and fennel, which are often targeted in home gardens. Larvae also feed on the foliage of carrot tops and parsnip. In wilder settings, the caterpillars use native members of the Apiaceae family, such as Queen Anne’s Lace.
The larval stage is dedicated solely to consuming and storing energy. Caterpillars eat voraciously and do not consume nectar or any other liquid food source during this phase. This extensive consumption of plant material provides the fuel necessary for their later biological processes.
Adult Nectar and Nutrient Sources
The adult Black Swallowtail shifts its diet to primarily liquid nectar. Utilizing a long, straw-like proboscis, adults draw nectar from various flowering plants, which provides the sugars necessary for flight energy and overall body maintenance.
Adults prefer flowers offering easily accessible, high-quality nectar, often those with flat-topped or clustered blooms. Sources include garden flowers like zinnias and milkweed, and wild plants such as thistles, clover, and purple coneflowers. This energy sustains activities like mating and egg-laying.
Adult males engage in “puddling,” gathering on damp soil or wet sand. This behavior allows them to absorb mineral salts and nutrients crucial for reproductive success. These minerals are distinct from nectar energy and are passed to the female during mating.
How Diet Fuels Metamorphosis and Defense
The specific plants consumed by the caterpillar form the basis of the Black Swallowtail’s chemical defense system. Host plants contain secondary chemical compounds, which the caterpillar ingests and stores (sequesters) in its body tissues without being poisoned. These sequestered toxins make both the larva and the adult butterfly unpalatable to most predators.
When disturbed, the caterpillar deploys a fleshy, forked organ called the osmeterium from behind its head. This organ releases a foul-smelling odor composed of concentrated chemicals derived from its diet, serving as an effective deterrent against birds and small predators.
The energy accumulated during the caterpillar stage is the power source for the entire process of metamorphosis, including pupation and adult emergence. Fat reserves built up from host plant consumption fuel the complete reorganization of the insect’s body inside the chrysalis. Since the adult only consumes maintenance fuel (nectar), the stored larval energy must be sufficient for the transformation.