Black swallowtails lay their eggs on plants in the carrot family because their caterpillers must eat the foliage of carrot-family plants to grow and become butterflies

Caterpillars that grow up to be black swallowtails must eat only plants in the carrot family if they are to survive and become swallowtails. So don't destroy all your carrot-family plants before winter. If they retain foliage, they may help provide for a new generation of black swallowtails.
Please click on image to enlarge view of the caterpiller of the Papolio polyxenes at World Peace Wetland Prairie.