The lily leaf beetle adult is a striking insect with a bright scarlet body and black legs, head, antennae, and undersurface. The adults are 6 to 9 mm (1/4 to 3/8 inch) long, and they will squeak if they are squeezed gently--a defense mechanism to deter predators. Adults and older larvae feed on leaves, stems, buds, and flowers of the host plant. Adults lay their eggs on the underside of leaves in an irregular line. The reddish/orange eggs take from 7-10 days to hatch under normal conditions. Females lay up to 450 eggs, sometimes over two growing seasons. Larvae resemble slugs with swollen orange, brown, yellowish or even greenish bodies and black heads. Larvae tend to cause more damage than adults. Larvae are distinctive and repulsive in that they secrete and carry their excrement on their backs. Younger larvae feed for 16-24 days, primarily on the underside of leaves.

Larvae enter the soil to pupate; pupae are florescent orange. New adults emerge in 16-22 days and feed until fall. They do not mate or lay eggs until they emerge the following spring in late March through June. Lily leaf beetles overwinter in the soil or plant debris in the garden or woods, sometimes a distance away from the host plants. Adults prefer environments that are shaded, protected, cool, and moist.