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All butterfly larvae, except some Lycaenidae (Miletini), eat plants. Lichens and fungi are eaten by a few Lycaenidae (the tropical tribe Liptenini), and ferns (Selaginella) are eaten by some tropical Satyrinae (Euptychia). All other butterflies eat advanced seed-bearing plants. The seed-bearing plants are divided into two groups, the gymnosperms and angiosperms. The gymnosperms have the seeds on the surface of a scale, like the scales of pine cones, without a special structure for receiving pollen. They include Cycadaceae (cycads include Zamia, eaten by a few Lycaenidae), Pinaceae (conifers, eaten by a few butterflies), and Cupressaceae (cedars and junipers, eaten by a few lycaenids).

Angiosperms, or flowering plants, have the seeds in a closed ovary that has one or more pollen-receiving structures (stigmas) on it. The flowers usually attract insects that pollinate them. The flower typically has small green leaflike sepals at the base, colorful petals, long stamens with pollen at their tips, and one or more stigmas to receive the pollen on a stalk above the ovary. The modifications of the flower parts are used to classify these plants. Flowering plants are divided into two groups, the monocotyledons and dicotyledons, reflecting the number of leaves in sprouting seed.

In the monocotyledons, the leaves usually have parallel veins, the stems are often hollow, the seeds are not split into two parts, and the flowers usually have three of each part (three petals, three sepals, three stamens) or a multiple of three. The monocotyledons eaten by butterflies are Liliaceae, Palmae, Sparganiaceae, Bromeliaceae, Musaceae, Cannaceae, Marantaceae, Gramineae, Juncaceae, and Cyperaceae. Most of these have obvious flowers, but the last three families have tiny inconspicuous flowers because wind pollinates them. Grasses (Gramineae) and sedges (Cyperaceae) are eaten by Satyrinae and Hesperiinae. Grasses have their leaves in two rows on the stem, and cylindrical or flattened and usually hollow stems. Sedges often grow in wet spots, their leaves are in three rows on the stem, and the stems are usually triangular in cross section and solid.

In the dicotyledons, the leaves usually have a network of veins going in all directions, the stems are solid, the seeds are split into two halves, and the flowers usually have five of each part (five petals, five sepals, five stamens). Some families of dicotyledons are important and easily recognized. The Compositae have many tiny flowers clumped into a head, each flower lacking sepals, the outer flowers and sometimes the inner flowers usually having petals. A dandelion has hundreds of flowers in each cluster, and the seeds have hairs to be blown by the wind, for instance. Cruciferae are weedy herbs ("herbs" are plants that die above ground in winter, not shrubs or trees or grasses) that have four petals, four sepals, and six stamens, two stamens shorter than the other four. In the Rosaceae, the sepals are joined into a tube or disk, with the petals and many stamens attached to the rim; they are often shrubs or trees. Leguminosae generally have the leaves split into many leaflets, pea-shaped flowers with a hood, some wings on the side, and a keel, and the seeds are "peas" inside a "bean" pod. The Umbelliferae are herbs with fernlike leaves, and the many tiny flowers are in a flat-topped cluster, from a spray of stalks at the top of a stem. Malvaceae are herbs with fanlike leaves, and the flowers have the stamens all joined into a column around the ovary. Labiatae are herbs that have square stems. Polygonaceae have a translucent sheath around the main stem where each leaf branches off. Scrophulariaceae have the petals joined into a tube having upper and lower lips, usually four stamens (often of different sizes) arising from the tube, and often a fifth stamen lacking pollen. Euphorbiaceae are herbs having milky sap, and their flowers lack petals and are clustered together above colorful leaves that look like petals. Asclepiadaceae are herbs also with milky sap, the flowers have extra petallike lobes above the true petals (like a crown), and the pollen is clustered into bundles that may catch on to butterfly legs. Salicaceae and Betulaceae, usually shrubs or trees, have the flowers clustered into catkins like squirrel tails. Crassulaceae are herbs ("succulents") with thick juicy leaves. Amaranthaceae and Chenopodiaceae are low weeds or shrubs that lack petals or obvious flowers; the Chenopodiaceae generally have gray leaves.

This catalogue lists the plants known to be eaten by the larvae of North American butterflies (the doubtful plants and errors are not listed). Plant genera and plant families are followed by their common names and the species of each butterfly known to eat them. The genera of hostplants within each plant family are listed after the common name of each plant family. Common names of hostplants are also listed, followed by their genus or species scientific names, so that you can look up the butterflies that eat them.
References are to species number, I'm still working trying to get them all in.