Many gardeners dutifully plant nectar sources for adult butterflies. But they may not be aware of the details of the life cycle of butterflies.  Most butterflies require native plants in order to reproduce. Butterflies start out in life as eggs, then hatch into caterpillars.  Most caterpillars are specialist feeders, and eat only a few species of native plants they coevolved with.  Butterflies will only lay their eggs on their native host plants; caterpillars will eat only these same host plants.  The typical North American garden full of alien plants such as periwinkle, hostas, Norway maple, etc., and even the so-called butterfly bush, will not feed our native caterpillars.  If you want butterflies, you’ve got to have caterpillars, and caterpillars require the fresh green leaves of native plants.  And, the flowers of native plants are excellent pollen and nectar sources for all pollinators.  Most of us are aware that the monarch butterfly requires native milkweeds for its caterpillar.  It is no exception.  If all gardens had host plants for caterpillars, butterflies might not be in decline.