Pyrrolizidine alkaloids occur in several plant families, attracting ithomiine and danaine butterflies that specialize on the flowers. I show that pyrrolizidine alkaloids in artificial nectar also inhibit at least one butterfly,Agraulis vanillae (Nymphalidae: Heliconiinae), a more general forager. Inhibition was demonstrated in two ways: quantity of nectar consumed and number of artificial flowers visited. The amount of sucrose solution consumed by individual butterflies was measured using a microcapillary tube. Number of flower visits was determined using an array of artificial orange and yellow flowers.A. vanillae drank less sucrose solution with monocrotaline, a pyrrolizidine alkaloid, than without. When monocrotaline was placed into yellow flowers,A. vanillae learned to visit predominantly orange flowers. Evolutionarily, pyrrolizidine alkaloids in nectar may represent an adaptation to exclude butterflies. Ithomiines and danaines, seeking these compounds in larval food plants, were not excluded. Alternatively, ithomiines and danaines overcame the nectar defense. In either case, the plant effectively increased floral constancy by inhibiting generalist butterflies and attracting specialists.