The milkweed plant also has high concentrations of cardiac glycosides in its sap. ... In addition, the caterpillar of the monarch butterfly dines exclusively on the sap of the milkweed plant, rendering both the caterpillar and butterfly mildly toxic as well.
Predators know that monarch butterflies are poisonous because of their bright coloration, which acts as a warning that they are poisonous.
This coloration warns potential predators that the animal contains poisonous chemicals. Warning coloration may work particularly well in adult butterflies because the hard body and wings allows a predator to bite the adult, taste the poison, and release the butterfly without killing it.