Milkweeds have become a popular garden plant the last several years. They are most commonly planted to help support monarch butterflies because milkweeds are the sole food source for monarch caterpillars.
Milkweeds contain toxic compounds (cardiac glycosides) to deter animals (insects and mammals) from feeding on them. However, monarchs have evolved to be able to feed on these plants. Additionally, they can take these chemicals and incorporate them into their bodies, making them unpalatable as well.