A Healthy Monarch Caterpillar (IMAGE) Emory Health Sciences Caption Monarch caterpillars feed on any of dozens of species of milkweed plants, including some species that contain high levels of cardenolides, which act as medicinal toxins. Credit Photo by Jaap de Roode, Emory University Usage Restrictions Must include photo credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.