Parasites are common in nature and by definition cause disease or death to their hosts. As a consequence, hosts have evolved defenses to protect themselves against parasites. The use of medication is one such defense, and is well-known in humans. However, whether animals also use medication is still poorly understood. The caterpillars of monarch butterflies eat milkweed plants, and some milkweeds are toxic to monarch butterfly parasites. This project will study if monarchs can use milkweeds as a form of medication by preferentially laying their eggs on toxic milkweeds that reduce parasite growth and disease in the monarchs' offspring. The researchers will use experiments to test for the medicinal effects of different types of milkweed, including different species and flowering versus non-flowering plants. These experiments will also be used to identify the chemicals that are responsible for the milkweeds' medicinal properties. The project will further test whether monarchs from four populations in North America and Australia have evolved the ability to use milkweeds as medication. Parasite risk varies greatly between these populations, and the project will test whether parasite risk affects the type of medication behavior that monarchs have evolved. In particular, it is expected that monarchs from high-risk populations always lay their eggs on medicinal plants (a form of prophylactic medication), whereas monarchs from low-risk populations only lay their eggs on medicinal plants when infected (a form of therapeutic medication). This project will enhance our understanding of how animals fight disease in nature, and will emphasize the role of parasites in the evolution of animal behavior. The project will also integrate research with education. Students of all levels (high school, undergraduate and graduate) and from underrepresented groups will participate in the design, execution and presentation of this work. The researchers will also host high school teachers in the lab to develop research projects for use in their classes to strengthen their ecology and evolution curriculum. Finally, the researchers will visit monarch butterfly festivals to teach the general public about science.