Why do monarch butterflies only lay eggs on milkweed?

Monarch Butterflies exclusively lay their eggs on milkweed because it is the only plant that provides the necessary nutrients for their larvae to thrive. Milkweed contains toxic compounds called cardenolides, which the caterpillars ingest and store, making them distasteful to predators. This specialized relationship ensures that the caterpillars have both food and a means of protection from potential threats. Additionally, milkweed is the primary host plant for monarchs, supporting their life cycle effectively.