How Do Chemicals in Milkweed Benefit Monarchs?
Early in the 20th century, Poulton (1909) suggested that the reason that monarchs feed only on milkweeds might be because chemicals in the milkweeds provide them with some protection from predators. However, it was not until the 1960’s that researchers discovered that cardenolides were the chemicals in milkweed that made monarchs toxic and bitter-tasting (Parsons 1965). Since then, several studies have shown that monarchs are able to sequester these toxic compounds. For example, Figure 1, drawn from data collected by Malcolm and Brower (1989) shows the range of cardenolide concentrations found in one species of milkweed, A. viridis, and adult monarchs that fed on that milkweed as larvae. Since the adults do not eat milkweed, this shows that they have stored the cardenolides that they ingested as larvae. Figure 1. Frequency distribution of cardenolide concentrations in A. viridis (collected in Louisiana) and monarch butterflies fed this species. This shows that monarchs are able to stor
