Milkweed Beetles

It is late June and milkweed flowers are in bloom. Common milkweed is famous for being a little ecosystem of its own. It may be most famous for its relationship to monarch butterflies. Monarch butterflies depend on milkweed to reproduce. They lay their eggs on milkweed plants. The caterpillars eat the leaves, and in so doing absorb and retain chemicals called cardiac glycosides. These chemicals make the caterpillars and adults toxic to predators, who learn not to eat them, an advantage to their overall survival. However, monarch butterflies are not the only creatures to take advantage of these plant toxins. Milkweed beetles also take advantage of these toxic chemicals.

Cardiac glycosides are produced by milkweed and other plants such as foxglove as a defense against the plant being eaten. Mammals which eat milkweed are sickened and learn not to eat them. Most insects are unable to eat milkweed leaves. However monarchs, red milkweed beetles, and milkweed bugs have separately evolved the ability to ingest the cardiac glycosides without ill effect, and to turn this to their advantage by becoming toxic themselves. In order for this to be effective, these insects are brightly colored as a warning signal, and the common feature is that they are all red and black. The adult monarch butterfly, of course, has these colors on the wings.

The red milkweed beetle has the same colors on its head, thorax, and forewings.

Birds which have learned not to eat red and black monarch adults are also warned off from eating red and black milkweed beetles which are similarly toxic. Milkweed bugs, which appear later in the year and eat milkweed seeds are also toxic and are red and black in color as well. This is an example of convergent evolution where distinct species of insects have separately evolved the same successful strategy of ingesting cardiac glycosides and have evolved the same color scheme which likely mutually reinforces the warning effect of the coloration. It is also an example of mimicry, where one species evolves an attribute, in this case coloration, which is already effective in another species. An example of pure mimicry is the viceroy butterfly which is not toxic and does not feed on milkweed, but is protected from predation by having a wing color pattern similar to monarch butterflies.

This evolutionary pattern also illustrates the fact that evolution is more an effect on the survival of a gene rather than on an individual. Some individual monarchs or milkweed beetles have to be eaten in order for the birds to learn that they are poisonous. The individual insects are killed, but other members of the species carrying the same genetic information are protected, and the genetic information lives on.