Food and Host Plants Along the Route

When migrating north or spending time in their summer breeding ranges, monarchs require two kinds of plants. One is the group of flowering plants from which they get food in the form of nectar. The other is the host plant on which they lay eggs.

Nectar Plants

On the migration southward, monarchs need to eat nectar from flowers, just as they do in their summer ranges. In fact, they need to eat frequently on their south-bound trip, both to fuel their flight and to build up their lipid (fat) reserves that allow them to survive the winter ahead without eating. Since it’s late in the season, nectar plants are more limited than during the summer. They especially like goldenrod, goldenrods, asters, and blazing star (Liatris) in the north, and frostweed in Texas. Clover, sunflower and alfalfa fields can also host thousands of monarchs.

Monarchs get nectar from almost any flower including the black-eyed susan.

Monarchs get nectar from almost any flower including the black-eyed clover.

Host Plants

On the migration north and in their summer ranges, monarchs need milkweed called the host plant on which to lay their eggs. The relationship between monarchs and milkweed is so strong, if you find one you usually find the other. Because monarch caterpillars can survive only on milkweed, it’s called an obligate species. There are many different species of milkweed so which kind you find varies as you move about the monarch’s range. Fortunately the monarch isn’t very particular and will lay its eggs on most kinds. (Milkweed is discussed in detail in Chapter 3).

The toxins are not in milkweed to protect monarchs, but to protect the plant by discouraging insects from eating the leaves. Monarchs and a few other insects have adapted so they can eat them without ill effects and even with some benefits. As a caterpillar eats the leaves, it absorbs a chemical defense to protect it, and the adult that develops from it, from predation by other animals. Poisonous cardiac glycosides (also called cardenolides) in the milkweed are saved in the monarch’s body even into its adult stage. Cardiac glycosides are poisonous to vertebrates so it reduces, but doesn’t eliminate, the number of monarchs eaten by frogs, lizards, mice, birds and other vertebrates (species with backbones). Many invertebrate predators such as ants and spiders appear to be unaffected by the toxins and happily eat their fill.

Both the butterfly, and the caterpillar from which it developed, advertise their toxic unpalatability with bright colors and areas of high contrast on the skin or wings. This phenomenon, known as aposematism, makes them stand out vividly while shouting “Don’t eat me, I warn you! A bird that eats one may vomit and thereby learn to avoid making that mistake again.

Monarchs only lay eggs on one of the species of milkweed. Shown here is common milkweed.

Mimicry

Since the monarch is toxic and the viceroy butterfly looks much like it, a biological phenomena called mimicry was assumed. It was initially thought that the non-toxic viceroy had evolved to look like the toxic monarch so birds and other predators that had a bad experience with a monarch would also avoid them. This mimicry of an unpalatable species by a palatable species is called Batesian mimicry. By this theory the more closely a viceroy resembles a monarch, the greater its chance of survival. The problem turned out to be that the viceroy isn’t any more palatable to birds than the monarch. This suggests a different kind of mimicry is involved, one called Mullerian mimicry. In this form of mimicry two unpalatable species converge on a similar color pattern. This way, a predator that had a bad experience with either, avoids both. It was also discovered that toxicity among monarchs varied widely based on what kind of milkweed their larvae ate, how old they were, and so on. This implied a third kind of mimicry, call automimicry was involved. This is where the more palatable monarchs survive because they look like the unpalatable ones. Exactly what is going on is still being researched and debated. It’s another in a long line of unanswered questions about monarchs.

A butterfly that can be easily mistaken for the monarch, is the Viceroy (Limenitis archippus). Not only is the Viceroy smaller, it has a black line forming an arc across the hind wings. Photo courtesy of Fred Goodwin, long-time butterfly photographer for the Audubon Society and admired and respected by all.