Perhaps, the brightly colored monarch butterfly is nature’s most beautiful creation. Their orange wings and black stripes are majestic and fortunately serve as a warning sign for their predators.

Its main food source is toxic, and while monarchs are able to safely eat milkweed, it makes them poisonous. When a hungry bird sees their bright colored wings, it looks elsewhere to find another meal.  

The monarchs travel habits and diet are as remarkable as their beauty and are described clearly in Anurag Agrawal’s enlightening book, Monarchs and Milkweed.  Much of the information in this column is found there.

Migration

The monarchs’ annual migratory cycle — perhaps the most widely appreciated fact about them — individual butterflies travel up to five thousand kilometers (three thousand miles), from the United States and Canada to overwintering grounds in the highlands of Mexico.

After four months of rest, the same butterflies migrate back to the United States in the spring. Come summer, their children, grandchildren and great-grandchildren will populate the northern regions of America.

Milkweed The Only Food Source

There is much more to the monarch’s story than bright coloration and a penchant for epic journeys. For millions of years, monarchs have engaged in an evolutionary battle.

The monarch’s foe in this struggle is the milkweed plant, which takes its name from the sticky white emissions that exude from its leaves when they are damaged. The monarch-milkweed confrontation takes place on these leaves, which monarch caterpillars consume voraciously, as the plant is their exclusive food source.

Defense

Milkweed gets its name from the thick, white sap that oozes from any wound inflicted on its leaves or stems, such as might be made by a chewing or sucking insect. The sap, or latex, is under pressure within the plant’s tissues, so any damage to those tissues causes the latex to gush out and spread around the wounded area. The latex is extremely sticky, and insects can become entangled and permanently trapped by it as it dries in just a matter of minutes.

The leaves and stems are also covered in a carpet of fine, white hairs called trichomes that make it difficult for insects, especially small ones, to feed on the edible tissues beneath them. But the most formidable defense against herbivores that milkweeds possess is the chemical composition of their milky latex. Milkweed latex contains a cocktail of toxic chemicals including cardenolides, a type of steroid that interferes with the function of animal cells and is lethal in high doses.

Given the triple threat of defenses that milkweeds possess, you might think that it is well protected from herbivorous insects.

Indeed, there are only a handful of species that have evolved to be able to bypass all three defenses and feed on milkweed. Interestingly, these species have become specialized in eating milkweed. The most famous of the insects that evolved to be able to eat milkweed is the monarch butterfly, Danaus plexippus. While adult monarchs can feed on nectar from the flowers of a wide range of different types of plants, the caterpillars have a much more particular diet — they can only eat milkweed.

Baby caterpillars

Not all caterpillars are able to overcome all of the defenses that milkweed evolved. Many baby monarch caterpillars — about 60% in one study — succumb to the sticky latex after their first nibble of milkweed.

Those that survive clear an area of its trichomes —those fuzzy hairs that coat the surface of milkweed leaves and stems — by cutting them near the base with their mandibles. They don’t eat the trichomes, but removing them from an entire area makes it much easier to feed on the tissues below.  

The next step is often to create what monarch biologist and author Anurag Agrawal calls a circle trench. By cutting into the milkweed and turning its body and cutting again, the monarch caterpillar creates a circle that quickly turns white as the latex oozes out. This leaves the area inside the circle with only a limited source of latex with which to defend itself. This strategy has the added benefit of making it hard for predators to reach the caterpillar, as it is surrounded by sticky latex.

Cardenolides

But the caterpillar still has to get past the last and most challenging of the milkweed’s defenses: its toxic cardenolides. Monarchs’ ability to feed on milkweed without being poisoned can be traced to a mutation in a gene that makes part of the sodium-potassium pump found in the cell walls of all animals. A mutation involving just a single DNA base change was enough to make it difficult for cardenolides to bind to the sodium pump, thereby minimizing its toxicity. As birds have learned, monarchs also manage to use these toxins to their advantage.

Final Thoughts

Monarchs face numerous threats, from extreme weather due to climate change to the liberal use of pesticides, like Monsanto’s RoundUp, to habitat loss and fragmentation. And without intervention, they are at risk of going extinct.

Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at  maxsherman339@gmail.com.