The Reigning Butterfly

They can travel 3,000 miles on a migration. They can glide at an altitude of 11,000 feet. You probably know them well, but there is more to them than meets the eye. The Monarch butterfly (Danaus plexippus) can surprise you.

Monarchs are naturally poisonous to predators, because the milkweeds their larvae eat contain cardiac glycosides, a heart poison. Their bright orange color is aposematic, warning predators that the butterflies are toxic. It was always thought that the Viceroy butterfly (Limenitis archippus) was a non-toxic mimic of the toxic Monarch, an adaptation known as Batesian mimicry. It turns out that both species are toxic! When one toxic species imitates another one, that’s called Müllerian mimicry.

One explanation for the origin of  the Monarch’s name is that early British settlers in North America were so impressed with the butterfly’s orange color that they named it after King William III, Prince of Orange.

There are actually three species of Monarch butterflies. Danaus plexippus is the species everyone knows in North America. It is also found in Hawaii, Australia, New Zealand, and the Pacific Islands. They are called “Wanderer” butterflies in Australia. The Southern Monarch (Danaus erippus) is found in tropical and sub-tropical South America. Some researchers believe the North American and South American Monarch were once the same species. The third species is the Caribbean’s Jamaican Monarch (Danaus cleophile), which ranges from Jamaica to Hispaniola.

Soon, in early September, only the “Methuselah generation,” the fourth generation to hatch in the eastern population, will migrate south 3,000 miles (more than 4,800 kilometers) to an isolated mountaintop—the oyamel fir forest in the mountains of southwestern Mexico. Amazingly, the butterflies have never been there before! Scientists are still trying to figure out how they find their way. During migration, a Monarch can soar to 11,000 feet (more than 3,300 meters) and spend a lot of time gliding, using warm, thermal air currents the way that hawks do. Many Monarch butterflies arrive at their overwintering sites in Mexico at the beginning of November, when local communities celebrate Dia de los Muertos, the Day of the Dead. According to traditional beliefs, the butterflies are the souls of the ancestors returning for their annual visit.

How do the butterflies know what path to follow on their migration? Researchers have found that the butterflies have a pair of molecules that are sensitive to the earth’s magnetic field, like a compass. They use these molecules and information from the position of the sun to find their way. You can follow the northward and southward Monarch migrations at Journey North.

I’ve seen Monarch butterflies laying eggs later in the season this year, and I’m not sure why. There are so many factors that can affect the population size and timing of hatches and arrival. Both the eastern Monarch population and the western population (that migrates along the Pacific coast to southern California) have been experiencing serious declines in the last 20 years. There are several reasons for this. Some farmers are using genetically modified crops that are resistant to herbicides, but the herbicides also kill milkweeds growing next to these crops. There is a major decline in milkweed populations due to habitat loss. Cars striking butterflies and habitat loss at overwintering sites are also factors. Climate change might cause Monarchs to migrate farther north, and that might make it harder for future last generations to reach Mexico.

What can you do? Planting a garden for them will help. Monarch larvae (caterpillars) eat native milkweeds (see my post Milkweeds Aren’t Weeds from June 11, 2020 for more on different species). Many nurseries should be having sales and now through fall is a good time to plant. Always ask whether the plants you buy are neonicotinoid free. Neonicotinoids are widely used systemic insecticides that will get into plant leaves and pollen and poison caterpillars, bees, and other insects.

Although larvae eat only milkweeds, adults feed on a variety of nectar plants, including Joe-Pye Weeds (Eutrochium sp.), Blazing Stars (Liatris sp.), Asters (Symphyotrichum sp.), Coneflowers (Echinacea sp.), Goldenrod (Solidago sp.), and more.

Many people raise Monarch caterpillars. Put them in a protected location (pots of milkweeds in mesh containers work well) outdoors rather than in your house. Studies have shown that caterpillars raised indoors fail to migrate properly.

Another way you can help is by tagging migrating Monarchs in the fall. Each tag is a small dot of glue-backed paper with a unique identifying number. You gingerly place the tag on the lower hindwing. Tagged butterflies help researchers learn about migration timing and movement. To order, learn more about how to tag monarchs, and order tags, go to Monarch Watch.

Let’s all help the Monarch by including milkweeds and nectar plants in our gardens or planting pots of them on our apartment stoops and patios. We can work toward creating connecting habitats along Pollinator Pathways.