The mysteries of monarch migration

By on June 24, 2015
A monarch butterly and its larva (caterpillar). - Original artwork by Al Spoo

A monarch butterly and its larva (caterpillar). – Original artwork by Al Spoo

One of my readers called me last summer to inform me that monarch larvae were eating her parsley; she said she can prove that they don’t stick to a milkweed diet. I tried to tell her that the larvae she was describing were black swallowtail caterpillars, but she wouldn’t give up and insisted that they were monarch larvae. Of course when I arrived at her home and she took me to her garden they were not monarchs, but were black swallowtails.

The monarch larvae should not be confused with any other caterpillar in our area; they will not be found feeding on any plant except for some species of milkweed. In North America, there are about 108 species of milkweeds and each of these contain a poison that is known as a glycoside, which attacks the heart muscle of animals that eat the plant. The monarch larvae are immune to this poison and when they eat the milkweed they absorb this poison and it makes the caterpillars toxic to whomever eats them. A few exceptions do exist — the black-footed mouse and snakes are not bothered by the poison, neither are some Mexican species of birds.

The larvae of the monarch are striped with trifold colors of black white and yellow and have no spots on them and also sport four black tails; two of them protruding from each end of the caterpillar. The mature larvae of the black swallowtail have no black tails and have their black bands marked with yellow dots.

You might be so accustomed to the story of the monarchs going to Mexico for the winter, but you probably do not realize how recent the discovery of their migration really is. The story of the monarch migration was unknown to man until in 1890, when Haggart first noted that the monarchs did fly southward in fall. By 1911 Jenny Brooks first confirmed that they did migrate, because they were seen on the move going southward, but no one knew where they really went. And by 1949 Fred Urguhart began putting small tags on some of them and other people found the monarchs with his tags on them and by these tags it was discovered that the monarchs went to Mexico for the winter, but he didn’t have any idea where they stayed.

In 1973, Urguhart ran an ad in Mexican newspapers asking anyone to respond if they knew where the monarch over-wintered. His ad finally received a response in 1975 from an American, Ken Brugger, who was living in Mexico had found the place where the monarchs had migrated. The first colony was discovered in the Sierra Madre Mountains on the second of January in 1975 and in 1976 in the August issue, the National Geographic first published the story of the monarch migration.

Monarchs east of the Rockies migrate to one of five sites in the Mexican mountains; those west of the Rockies go to southern California and a few stay in Florida. Those that migrate to Mexico remain for about six months, but a monarch’s normal lifespan is only about six weeks and no one knows the reason why those that migrate are able to live a full six months — 30 times their normal lifespan.

At one time, various estimates of the monarchs per site are somewhere between 13 million to 100 million, and they were so numerous on the trees that they may cover the trunks or look like leaves on the branches. Like many other things in our world, things have changed and the depletion of the monarchs has caught the attention of the Fish and Wildlife service and now the once common monarch is being considered as a candidate for the endangered specie list.

No one knows for sure what is causing this depletion and there are various ideas put forth such as the use of generically engineered corn, which gives off poison that may precipitate on the nearby milkweeds and kill the larvae. Other ideas such as sprays and the mowing of milkweed along roadsides may contribute to their decline. In spite of these laws, some of the Mexican people are cutting down the trees on which the monarchs roost and are using them for firewood. This clearing of the land may eventually deplete the population.

Look around this summer and see how few monarchs you’ll see compared to what there were ten years ago!

Spoo is a longtime Rothsville resident and has had a lifelong interest in wildlife and insects. He was the first prize winner of the Lancaster County Science Fair in 1953. He frequently lectures and speaks to schools, clubs and other organizations. If you have a question for Al, he can be reached at 626-2684, or by e-mail at


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