The missing monarchs of Mexico

Why a magnificent butterfly population is in decline, and how we can join Michelle Obama in an effort to do something about it.

I’ve had a special relationship with monarch butterflies all my life. Call it an obsession — I don’t mind. As a young lad I collected and bred monarch caterpillars, mounted and attempted to classify the butterflies, and then had my entire collection stolen by a visiting lepidopterist (see my article “Vanishing Monarchs”). I have also told you the story of my visit, half a century later, to the winter breeding grounds of these beautiful creatures (“Monarchs in Mexico — millions of them”).

I have been reading Scientific American for all of my adult life. So it was most gratifying to find an article in the latest issue that described and confirmed my second story. “What Is Really Killing Monarch Butterflies?” describes how in the late 1970s, after a long search, scientists had found the breeding ground in Mexico. Quite incredibly, monarchs from the northern United States and even Canada each year undertake a journey of many thousands of miles to find a specific forest on the slope of a mountain in Michoacán. The article tells us how ecologist Karen Oberhauser scrambled up the mountain in 1997, ten year before my own very memorable visit to the refuge. There she, like me, saw “millions of monarchs draped like living jewels on fir trees that hugged the slopes.”

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Credit: Nigel Hawtin; Sources: “Mechanisms behind the Monarch’s Decline,” by Anurag A. Agrawal and Hidetoshi Inamine, in Science, Vol. 360; June 2018; World Wildlife Fund, Mexico (2018 winter population data).

Above is the migration pattern described in my article on monarchs in Mexico. In the late summer the northern American monarchs don’t mate or lay eggs, but feed vigorously in preparation for the journey to the south. They set out and are able to locate the 18 hectares breeding forest, where they congregate on the same pine and oak trees as the generations before them have done. After mating they begin the northern migration, only making it part of the way. The females lay their eggs on milkweed bushes, mainly in Texas and Oklahoma, and the new generation continues on the next leg of the migration. The normal lifespan of these monarchs is less than two months, and in the fall the third generation arrives in the northern breeding locations. Then the fourth generation can embark on the very long journey south, all the way to that same little spot in Mexico.

Unfortunately Oberhauser and other ecologists have discovered that Mexico’s butterfly population is crashing. During her first visit to Michoacán there had been about 300 million butterflies, but just over a decade later there were fewer than 100 million. The scientists concluded that the migrating butterflies found fewer milkweed plants in the farm fields to the north, which meant fewer eggs, and fewer adults returning to Mexico. In 2012 Oberhauser published a paper describing her “milkweed limitation hypothesis”: The main cause of the monarch decline was the extensive use of a particular herbicide. The agricultural company Monsanto had engineered corn and soy plants to withstand exposure to the glyphosate, better known by its trade name Roundup. Farmers were dousing their fields there with the weed killer, which leaves the agricultural crops unharmed but kills nearly everything else in a field. For farmers, “Roundup Ready” money crops were boons, for other plants they were a death sentence.

Oberhauser and others called on the general public to alleviate the situation by planting milkweed in large amounts, to make up for the Roundup losses. Thousands of citizen conservationists answered the call., and more than 10,000 “monarch way stations” sprouted around the country.

Subsequently President Barack Obama (and his Mexican and Canadian counterparts) promised to protect the butterfly, and in a well-publicised action First Lady Michelle Obama joined the effort. In 2014 she introduced pollinator plants, including milkweed, in the White House Kitchen Garden [photo via Obama Foodorama Blog]. They were inedible to humans, but were meant to “encourage the production of bees and Monarch butterflies.” What a fine effort by such a fine lady.

Some scientists have, however, called the pure milkweed case into question. Many of the dwindling Michoacán butterflies come from U.S. areas which do not have Roundup-soaked crop fields. Something else must be taking them out on their way to Mexico. The identity of that something, however, remains elusive. Perhaps it is the loss of nectar plants that adult monarchs feed on during their southward journey, or it may be a yet unidentified parasite infection that is decimating the migrants. The milkweed loss may be cause number one, but other factors are also present, factors that make helping the insects more complicated.

If you are interested in the complex subject of monarch decline read this March 2020 Scientific American article.

Written by

Frederic Alois Friedel, born in 1945, science journalist, co-founder of ChessBase, studied Philosophy and Linguistics at the University of Hamburg and Oxford.

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