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MEXICO CITY — Every winter of my childhood in Contepec, Michoacan, millions of orange and black monarch butterflies magically arrived to the oyamel fir forest at the summit of Altamirano Hill. When the sun was out, rivers of butterflies would stream through the streets in search of water. Altamirano was one of the five mountain ranges protected by the original 1986 decree that created the Monarch Butterfly Special Biosphere Reserve, for which I had petitioned Mexican President Miguel de la Madrid. In 2000, another decree enlarged the reserve’s size and added protection for more butterfly colony sites.

I was born in 1940 and grew up with (and wrote about) the monarchs, but it was only in 1975 that their overwintering forests in central Mexico’s Transvolcanic Belt were “discovered” by Canadian scientists.

On Jan. 29, news was released of a dramatic plunge in the monarch butterfly population that overwinters in Mexico after flying thousands of miles south from the northern and eastern United States and southern Canada. This season’s population, calculated by measuring the area of occupied trees, covers a tiny 0.67 hectares — the smallest ever since these measurements began 20 years ago — and a huge drop from the 1996 high of 21 hectares. The population has plummeted from an estimated 1 billion in 1996 to 33 million this year, scattered over seven sites. There have been no monarchs in Contepec for years.

In the past, most of the blame for the steady decline in monarch numbers has been placed on logging in the core and buffer zones of the reserve, out-of-control tourism and devastating climate events such as the 2002 storms in Michoacan or the severe 2011 drought in Texas. Now, it has become all too apparent that industrial agriculture in the U.S. Corn Belt (and less in Canada) is largely responsible for the dizzying drop in butterfly numbers. In the last decade, there has been a huge increase in additional land planted with corn to satisfy the demand for federally-mandated corn-based ethanol.

The main culprits are the herbicides relentlessly applied to genetically modified corn and soybean plants (grown from Monsanto’s Roundup Ready seeds, among others) that guarantee sterile fields where only the engineered crops are allowed to flourish. Ninety-three percent of total soybean acreage in the United States is now herbicide tolerant (HT), while 85 percent of corn is HT. Graphs show an alarming correlation between increased planting of HT crops and dwindling monarch populations, for milkweed — the only plant on which adult monarch butterflies lay their eggs and the only one the hatched larvae (caterpillars) will eat after devouring their own eggshells — is a main victim of these deadly glyphosate herbicides.

Common milkweed (Asclepias syriaca), so named for the sticky white sap that oozes from broken stalks, branches or leaves, is a summer-blooming perennial pollinated by insects such as bees, butterflies, moths or ants that land on the fragrant pinkish-purple flowers to suck the sweet nectar. The sap itself is toxic, and when the monarch larvae feed on the leaves, they become toxic to monarch butterfly predators.

Until now, the Mexican government has denied that the monarch butterfly migratory phenomenon is in jeopardy, despite warnings since the 1990s from preeminent monarch expert Lincoln Brower, but for the first time has acknowledged the possibility of its disappearance.

When I was Mexico’s ambassador to UNESCO and lobbied representatives of the 21 countries on the World Heritage Committee to vote in favor of including the Monarch Butterfly Biosphere Reserve on the World Heritage List, my colleagues from the United States and Canada were reluctant to do so, alleging opposition from their governments. They finally did give their votes, as a personal favor to me, along with the other 19, and the reserve is now listed.