On a wing and without a prayer – the decline of the monarch butterfly

Out of America: More GM crops mean more herbicides – which destroy the food the insects need for their epic migration

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As you read this, the first intrepid invaders are fluttering northward across the Rio Grande that separates Mexico from the United States. Within days, they will be followed by millions of others, in one of earth's most extraordinary rites of spring. The great migration of the monarch butterfly is under way – and never has one been watched so closely, and so anxiously.

Forget the politicians and the rest of the human activity that makes Washington tick. The real wonders of this place, I have come to realise, are natural – and two of them in particular. The first are the Brood X cicadas, the larvae of which grow underground for 17 years before emerging in late spring to spend six short weeks in our sunlit world, in which time they mate, breed and die. They are sluggish creatures and seemingly half blind, to judge by the way they splatter into walls and windows. They also make a deafening din that makes summer's standard cicada buzz a whisper. Cicada-fanciers say they are delicious deep fried, but to my shame I've never dared to try one. More to the point, they appear a mere five times in the average human lifetime. Currently the larvae, which last surfaced midway through the George W Bush presidency, have served slightly over half their subterranean sentence and will reappear in 2021 (when, who knows, Hillary Clinton may be embarking on her second White House term).

But the glorious visitation of the monarchs takes place every year. Here in DC, we're on the edge of the migration, but in May a few northbound ones, instantly recognisable through their gaudy gold and black colouring, usually drop into the garden to refuel. More may be seen in late September or October on their way back to Mexico, still a month or two's travel away. This year, however, there may be fewer than ever. The monarch, alas, is in big trouble.

In its way, a monarch's life cycle is even more extraordinary than that of Brood X cicadas. It winters in one of just a dozen patches of fir forest in the Sierra Madre mountains, to the west of Mexico City. In early to mid-March it begins the epic northward journey. When the monarchs reach the southern US they lay their eggs, and a new generation is born, with an overall life cycle of up to eight weeks.

Twice more this process – egg to caterpillar to chrysalis to butterfly – repeats itself as the monarchs fly north to the Canadian border and beyond, where a fourth generation of monarchs is born. But this final generation of the year is different. It lives for eight or nine months, not weeks, and does not breed until the very end. Its mission is to fly back to those same fir trees whence its great-great-grandparents departed eight or nine months earlier. There they wait out the winter before the whole cycle starts again.

How the monarchs navigate their 4,000-mile round trip no one is quite sure. Creatures of many species migrate, but usually with a parent to show them the way. No such luck for the monarch; the last members of the species to ply the route were four generations ago. Yet somehow they manage; year after year, monarchs west of the Rockies go back to a few similar acres of highland forests in southern California, while east of the Rockies the butterflies return to Mexico.

These latter could take a wrong turn to Florida; they could get lost over the Gulf of Mexico and drown. But most don't. Instead, they home in on a 50-mile-wide corridor across the Rio Grande, some 200 miles due west of San Antonio, that leads to safety. Instinctively, therefore, they can measure both latitude and longitude – something, incidentally, that humans couldn't manage until the 18th century, when John Harrison invented his famous clock for seafarers.

Last week, however, brought alarming news. According to the new census by the Mexican authorities, the number of returning monarchs last winter was the lowest in two decades. Obviously, the number of butterflies cannot be counted, but the area of forest occupied by their colonies dropped by 60 per cent from the year before, to just 2.94 acres. And while year-on-year fluctuations are normal, a long-term downward trend is unmistakable.

The reasons are several. A prime one used to be logging that destroyed the butterflies' winter habitat. But the Mexican authorities have created a 200 square mile biosphere reserve where tree harvesting is banned. More important, local people have realised that the butterflies are a tourist attraction far more valuable than hardwood.

Another factor, inevitably, has been climate change. On their journey north, the butterflies were assailed by the hottest year on record in the US. The early onset of heat upset the monarchs' breeding rhythm, while the scorching summer dried out eggs, and reduced the nectar content of flowers. On the way back, they encountered Texas's worst drought in decades.

Most serious of all have been farming developments in North America, where genetically modified corn and soybean allow the use of herbicides that wipe out the milkweed whose nectar feeds the caterpillars. Milkweed is not only an essential food; it provides a poisonous toxin that monarchs store in their bodies, making them unpalatable to birds and other predators. Indeed, the gaudy stripes of both caterpillar and mature butterfly are nature's way of saying "don't touch".

All may not be lost. Sooner or later the rains will return, and midwestern summers may turn milder for a while. Local US authorities may replenish the milkweed to help to preserve a creature that is a minor national institution, a piece of Americana in its own right. And every fan of the monarch, from DC to the Rockies, is demanding it succeeds.

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