The number of monarch butterflies that migrated from the United States to the forest-covered peaks of the Sierra Madre in Mexico fell again this year. In December 2013, the migration of the most popular butterfly in all of North America involved only 35 million individuals, covering an area of 6,700 square metres of forest according to data from the Mexican government and WWF. To put that into perspective, a football pitch is approximately 7,000 square metres.

This is the smallest figure ever recorded, far removed from the positive record set in 1996, when the butterflies covered an area of more than 182,000 square metres.

The decline of the species has been ongoing for years, but according to Karen Oberhauser, a biologist at the University of Minnesota who has been studying monarch butterflies for decades, "this is the third consecutive year in which such a significant drop has occurred — it's truly frightening." According to Oberhauser, the very phenomenon of migration is at risk.

The causes of the decline

The decline in the monarch butterfly population is said to be partly due to a couple of poor seasons from a meteorological standpoint, particularly in Texas. But the main cause is said to be the agricultural boom that has affected tens of millions of hectares of the American Midwest.

This argument is supported by the study Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population, conducted by Oberhauser together with John Pleasants of the Department of Ecology at the University of Iowa. Maize and soya cultivation has grown exponentially, above all following the increase in demand for biofuels.

The explosion in their use has led to an increase in the use of weedkillers, in particular Monsanto's agrochemical Roundup, sprayed directly onto the leaves of genetically modified plants. In 2013, 83 per cent of all GM maize and 93 per cent of all GM soya grown in the United States (nearly 63 million hectares) was treated with this herbicide, which destroys a particular type of plant greatly favoured by the butterflies: milkweed.

No coincidence, then: "We have found the smoking gun. This is the only thing we have been able to correlate with the decline in butterfly numbers," said Oberhauser. The data are clear, but there is some cause for hope — such as the prospect of favourable weather forecasts for the coming Texas spring, or the possibility that conservation organisations will take an interest in the problem and consider campaigning for the creation of weedkiller-free zones where every type of plant can grow freely.