India, Sept. 14 -- L ike most insects, butterflies are not terribly good fliers. They often rely on wind currents to help them to their destinations. As rising temperatures affect forests, vegetation and Earth's massive wind systems, the monarch butterfly is finding itself caught in the crosshairs. It isn't the only one, but it is a good bellwether of what is unfolding in the world of migratory insects, because few species have been studied so extensively, for so long. The wealth of data gathered on these lepidopterans since the 1950s is giving researchers unique insight into how hard these butterflies have been hit. There was a time when they arrived in California in such large numbers, one could hear them congregate. As millions of the winged things settled in, after a 1,000-km journey from the Rocky Mountains, it was said they made a sound like that of a rippling stream or a summer rain. They don't make this sound any more. There simply aren't enough of them. The numbers that arrive in California now stand at 5% of what they were in the 1980s, according to a 2024 report by the Xerces Society for Invertebrate Conservation in Oregon. The dip has been similar in the east- ern population, which flies 4,000 km from the Rocky Mountains to the fir forests of central Mexico, says Emma Pelton, senior conservation biologist at the Xerces Society. These butterflies once covered a staggering 45 acres of these forests, in 1995-96. (Monarchs are so small and numerous that their population is calculated by the area they occupy.) By 2003-04, that area had fallen to 27.5 acres. In 2024-25, they occupied just 4.42 acres. "Forest loss in Mexico has been one of the major causes of the decline," says Leslie Ries, an ecologist at Georgetown University and head of the North American Butterfly Monitoring Network. Herbicides for genetically modified crops made things worse in the early 1990s, she adds, by killing off the native milkweed that is the larval host plant for the monarch butterfly. "The most likely culprits impacting them now are insecticides and climate," she adds. Efforts are being made to stem the loss. A 56-hectare Monarch Butterfly Biosphere Reserve was notified by Mexico in 1980, and declared a UNESCO World Heritage Site in 2008. In California, a number of state parks serve as protected areas too. Across the US, organisations distribute monitoring kits and citizen scientists set up monarch waystations, complete with milkweed plants. These may help, but they cannot, of course, replace the forest networks that supported this species over tens of thousands of years....