The record heat of 2026 made this summer exceptional for butterfly-watching. More than 100,000 volunteers fanned across Britain with notebooks this summer, tallying nearly 1.4 million common butterflies and day-flying moths. This was the second-best butterfly season of the past 16 years, according to the Big Butterfly Count. Painted ladies surged by 335% compared with 2025. But this butterfly season could be a warning, according to experts.
Britain sees a butterfly boom after a summer of record heat
The record heat of summer 2026 has delivered the second-best butterfly season of the past 16 years in Britain. More than 100,000 volunteers counted almost 1.4 million common butterflies and moths this season. Painted ladies, a migratory species that island-hop from sub-Saharan Africa through several generations each summer, did the heavy lifting. There was a 335% rise in their numbers. The Butterfly Conservation organisation recorded their second-highest numbers since the count began in 2010. Other species, such as the holly blue and gatekeeper, also claimed their highest counts ever. The small copper landed third-best in its recorded history. The overall results matched the strength last seen in 2019, a summer that still ranks as the best in the dataset.
Butterfly boom may come at a cost
So what brought the butterfly boom? Adult butterflies want sunshine and dryness. They got both, in abundance. The five-heatwave season turned out to be perfect for feeding and mating. The significant surge also grabbed human attention. Migratory species like painted ladies rode favourable winds and found conditions ideal for breeding. In pure adult butterfly terms, the weather delivered. But experts believe that the boom may have simultaneously seeded a population crash for next year. Adult butterflies appreciate the bright summer, but for the caterpillars, it’s not really promising. The grass, wildflowers and other plants that caterpillars eat withered in the drought.“Adult butterflies love hot, sunny weather for flying, feeding and breeding, and that’s exactly why people saw so many this summer. But a good summer for adult butterflies is not the same as a good summer for butterfly populations,” Richard Fox, head of science at Butterfly Conservation, told The Guardian.“What matters far more is what’s happening on the ground, to the butterflies’ caterpillar offspring and the plants they depend on. The very same weather that made it a good year for butterflies – lots of sunshine and very little rain – could mean we’re actually on course for a population crash, which could take years to recover from.”
Population crashes followed the excellent butterfly years of 1976 and 1995, both linked to drought stress. But the trend may not always follow. The droughts in 2018 and 2022 failed to trigger comparable collapses. However, experts still confirm that such hot summers are often bad news for butterflies.But no two droughts are the same. Matthew Oates, an independent naturalist who has kept meticulous butterfly diaries since 1964, pointed to the difference. The 1976 drought was brutal for butterflies, partly because it followed a year of already-dry weather, priming plants to fail catastrophically. This summer was different. Winter 2025 was wet, and early June saw significant rainfall before the drought kicked in. These conditions may have given butterfly foodplants enough water during critical growth stages. He said he was hopeful that last year’s wet winter and wet week in early June might prevent the population crash next year.Though it has been a strong summer for migratory butterflies, drought-sensitive species struggled. The speckled wood, green-veined white and ringlet were all recorded in below-average numbers during the Big Butterfly Count, with declines of 44%, 54% and 39%, respectively, since 2011.Fox thinks there are winners and losers in hotter summers: “There will be very few butterfly species that can thrive in the drought situations we had this summer across much of the UK.”Both experts noted that Britain is entering unknown territory in terms of how climate extremes would impact native wildlife and flying insect populations.“If we get another summer of drought or excessive heat that’s really moving into a new future for us and our wildlife, including butterflies,” Fox said.“These last two summers have been way beyond our previous experience. We are out of our comfort zone and we can’t speak with any confidence any more. A lot of our butterflies are behaving more like they behave much farther south in Europe. In some ways it is exciting, but I worry about how relevant our existing, treasured knowledge is. We need every entomologist we can get in this country,” Oates added.