“Insectageddon” has not occurred, but there has been a loss of butterfly diversity over the past half a century, according to the world’s largest insect monitoring scheme.

More than 44m butterfly sightings scientifically collected in Britain since 1976 show that of the 58 native species recorded, 33 species have declined and 25 have increased in number.

Global heating is proving a boon to some butterfly species: the purple emperor (up 136%), red admiral (up 330%) and comma (up 178%) are moving northwards through Britain owing to warmer weather.

But a number of rarer, “habitat specialist” butterflies – which rely on specific plants or habitats such as wetlands, woodlands or flower-rich grasslands – have sustained serious declines, including the high brown fritillary (down 66%), pearl-bordered fritillary (down 70%) and white-letter hairstreak (down 80%).

“It’s not the insect Armageddon picture that was put forward a few years back,” said Richard Fox, the head of science at Butterfly Conservation. “But the data show very clearly that butterflies have declined in the UK over the last 50 years, and in particular habitat specialist butterflies have declined.”

Pearl-bordered, small pearl-bordered and high brown fritillaries have all become rare butterflies because of the loss of traditional woodland management such as coppicing over the past century, which has reduced the floral-rich, sunny character of many woods.

Habitat loss and pollution are driving declines, but global heating is often playing an indirect role. According to Fox, the grassland-dwelling wall brown and grayling are likely to be vanishing because a combination of warmer weather and nitrogen pollution mainly from vehicles is causing grass to grow taller, making it too cool for some caterpillars to thrive.

“The rain is now dilute fertiliser and that’s raining down on our plant communities and causing them to change,” Fox said. “Changes in the composition, structure and even chemical makeup of plants is bound to have knock-on effects on specialist herbivores like our butterfly caterpillars.”

Rare and declining habitat specialist butterflies have been well-studied. Fox said conservationists generally knew what kind of conservation management could boost numbers of the declining species. “We just need to do a lot more of it,” he said.

The 50-year UK Butterfly Monitoring Scheme (UKBMS) dataset, jointly run by Butterfly Conservation, the UK Centre for Ecology & Hydrology, the British Trust for Ornithology and the Joint Nature Conservation Committee, reveals notable conservation successes with some habitat specialist butterflies.

The biggest winner of the past half a century is the large blue, which was declared extinct in Britain in 1979 but was reintroduced to specially managed grassland in Somerset. Its numbers have soared by 1,866% since 1983. Conservation work has also helped rare butterflies such as the silver-spotted skipper (up 300%) and the black hairstreak (up 844%).

Volunteer citizen scientists have walked more than 1.5m kilometres (900,000 miles) weekly during the summer months since 1976 at more than 7,600 sites across Britain to produce the data.

Prof Chris Thomas, the director of the Leverhulme Centre for Anthropocene Biodiversity at the University of York, who is not connected with the UKBMS, said: “The UKBMS scheme itself is exceptional, and should be celebrated. We can all agree that there has been a lot of change, and it is thanks to the UKBMS that we know this. The fact that 43% of species with data have increased over the last 50 years is fairly remarkable given the rates of environmental change.”

The UKBMS results for 2025 revealed that despite Britain experiencing its sunniest year on record, it was only the 20th best year for butterflies since 1976. Not a single species recorded its best year.

Fox added: “It’s not just that the total number of butterflies has gone down, it’s about the diversity that’s being eroded. Just as we have lost family-run shops and traditional skills from the nation’s high streets, so we’ve lost variety and diversity in the butterfly communities that can exist in our damaged and simplified landscapes.

“We have some remarkable species in this country, and we know what we need to do to help them: create more habitat. That is what Butterfly Conservation has been doing for more than 50 years, but we are fighting an increasingly urgent battle and we need more help.”