UK butterflies bounced back last year after a string of poor years as the country basked in a summer heatwave, conservationists have said.
More than two thirds of UK species were seen in higher numbers than in 2017, the annual UK butterfly monitoring scheme reveals.
Two of the rarest butterflies, the large blue and the black hairstreak recorded their best year since records began in 1976, as they benefited from warm and sunny weather when they were flying in the early summer.
Large blue butterfly numbers were up 58% on 2017 levels and black hairstreaks rose more than 900%.
Brown argus and speckled wood butterflies also thrived, recording their third best year on record, while it was a good year for the common white butterfly species after a run of below-average seasons.
Large whites, small whites and green-veined whites all saw numbers increase year-on-year due to warm and sunny weather from April to the end of July.
But despite the upturn on the previous few years, 2018 was still only an average year for UK butterflies, and two thirds of species remain in a state of decline over the long term, the experts said.
The hot spring and summer weather did not suit some grassland butterflies, with the drought conditions drying out caterpillar food plants.
Gatekeepers, small skippers and Essex skippers all saw their numbers fall compared to 2017.
And conservationists said it was a surprisingly poor year for some garden favourites, with small tortoisehells down 38% compared to the previous year and peacock butterflies down 25%.
Red admirals numbers crashed 75% from a good year in 2017.
But there was better news for the threatened Duke of Burgundy, which saw numbers rise 65%.
The species has been the subject of intensive conservation efforts in recent years, and the butterfly’s population has stabilised over the last 10 years in the face of significant long-term declines.
The UK butterfly monitoring scheme, run by Butterfly Conservation, the centre for Ecology and Hydrology, British Trust for Ornithology and Joint Nature Conservation Committee, involves thousands of volunteers collecting data throughout the summer.
Professor Tom Brereton, at Butterfly Conservation, said: “2018 brought some welcome relief for butterflies following five below average years in a row.
“But, there were not as many butterflies around as we might have expected given the fabulous weather over much of the butterfly season and overall 2018 ranked as barely better than average.
“This and the fact that two thirds of butterflies show negative trends over the long term, highlights the scale of the challenge we face in restoring their fortunes and creating a healthier environment.”
He added: “It remains to be seen what the knock-on effects of the 2019 heatwave will be.
“We know that extreme events such as this, which are set to increase under climate change, are generally damaging to butterflies.”
Dr Marc Botham, from the Centre for Ecology and Hydrology, said the results showed the positive impact that suitable weather conditions could have if there was the right habitat in place for butterflies to thrive.
“Thanks to ongoing habitat management, many of our threatened species can benefit from the good weather like that of summer 2018, but more still needs to be done to improve the condition of the wider countryside as a whole so other species can also take advantage.
“This can start in our own back gardens, by leaving areas unmown and planting native wildflower species, for example,” he said.