UK butterflies bounced back in 2018 following a string of poor years, thanks in part to last year’s heatwave summer, a new study has revealed.

More than two-thirds of UK butterfly species (39 of 57) were seen in higher numbers than in 2017, with two of our rarest, the Large Blue and Black Hairstreak, recording their best years since records began. Numbers of the threatened Large Blue rose by 58 percent from 2017 levels and the endangered Black Hairstreak was up by more than 900 percent. Both species benefitted from warm and sunny weather when they were flying in the early summer, whilst the cold spell in February and March may have also helped by improving survival of caterpillars and chrysalises.

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Numbers of the threatened Large Blue rose by 58 percent. image:Neil Hulme, Butterfly Conservation

The results have come from the annual UK Butterfly Monitoring Scheme (UKBMS) led by Butterfly Conservation, the Centre for Ecology & Hydrology (CEH), British Trust for Ornithology (BTO) and Joint Nature Conservation Committee (JNCC).

They also revealed that Brown Argus and Speckled Wood butterflies had thrived, recording their third best year on record.

Common White butterflies experienced a good year after a recent run of below-average seasons with Large White annual abundance up 118 percent, the Small White rising by 155 percent and the Green-veined White increasing by 63 percent, again due to warm and sunny weather from April to the end of July.

The threatened Duke of Burgundy was up 65 percent. This butterfly has been the subject of intensive conservation efforts in recent years from Butterfly Conservation and partners, with some of the biggest annual increases seen at conservation sites. The butterfly’s population has stabilised over the last 10 years in the face of a significant long-term decline.

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The threatened Duke of Burgundy was up 65 percent. image: Bob Eade, Butterfly Conservation

But despite the upturn, 2018 was still only an average year for the UK’s butterflies. Around two thirds of species (36 of 57) show an apparent decline since records began 43 years ago, with 21 of these showing significant long-term declines.

The hot spring and summer weather was not ideal for all species. Some grassland butterflies struggled, not helped by drought conditions drying out caterpillar food plants. The Gatekeeper dropped by 20 percent from 2017 levels and the Small Skipper and Essex Skipper were down by 24 percent and 32 percent respectively.

It was also a surprisingly poor year for some garden favourites. The Small Tortoiseshell slumped by 38 percent compared to the previous year and the Peacock was down 25 percent, whilst the migratory Red Admiral crashed by 75 percent after a good year in 2017.

Professor Tom Brereton, Associate Director of Monitoring 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".