Big Butterfly Count records worst summer ever for the species, despite the heatwave providing a boost to most others

This article is more than 2 years old

This article is more than 2 years old

Fears are growing for the small tortoiseshell butterfly after this once-common garden insect continued its baffling decline despite the hot summer proving a boon to most species.

The small tortoiseshell suffered its worst summer in the history of the Big Butterfly Count with sightings falling by 32% compared with last year, according to the charity Butterfly Conservation.

The hot, dry weather is believed to have boosted many of Britain’s 59 native butterfly species this summer as a record 100,000 people spotted nearly one million butterflies during the three-week count, the world’s largest butterfly survey.

Most common was the small white, with numbers up 161% on the previous year, while the large white came second, up 104%. The three “cabbage whites” – the large white, small white and green-veined white – made up 55% of the total butterflies recorded.

“It was definitely a white summer and the blues did really well too,” said Richard Fox of Butterfly Conservation. “We suspect it was a good summer for most species.”

After fears over the decline of the common blue, this small butterfly bounced back with its best results since 2010, while the commonest blue butterfly seen in gardens, the holly blue, recorded its highest ever numbers in the count’s history.

Facebook Twitter Pinterest The holly blue recorded its highest ever numbers in the count’s history. Photograph: Tim Melling/Butterfly Conservation

Overall, however, the good weather did not produce a butterfly bonanza for participants: on average, during the 15-minute counts in local parks and gardens, people saw 11 butterflies of the 19 common species targeted in the project. This abundance was only marginally higher than last year – the worst in the count’s history.

According to Fox this was because hot weather caused many common species – including the resurgent marbled white, the ringlet and meadow brown – to emerge earlier in the summer, before the count began at the end of July.

Populations of frequently-seen garden butterflies the red admiral and comma were well down on last year but Fox said that was to be expected after both species experienced exceptional years the previous summer.

The small tortoiseshell’s decline is not following a typical insect boom-and-bust cycle, however, with the population of this once-ubiquitous species falling by 75% since 1976.

Facebook Twitter Pinterest Small tortoiseshell caterpillars feed on nettles. Photograph: Frank Hecker/Alamy

Its decline is a particular puzzle, according to Fox, because its caterpillar’s foodplant, the stinging nettle, is almost certainly more widespread in Britain than at any point in history. The nettle thrives on nitrate-rich land caused by vehicle emissions and farm fertilisers. Studies show that the small tortoiseshell caterpillars thrive on nitrate-rich nettles as well.

Fox said: “We know a lot about the decline of rare specialist species in Britain but the once-common ‘wider countryside’ species such as small tortoiseshell, gatekeeper and small skipper are undergoing big declines which we don’t understand. It might be to do with climate change or general land management such as pesticide use.

“If that rate of decline continues then the small tortoiseshell may become a threatened species.

“How our wider countryside species are faring is very much one of Butterfly Conservation’s priorities, and the Big Butterfly Count is really useful in providing data about them.”