NUMBERS of one of our best known garden butterflies have plummeted this summer despite the record-breaking hot weather, Dorset conservationists have revealed.

The Small Tortoiseshell suffered its worst summer in the history of the Big Butterfly Count with sightings falling by 32 per cent compared to last year.

Just 23,000 Small Tortoiseshell were counted by participants across the UK during the three-week recording period, which coincided with the joint hottest summer on record.

The population of the once common and widespread Small Tortoiseshell has collapsed by 75 per cent since the 1970s and there are now growing concerns among scientists for the butterfly’s long-term future. Reasons for the ongoing decline are being investigated with climate change, pollution and parasites all possible culprits.

The hot, dry weather experienced across the UK this summer should have helped most butterflies, but the Small Tortoiseshell was not the only species to suffer a slump.

Populations of Red Admiral and Comma were down 73 per cent and 40 per cent respectively, compared to the high numbers seen in the same period last year, and Gatekeeper fell by 54 per cent.

But many other common species were seen in improved numbers in response to the glorious summer weather.

The three white butterflies enjoyed a bumper summer, each recording large increases compared to last year.

They accounted for more than half of the 964,000 butterflies and moths recorded.

Meanwhile, the Holly Blue recorded its highest ever numbers in the history of the project and was up 122% compared to 2017. The Common Blue was up 51% on the same period last year and had its best results since 2010.

Despite the good weather, an average of just 11 butterflies of the 19 target species were seen per count this year, a figure only marginally higher than last year - the lowest in the count’s history.

The low numbers are thought to be a result of some species emerging earlier than usual in response to the heatwave, which meant numbers were declining before the count started.

Richard Fox, associate director of recording and research at Lulworth-based Butterfly Conservation, said: “The ongoing decline of Small Tortoiseshell is clearly a concern, especially given the generally good weather this summer, but it was good to see the high numbers of whites and Common Blue being reported across the UK.

“This year’s count has been a phenomenal success with more than 100,000 people enjoying beautiful butterflies, contributing towards conservation and reaping the mental and physical benefits of being outdoors in nature. Participants’ records are really important as they help us find out how the UK’s common species are faring and how to best protect them in the future.”

More than three-quarters of the UK’s butterflies have declined in the last 40 years with some common species suffering significant slumps.