A once-in-a-decade mass emergence of painted lady butterflies saw nearly half a million recorded across the UK.
Results from the Big Butterfly Count, which took place over three weeks this summer, showed 30 times more painted ladies arrived in the UK than in 2018.
The last big influx of migratory butterfly took place in 2009, when 11 million painted ladies were seen.
The count also found other common species had a good summer, helped by fine weather.
Peacock butterflies had their best summer since 2014, with a 235% increase in numbers sighted compared with last year, while the marbled white saw a 264% increase.
Red admirals were up 138%, gatekeepers up 95%, and there was a 64% rise in sightings of the six-spot burnet moth, one of two day-flying moths counted in the survey.
And the struggling small tortoiseshell had its best result since 2014, with around 70,000 spotted during this summer’s count.
Dr Zoe Randle, a surveys officer at Butterfly Conservation, said the high numbers were significant because butterflies are “vital food” for other wildlife, including birds, hedgehogs and dragonflies.
She added that the “clouds of painted ladies” that were seen this summer – 500,000 in just three weeks – were “fantastic news”.
Experts are still worried about the butterfly and have suggested climate change could be having an impact on its fortunes.
Butterfly Conservation’s Richard Fox said: “Last year the small tortoiseshell experienced its worst summer in the history of the Big Butterfly Count, so to see its numbers jump up by 167% this year is a big relief.”
He added that the results showed the species performed far better in Scotland and Northern Ireland than in England and Wales.
“We’re still trying to establish what is behind the long-term decline of the small tortoiseshell and, while it is good news that the butterfly fared better this summer, the poor results in southern England in particular suggest that climate change may be having more of an impact on this species than we have previously realised.”
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Mr Fox added: “The painted lady obviously stole the show this summer, taking the top spot in England, Wales, Scotland and Northern Ireland, but 2019 has also been the most successful Big Butterfly Count in its 10-year history, with more people taking part and more counts being submitted than ever before.”