Up to 200 red admiral butterflies are being released on Sunday at a Conwy county nature reserve.
The event, which takes place at 14:00 BST at Pensychnant, near Conwy, aims to raise awareness of the decline in butterfly numbers across the UK.
Ray Sandiford bred the insects at his butterfly house in Bolton, Greater Manchester, from caterpillars he collected in north Wales.
“The decline in the numbers is coming fast and furious,” he said.
Mr Sandiford has been fascinated with butterflies since he was a boy.
He founded the north Wales branch of the Butterfly Conservation Society when he lived in Betws-Y-Coed, Gwynedd in the mid 1990s.
Each spring, he returns to north Wales – usually Criccieth and Anglesey – to hunt through nettle patches where some butterfly species lay their eggs.
Any caterpillars he finds, he takes home.
“I breed them through and when they come into butterflies, they will mate here, lay their eggs and these eggs I breed through into pupae, back into butterflies and then in September I do butterfly releases,” he said.
“I’ll probably be taking between 150 to 200 to release in Pensychnant and, when they fly off, it’s the most fantastic thing. People just go ‘wow’.
“We get quite a few families and I give the children bits of banana to put on their fingers, so the butterfly will land and stay there for a while to feed on the banana. It gives people a chance to take photos.”
Mr Sandiford has managed to breed about 1,000 red admirals over the summer – a far greater number than would have survived in the wild, he said.
“That’s a fantastic year,” he added. “I haven’t had a year like this for seven years because the caterpillars just haven’t been there.”
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But other species have not fared so well – Mr Sandiford has been unable to find a single peacock caterpillar, despite trawling north Wales and the Bolton area.
“There’s a general decline in butterfly numbers and a lot of it is down to habitat loss. So many nettle patches are overgrown by balsam, blackberries and tall grasses. We need to raise awareness of the problem.
“People don’t like nettles in the garden but if you grow nettles in a pot and then have some buddleias to attract the butterflies, hopefully you should have some laying eggs in the nettles.
“I just think that they’re so beautiful, why would we want to lose them?”