Wainwright has been searching for new English colonies since he started working for Butterfly Conservation in 2004.
He even visited the Wallington estate that year but found none.
But on his most recent visit, Wainwright said they found one “within a few minutes”.
“I was hopeful – I’m always hopeful- but I definitely didn’t expect it because I’ve been to so many sites to look for this species over the years, so my heart and head were in two different places,” he said.
“It was one of the highlights of my career.
“Overwhelmingly it was just a feeling of relief that we weren’t just fighting the fight at one site anymore.”
The moth’s caterpillars will only eat two plants – aspen and creeping willow – which makes it a challenge to conserve, according to Butterfly Conservation.
Wainwright said Wallington was a different environment to Strensell, which conservationists had previously based their idea of a “good habitat” for the moths, opening up new possibilities.
“The thing that’s kept me going all these years is I always felt there was a chance of finding dark bordered beauty on the Northumberland/Cumbria border – now it feels like there could be other colonies out there waiting to be discovered which would be brilliant,” he said.
The National Trust, which runs Wallington, said it hoped to expand the potential habitat for the moth.
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