The checkered pattern remains uncommon but died in 1976 due to adjustments in forest management.
The descendants of adults released at Rockingham Forest, Northamptonshire, in 2018 were last summer.
Susannah O’Riordan of the Back from the Brink project said, “We haven’t done everything we expected this year.”
But she said she was “very pleased to verify that [butterflies] are still present” in the forest.
The decrease in scrub and the increase in conifer plantations that were suitable for the butterfly led to its extinction in England.
Butterflies caught in Belgium and taken to the UK in Eurostar were released into the secret forest two years ago as a component of the Butterfly Conservation charity project.
Another batch of 24 checkered Belgian employers brought in last year, but Ms. O’Riordan said they “couldn’t make another outing as planned,” which would have been the last of the project.
She said that once the lockout restrictions were lifted, the assignment team was able to return to the forest and “recorded a moderate number of butterflies.”
“Everything looks promising, which is great,” he added.
And a credit for the blockade, he said, was that butterflies “will have benefited from an intact habitat.”
Ms. O’Riordan said the team would remove paints from the habitat in the fall and the butterfly larvae.
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