Controversy surrounds Black-veined White sightings in London

The appearance of Black-veined Whites at a London reserve has sparked controversy over the butterflies' origin.

The species has been extinct in Britain for almost 120 years. As a result, the sudden appearance of the insects at Hutchinson's Bank, Croydon, in late May and early June was treated with suspicion and even some frustration from conservationists.

Black-veined White at Hutchinson's Bank (Tim Ballard).

Some commentors on Twitter expressed disappointment at what they considered to be an illegal release of captive-bred butterflies, with many explaining that such a 'chucking out' can threaten the balance of a native ecosystem, as well as other issues such as where the butterflies have come from and how they will fare.

One Twitter user, @foggyknitter, said: "What happened is an open secret locally."

Butterfly Conservation issued a statement which said that "these sightings are the result of an unofficial release", adding that it is "supportive of reintroductions when they are based on sound research". The statement went on to say: "Undocumented releases confuse existing conservation effort. They disrupt the recording of species' natural ranges and trends, and Butterfly Conservation does not support these unofficial releases."

Hutchinson's Bank has a history of unexplained releases of butterflies, including Marsh and Glanville Fritillaries and Duke of Burgundy.