The large tortoiseshell has been spotted at several sites in Hampshire and across southern England in recent weeks, raising hopes of its return as a resident species.
Sightings have also been recorded in Kent, Sussex, Dorset, Cornwall and the Isle of Wight.
Professor Richard Fox, head of science at Butterfly Conservation, said: “This is great news about a beautiful and charismatic butterfly: if the individuals that are being seen can survive and reproduce then there’s no downside to the Large Tortoiseshell coming back, and we can look at what we might be able to do to help it.”
Large Tortoiseshells (Image: Butterfly Conservation)
The Large Tortoiseshell, Nymphalis polychloros, is a big, colourful butterfly with orange, black and blue wings. It is closely-related to the Small Tortoiseshell but has different markings.
Once widespread across England and Wales, the butterfly was last recorded as a resident, breeding species in the UK in the 1980s.
Its decline has been largely attributed to Dutch elm disease, which devastated elm trees – the primary host plant for its larvae.
Although rare sightings have occurred since its extinction, they are becoming increasingly common.
Butterfly Conservation says that if it were to update its official endangered species list tomorrow, it would probably class the Large Tortoiseshell as no longer extinct.
Large Tortoiseshells (Image: Butterfly Conservation)
Mr Fox said: “I would imagine that, if things carry on with this trajectory, then when we compile our next GB Red List we not classify the Large Tortoiseshell as regionally extinct.
“I think it’s reasonable to assume that the species is now established in several parts of Britain.”
Sustained growth in populations in the Netherlands has led ecologists to believe that the butterflies may be crossing the sea to England.
The large tortoiseshell overwinters as an adult and emerges from hibernation on the first warm days of spring, which is why lepidopterists have been reporting sightings in the past two weeks.
The first reported sighting this year was on the Isle of Wight on February 24.
At least 20 more sightings have since been reported across the Isle of Wight, Kent and Hampshire.