A BUTTERFLY feared to be declining rapidly is on the rise in Dorset, experts said.

According to Butterfly Conservation, the small tortoiseshell has become increasingly rare across the south of the country in the past five years - possibly as a result of a parasitic fly.

But a recent influx of the attractive butterfly, thought to be from mainland Europe, could offer fresh hope for the tortoiseshell.

Butterfly Conservation said hundreds of small tortoiseshells were seen in late August and early September on the east coast, while large numbers were spotted inland in central and southern counties in the first half of September.

Large numbers were also reported in gardens across Dorset in the second week of September, and it is thought the benign weather this month could have helped the butterflies cross the Channel. Other migrants including the red admiral and large white butterflies have also been seen in large numbers on the south coast.

The conservation group said the small tortoiseshells were falling victim to a parasitic fly whose eggs are laid on nettles and are then eaten by the caterpillars who feed on the plants. The eggs hatch and develop inside the caterpillars, eating them from inside before bursting out of the cocoon.

Butterfly Conservation said the small tortoiseshell was one of a handful of butterflies which hibernate, and that the critical time for the new arrivals would be next spring.

Conservationists are hoping the new arrivals' caterpillars will not fall victim to the same fate as the British small tortoiseshells.

Dr Martin Warren, chief executive of Butterfly Conservation, said: "I was thrilled when my own garden suddenly became alive with small tortoiseshells.

"There were over 20 jostling for position on just one patch of sedum."