During the years 2003-2008 the Small Tortoiseshell suffered a major decline and one reason might be the arrival in Britain of the parasitic fly. The fly Sturmia bella ( Tachinidae ) occurs mainly in warmer climates, and first arrived in Britain in 1998. It lays it's eggs on nettle plants. When the Small Tortoiseshell caterpillars eat the nettle leaves, the microscopic eggs are ingested undamaged and pass into the caterpillar's gut. There they hatch, and the resulting grubs bore their way through the soft flesh, consuming non-vital body tissues. When the grubs are almost full grown they eat the vital organs, and then break out through the skin of the dying caterpillar to pupate.
Dr Owen Lewis of Oxford University has been studying the role of the parasite in the decline of the Small Tortoiseshell and the findings so far are not conclusive.
It would seem that there is little doubt that more Small Tortoiseshell caterpillars are dying now that Sturmia bella has invaded UK than was the case before. The parasite undoubtedly has played a role in the decline but Dr Lewis believes other factors are also involved.
There is a small recovery in the numbers and on days like today it was possible to watch a few of these wonderful insects as the nectared on bramble and honeysuckle, just on the edge of the village. The Small Tortoiseshell may be the worlds oldest butterfly species. A fossil found from the mid-miocene period is estimated to be 15 million years old and is virtually indistinguishable from today's species.