The Small Tortoiseshell is probably our most familiar “coloured” butterfly.
Often you can find writhing masses of their caterpillars on stinging nettles.
However, despite such large broods being laid, most never make it to adulthood.
It seems the main enemies of Small Tortoiseshell caterpillars are various species of tachnid flies.
Here is one of at least three Pelatachina Tibialis tachnid flies I found among a brood of such caterpillars recently.
Tachnids lay eggs close to or even on the caterpillars.
These eggs hatch into grubs which burrow into and live inside the unfortunate caterpillar. From then on the caterpillar is destined to turn into a fly rather than a butterfly!
Parasitism by tachnid flies is so common, it can severely limit numbers of Small Tortoiseshells.
It is not uncommon to find a large brood of healthy caterpillars, only to discover them all dead or paralysed next time you check.
Of course, it is all part of the intricate web of nature, whereby every species seems to support many more species which live on or in it.
As if that wasn’t enough for Small Tortoiseshells to put up with, a continental tachnid has hopped the channel in recent years and rapidly become established throughout the country.
So next time you spot that bonnie wee butterfly, wish her luck on her way.
She will need it!.
By George Hogg. Hogg Estate Services, Wildlife Management