A) A nasty parasite (Sturmia bella) was the reason for the huge decline of the small tortoiseshell butterfly throughout much of the UK. This parasite is a tachinid fly that parasitises this butterfly, laying its eggs near the small tortoiseshell’s caterpillars on nettles. The unsuspecting caterpillar ingests the egg, which then develops within the caterpillar, killing its host.
Sturmia bella spread to the UK from the Continent at the start of this century, and soon halved the small tortoiseshell population. However, it seems that this parasite doesn’t like cold winters, which allows the butterflies a reprieve and a chance to rebuild their numbers. The small tortoiseshell is a highly successful and prolific butterfly, and has been quick to recover.