After some years of very low numbers, the small tortoiseshell butterfly (Aglais urticae) seems to be back in numbers. Today I saw several, including the one below, in a rough field at Netherfield in East Sussex wheeling about and buzzing one another in the warm afternoon sunshine.
This recent decline has been linked to the parasitoid tachinid fly Sturmia bella which was first recorded in Britain in 1998 at about the time the decline of the small tortoiseshell began. The butterfly caterpillars swallow the eggs of the fly while they are feeding and the parasitoid larvae develop inside them, eventually killing them.
Some research has been done on this and large numbers of small tortoiseshells have undoubtedly been attacked by Sturmia, but results are not so far conclusive.
It may be that we will now see cycles of abundance and scarcity of this species (as with the holly blue and its parasitoid). More butterflies means more tachinids, but this leads to less butterflies and less tachinids and so on round and round