Small Tortoiseshell (Aglais urticae)
The Small Tortoiseshell A. urticae is one of our most familiar and iconic butterflies, appearing in gardens throughout the British Isles. Unfortunately, this beautiful insect has suffered a worrying decline, especially in the south, over the last few years. It has always fluctuated in numbers, but the cause of a recent decline is not yet known, although various theories have been proposed. One is the increasing presence of the parasitic tachinid Sturmia bella (Diptera: Tachinidae), due to our warming climate - this species being common on the continent. The fly lays its eggs on leaves of the foodplant, close to where larvae are feeding. The tiny eggs are then eaten whole by the larvae and the grubs that emerge feed within their host. The larva of S. bella eventually kills its host and emerges from either the fully-grown larva or pupa before itself pupating. Although S. bella attacks related species, such as the Peacock and Red Admiral, it is believed that the lifecycle of the Small Tortoiseshell is better synchronised with that of the fly and it is therefore more prone to parasitism.
A recent visit to Ferring Rife in West Sussex produced around 20-25 overwintered A. urticae. Considering counts of up to 194 were recorded by Neil Hulme in 2013, with good numbers also recorded during the 2014 and 2015 seasons, the site appears worryingly less productive this year. Whether this is due to what appears to have been a change in management regime remains to be seen. What we do know is that this site has supported the largest population of A. urticae in West Sussex and has doubtless played a fundamental role in the species’ local recovery since at least 2012.