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.
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Can't imagine any year being worse for butterflies than last year, so that is worrying
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