The last few weeks have seen a noticeable upturn in the numbers of butterflies on the wing, with small tortoiseshell perhaps the most abundant of these. After the slow start to the year for many species, the ‘good’ summer that has followed has given our butterfly populations a real boost. The presence of so many small tortoiseshells is particularly welcome given the fortunes of this familiar butterfly over recent years. Numbers, which normally show an increase towards the end of the summer, have been lower lately and comments have been made about the possibility of a longer-term decline.
Small tortoiseshell populations here in Britain are influenced by a number of different factors, one of which is the weather. Periods of hot and dry weather during the caterpillar stage lead to faster growth rates and a larger emergence of adults. However, parasite populations also shape tortoiseshell numbers and the arrival of a new parasite was thought to have increased the pressure on British populations. Until relatively recently, small tortoiseshells were only parasitized by a number of generalist parasitic flies and wasps, parasites that also target other species. The arrival of a new parasitic fly, called Sturmia bella, just over a decade ago was thought to have changed things somewhat – at least over the short term. This species, which is more of a specialist, lays its eggs near to small tortoiseshell caterpillars. These are ingested along with the plant material on which the caterpillars are feeding. The eggs then hatch inside the caterpillar, before the resulting larvae go on to kill it. The larval fly (a white maggot) emerges from the butterfly’s chrysalis and descends to the ground on a silken thread.
Although the arrival of this new parasite may have had some initial effect, evidence from elsewhere suggests that the recent declines in small tortoiseshell populations here are not linked to the parasite but are, instead, the result of a run of poor weather. While a hot and dry summer may boost the numbers of adults seen, summer drought can reduce the availability of nettles (and in particular fresh nettle growth), used as a food plant. It will be fascinating to see how populations fare going into 2014.