The recovery of the Small Tortoiseshell Butterrfly
As Butterfly Conservation releases its results from the Big Butterfly Count, National Trust’s Matthew Oates, looks at some of the highlights.
It was great to learn from Butterfly Conservation’s speedy analysis of the 2014 Big Butterfly Count data that the Small Tortoiseshell is continuing to recover. It is the quintessential garden butterfly, one of the nation’s favourites – but we took it for granted until it inexplicably started to nose-dive during the early noughties.
One theory is that Small Tortoiseshell larvae suddenly became infested by the grubs of a parasitic fly called Sturmia bella, a new colonist from the continent that was first recorded in the UK in 1998 and which has spread rapidly. The caterpillars, which feed gregariously in silk tents, already had to deal with the ravages of native parasitic flies, notably a common black and red one called Phryxe vulgaris.
But perhaps these parasitic flies took a worse knock from the horrendously wet summer of 2012 – when it rained and rained and rained – than the host butterflies, enabling the gap between host and parasite to widen, and for host numbers to increase? We don’t know, of course, as the parasites are not monitored, only the status and distribution of the host butterflies. It is interesting that the Large White and Small White were unusually abundant in 2013, perhaps because the tiny parasitic wasps which readily bestialise their larvae suffered disproportionately badly during the 2012 summer rains?
Small Tortoiseshells are often most numerous in late summer and early autumn when they congregate in gardens, to feed up on nectar prior to hibernating, unmated. Late-flowering Buddleia bushes and herbaceous perennials such as Michaelmas Daisies and Verbena bonariensis are strongly favoured. Those that survive the winter pair up in early spring, to produce fresh broods around midsummer and again in late summer.
National Trust gardens are one of the best places to see butterflies during September and early October, as showy species like the Comma, Red Admiral and Small Tortoiseshell congregate around herbaceous borders, feeding up prior to hibernation.