Small tortoiseshell: This familiar butterfly hibernates as an adult in September or October in sheltered places in woodlands and woodpiles or in our garages, barns, lofts, and church rooves, where its dark shape, with antennae folded back, can be a familiar sight. On sunny days in January or February they may wake and fly for a short while but soon return to hibernation. In March, as it warms up, they leave hibernation and seek mates and suitable patches of nettles. They are quite fussy about the choice of location and the growth stage of nettle patches. These butterflies are strong fliers and may travel a 100 or more miles in their lifetime. Many are somewhat migratory in that the spring and early summer adults fly generally NNW and the second generation from mid-August fly approximately SSE. There are influxes from the continent each year, but it is believed that most small tortoiseshells are ‘homegrown’ unlike red admirals and painted ladies. After mating the female lays several batches of 60-100 eggs on the undersides of young nettle leaves. The caterpillars hatch quite soon and feed communally and build a somewhat protective web. Once that bit of nettle is eaten out the larvae will move on and spin a new web. When ready to pupate, they will wander away from the nettle patch or find a suitably sheltered spot within it, to form their suspended pupae. In less than a month adult butterflies emerge – which is happening here and now at the end of May! Most in our part of the world go on to find mates and lay eggs which will become the second generation. These are the butterflies we see on our Buddleia and Michaelmas daisies in the autumn. They are feeding up on nectar before finding hibernation sites in September or October to pass the winter. A few of this first generation may go straight into an early hibernation as they would nearly all do in Scotland. Small tortoiseshells are nowhere near as abundant as they used to be, particularly in the south of England. The reasons for this are rather unclear but climate change may have favoured a tachinid fly that lays its eggs on the nettle leaves that get eaten by young caterpillars. These eggs then hatch inside the caterpillar and eat it out from the inside without killing it until the fly’s development is complete when it leaves the dead caterpillar to form its own pupa, from which comes a new fly. Other circumstances, like the summer drought of 2018, have meant there were few suitable nettles for this first generation to lay eggs on in June to provide the second generation that are the autumn butterflies that will go into hibernation. If the season becomes really advanced many may try for a third generation which fails completely because suitable nettles and nectar sources are not available. The factors affecting how many small tortoiseshells there are in our gardens, and in the wider countryside, from year to year, are clearly complicated.
Photos: Mick Fletcher, John Ball, Peter Bright