Butterfly of the Week 22nd July 2020

Chalkhill Blue:   This elegant butterfly, somewhat larger than the common blue, is confined, as its name suggests, to unfertilised chalk and limestone short grasslands.  Ideally, and perhaps nearly essentially, rabbit grazing to create this short sward height is important.  Certainly, scrubbing up of such habitats, as rabbit numbers have reduced, is connected to reduction in Chalkhill blue numbers.  There may also be climate effects as many populations are struggling, at the moment, across England.  The male is a stunning and unmistakeable milky blue colour while the females are milk chocolate brown.  Both male and female have conspicuous dark lines through the white fringes on edges of the wings that the smaller common blues do not have.  The butterfly is entirely dependent on horseshoe vetch which is, in turn, dependent on suitable grazing to keep the sward relatively short.

Horseshoe vetch flowers obviously in late May but otherwise the fine leaves of the plant are very inconspicuous. The caterpillars are also dependent on attendant ants that they supply with attractive secretions, who, in turn, benefit significantly from these as a food source.  When the butterflies emerge in mid-July and perhaps flying into September the males are particularly conspicuous covering the colony area searching out unmated females.  These females can be inconspicuous but are easily found nectaring on suitable plants such as marjoram and knapweeds and show off the dark lines through the white fringes of their wings.  The eggs are laid on or near the foodplant but seem to fall easily to the ground, where they are invisible in the chalk or limestone rocky soil, and they spend the winter at this stage.   The egg hatches in the following April and the young larvae begin their mutual relationship with the local ants such as the Yellow Meadow ant while feeding on the horseshoe vetch leaves and later flowers.  It is possible that pheromones from ant trails help determine exactly where the female butterflies lay their eggs.  The pupa is formed in June or July and is probably buried by its ant partners that are still collecting its secretions.  Like other Lycaenids the pupa is believed to make ant-attractive sounds!   The single generation is then ready to emerge in late July and onwards.  In Somerset, Chalkhill blues are confined to the Mendips from Wookey Hole Fields to Brean Down and to the reserves on the Poldens.  Most of the sites are in Nature Reserves of one sort or another where suitable grazing can be maintained.  We are fortunate – if you want to see this spectacular, and Near Threatened, butterfly, that Westbury Beacon, Stoke Camp and Draycott Sleights are all good sites even if numbers have fallen in recent years.  They could also be expected on Deerleap and Cook’s field if the food plant survives there as well.  Please let me know if you see any, particularly in out of the way places!

Photos  Peter Bright

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