Only one ‘Blue’ Butterfly is generally abundant in our region, and that is the aptly named ‘Common Blue’. The larvae feed on plants such as Bird’s-foot Trefoil and other members of the pea family, in open flowery grasslands. I still recall my excitement as a ten-year-old child finding my first Common Blue colony on a patch of relict grassland that had somehow survived on derelict land close to where I lived. By the following year, it was under tarmac.
However, there is another ‘Blue’, which can turn up in urban areas and well away from grass and meadows. In fact, the occurrence away from such areas is a key help in its identification. This is the Holly Blue, and a denizen of shady gardens, woodland edges and rides, and overgrown hedgerows. They lay their eggs on plants such as Holy and Ivy, and in gardens, on Berberis and the like as well. The butterfly has typical lilac blue upper parts but a strikingly silver-white underside – very distinctive, especially on a sunny day. One in my garden recently, looked almost like a fragment of tin foil blowing on the light breeze. They are a delightful addition to the garden wildlife. Holly Blue has spread north in recent years from a situation in the 1970s, when it occurred mostly to the south. For Sheffield entomologists, the Holly Blue, was a formerly a butterfly of woodland habitats on the Derbyshire fringe. I have already had records of the ‘springtime’ Blue this year.
As well as spreading northwards, this Blue has a particularly interesting ‘boom-bust’ lifecycle. Affected by a very specific and fatal parasitic wasp preying on the larvae, the numbers rise if the parasitoid is low in numbers. As the butterflies increase, then over the next few years, the wasp increases too, and the butterfly numbers crash as a result. The wasp parasite follows the nosedive of the Holly Blue, and the cycle begins once more, giving a periodicity of 5-7 years.
The Blue has two broods each year in spring and then summer, generally peaking about mid-August. Weather conditions affect the Holly Blue, but now is their peak, and a good time to look out for them. The annual Butterflies & Moths report for Yorkshire shows them distributed across the whole county. Do send your records into the Yorkshire Branch of the Butterfly Conservation Society so we can get a better picture of the ups and downs of this overlooked beauty.
Ian Rotherham, Writer, Broadcaster, Professor of Environmental Geography, Reader in Tourism & Environmental Change