Chalkhill Blues buck the trend


The awful weather conditions that until recently have blighted the 2012 spring and summer have unsurprisingly been pretty disastrous for most of our butterflies. However, every cloud has a silver lining; in this case a silvery-blue one. Phenomenal numbers of beautiful Chalkhill Blues emerged on some areas of the South Downs at the beginning of August, providing a spectacle seldom witnessed since Victorian times. It seems likely that the excessively wet conditions have resulted in unusually lush growth of nitrogen-rich Horseshoe Vetch plants, capable of supporting a veritable army of the butterfly's green and yellow slug-like caterpillars. In most summers competition for limited resources dictates the size of the adult population, particularly when drought conditions restrict the availability of larval food-plants.

Chalkhill Blue male (Neil Hulme).

On 2nd and 3rd August, Michael Blencowe and Mike Mullis from Butterfly Conservation Sussex Branch joined me to help count butterflies in random metre squares on Friston Gallops (Butchershole Bottom) near Eastbourne. By using satellite images of the site we were then able to make a conservative estimate of the population, amounting to an incredible 820,000 Chalkhill Blues. At the northern end of the huge grassland strip, butterfly densities reached 33 insects per square metre in places and at the height of the emergence the turf appeared to shimmer silvery-blue as they took to the air each morning or after a cloudy spell. Mating pairs were seen in their hundreds, with huge bundles of amorous males all vying for the opportunity to mate with each female.

Chalkhill Blue female (Neil Hulme).

In most species of butterfly it is the females that take on the responsibility for searching out new areas of suitable habitat and potentially setting up new colonies. This seems logical, bearing in mind that they will be carrying a cargo of fertilised eggs. However, although more difficult to explain, it is well documented that male Chalkhill Blues will wander great distances from their place of birth. As the population at Friston Gallops began to burgeon, we received reports of Chalkhill Blue males well away from the Downs, at Hailsham Country Park, Horam and Coggins Mill near Mayfield. These three sites lie in a straight line trending south to north from Friston Gallops, with the greatest distance of travel being Coggins Mill at 17.5 miles as the Chalkhill flies.

Chalkhill Blue male (Neil Hulme).

On 4th August I returned to Friston Gallops specifically to search for aberrant forms: individuals that differ significantly from the normal pattern due to either genetic or environmental influences. Such irregular forms were highly prized by collectors, often fetching significant sums, although these days most prefer to collect them as digital images. Unsurprisingly, when exceptional numbers appear, the variety in the gene pool comes tumbling out and as the butterflies settled down to roost I found seven aberrant examples in about 45 minutes. Many of these have been given scientific labels and all of those I found are named, rather descriptively, ab. postcaeca, meaning the rear wings are 'blind' (lacking the normal spots).

Chalkhill Blue male ab. postcaeca (Neil Hulme).

Chalkhill Blue female ab. postcaeca (Neil Hulme).

Within a week the huge numbers had dropped significantly, as the rate of mortality increasingly outstripped the rate of emergence and many more butterflies dispersed away from the overcrowded site. Although the best part of this marvellous spectacle was over all too soon, Chalkhill Blues were doing rather well elsewhere. On 9th August I met Simon Mockford of the South Downs National Park Authority to assess numbers on another Sussex site where the population has exploded this year. In a couple of seldom-visited valleys on the Downs near Amberley, I was once again astounded by the unprecedented abundance of this species. The isolated flowery slopes were covered in Chalkhills and by using a similar technique I estimated the colony to be 175,000 strong. Unusually high numbers of Chalkhill Blue have been recorded from further afield too. Denbies Hillside in Surrey and the Butterfly Conservation reserve at Magdalen Hill Down in Hampshire have also been blessed with shimmering fields of blue. In a generally poor season there can be little doubt that this species has earned the title 'Butterfly of the Year'.

Downs near Amberley (Neil Hulme).

Written by: Neil Hulme