HTAG Guildford - PDCV Update Dec 06

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PDCV Update Dec 06

Chalk-hill Blue Butterflies Boom on Pewley Down

If you walked on parts of Pewley Down during late July or Early August clouds of blue butterflies would have risen at you feet. These blue butterflies were Chalk-hill Blues and numbers this summer on the Downs were higher than at any time in recent memory. The male Chalk-hill Blue is one of the easier blue butterflies to recognise. It is slightly larger than the other blues (with the exception of the very rare and reintroduced Large Blue) with a distinctive pale, powder, blue colour and a fairly broad black sub terminal border to the upperwing. It is a very local butterfly which has a southerly distribution in this country and, as its name implies, it occurs exclusively on chalk. The larvae of the butterfly only feed on Horseshoe Vetch plants. Notwithstanding these rather exacting habitat requirements the butterfly can occur in very large numbers when conditions are right.

Numbers on Pewley Down

Chalk-hill Blue has almost certainly occurred on Pewley Down for very long time. However, with cessation of regular cutting and management on the Downs in the 1980's, scrub started encroaching across the Down slopes shading out the Horseshoe Vetch. As the number and quality of the vetch started declining so did numbers of Chalk-blue. Recent conservation work on the Down has cleared much of the scrub and, as a consequence, the Horseshoe Vetch has recovered. Indeed, currently, in early summer, the yellow of the Vetch flowers is one of the most prominent and attractive features of the Down slopes. Happily numbers of Chalk-hill butterflies have also recovered along with their food plant. For the last two years we have good information of Chalk-hill numbers from sweep counts over the Down, conducted by volunteers, and from a butterfly Conservation transect carried out on the Downs. Earlier estimates of numbers are based on hearsay and surmise! The sweep counts have been carried out in slightly different ways and over slightly different areas which means that the results should be used with some caution. Nevertheless the counts do give a very good impression of the recent growth in numbers and the huge numbers present this year. A sweep count of Chalk-hill Blue Butterflies in 2005 = 400 plus, and in 2006, 2,804.

The Transect surveys, which are carried out as part of a national enquiry into butterfly numbers, are conducted by a standardised method on a predetermined route over the Down. The results from this source are more strictly comparable between years and confirm the large increase in butterfly numbers over the two years. Peak counts from Butterfly Conservation Transect in 2005 = 261, and in 2006 = 665.

The Future

The very good season enjoyed by the Chalk-hill Blue butterfly has two principle causes. Firstly, the very hot weather in early June undoubtedly favoured the development of the heat loving larvae and secondly, the very good growth of Horseshoe Vetch gave the larvae plenty of food. In the future, global warming should favour this southern species. However this will count for nothing unless the habitat on the Down remains right for them. The preservation of the habitat depends, in the short term, on the continued efforts of Guildford Borough Council's countryside team and the Pewley Down Volunteers in keeping the scrub on the Down in check. In the longer term grazing by animals is probably the only way that the Downs can be kept open and in good condition. Provided this happens the future of the Chalk-hill Blue on Pewley looks bright.

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