As in several recent years the Chalkhill Blue was seen very early (compared with text book emergence dates) after a gloriously warm and sunny Spring. Upper Thames Branch couldn't compete with the earliest UK dates but still managed a report of over 200 separate butterflies at Lardon Chase (Berks) on 14th July. Berkshire sites were some way ahead of those in Oxfordshire and Buckinghamshire and the warmest south facing Chiltern slopes equally ahead of those with a more northerly aspect.

It was a very mixed year for the species. Sites with the most benign aspect had earliest emergence and the numbers were good (or even very good). Sites that are less sympathetic to developing larvae, where emergence began later as the Summer turned ugly, had poor numbers. Having said that, I didn't hear of any sites with established breeding colonies which produced no sightings at all. There were single visits to sites that have produced very low numbers in previous years, where none were seen, e.g. Shirburn Hill (Oxon) and College Lake (Bucks), but quite possibly previous sightings were of vagrants from nearby colonies. Equally, visits to sites with some Horseshoe Vetch and no known colony, did turn up the odd male, also presumed to be wanderers, e.g. the confusingly named Coombe Hill near Aston Clinton (Bucks) where in early August there were two males together.

A male Chalkhill Blue at UTB's
Holtspur Bottom Reserve
Photo © Brenda Mobbs

Probably the best news was that the species is believed to have bred at Holtspur Bottom (our own reserve near Beaconsfield) for the first time in approximately 70 years (click here to read the separate report). The odd wandering male has been reported during those 'empty' years. Currently amounts of Horseshoe Vetch Hippocrepis comosa larval foodplant may be too little to support a colony. So efforts are being made to increase the number of plants capable of supporting larvae.

Left: November 2011 - work party planting Horseshoe Vetch
at Holtspur Bottom reserve.

There was a late flourish at several sites (again largely those that present the most beneficial conditions) with our latest UTB sighting on the 11th September when four were reported from Yoesden Bank (Bucks).