General Distribution and Status

Although our smallest butterfly has a very wide distribution occurring in north Scotland and south England its strongholds are in the south. Colonies further north are more isolated and mostly found in coastal locations. Colonies usually consist of very few adults but some consist of thousands. It was more common in the early part of the 20th century but ploughing up of grasslands and agricultural improvements have attributed to many colonies being lost. However, since the turn of the 21st century there are signs that the butterfly is making a comeback which may partly be due to the planting of the larval foodplant Kidney Vetch Anthyllis vulneraria at many suitable sites around the country. In Hertfordshire and Middlesex, there were concerns that the insect became extinct in the 1990s but it now appears to be doing well at Aldbury Nowers, Hillbrow (west Letchworth), north of Clothall Common and the edge of the former Butterfly World site near St. Albans. Confirmed sightings also noted at the A41 Bourne End junction, Tring and the Hilfield Park Reservoir (Wood, 2020). New colonies found on seeded Kidney Vetch east of Royston and at Flamstead (Wood, 2022).

UK distribution map
UKBMS Species summary (2020)

Habitat Requirements

This species frequents warm and sheltered situations where the larval foodplant Kidney Vetch A. vulneraria grows on chalk and limestone grasslands, woodland clearings and coastal undercliffs (Riley). Quarries, railway embankments and roadside verges can also harbour this butterfly.

Larval Foodplants

Kidney Vetch A. vulneraria.

Adult Food Sources

Kidney Vetch A. vulneraria (16), Common Bird's-foot Trefoil Lotus corniculatus (5), Horseshoe Vetch Hippocrepis comosa (3).

Historical Records

In Gibbs' 1902 report he writes 'it appears to be particularly abundant at certain spots in the Tring district' with other records from Hitchin, Hertford, Letchworth, Aldbury Nowers and Royston. Foster's 1934 report describes it as 'used to be plentiful in around Hitchin, notably on the railway banks along the Cambridge line between Walsworth and Letchworth, but has disappeared from this spot'. Ray Palmer in his diaries reported seeing it in the Knebworth Woods area on 29 July 1934. There is a record of it at a railway bank in Stevenage (TL22H) in the 1940s (Sawford) and at Knebworth station by S.R. Bowden but no date given. S. Woodhall found it at a railway cutting in Ickleford on 30 May 1976.

Local Distribution and Abundance

Only one record of the species since 1995: at St. Nicholas churchyard, Stevenage on 1 August 2015 although I almost certainly spotted a specimen on 8 June 2018 on shrubby vegetation at the bottom of the chalky bank north of Martins Way. The colour, size and flight behaviour suggests that it was a Small Blue but I did not see it settle. Confirmation is needed at this site. The nearest known colony is at Hillbrow, west of Letchworth so could it have dispersed from that site? An example of dispersal is in Somerset where newly-created areas of Kidney Vetch A. vulneraria were colonised in the first year with the nearest known colony 20km (12 miles) away; female Small Blues can detect Kidney Vetch A. vulneraria, especially where it grows in profusion, across very appreciable distances (George). The seed-planting scheme off the Baldock bypass included the sowing of Kidney Vetch A. vulneraria seeds in the latter half of the 2000s and although the plant grows well there now no Small Blue colonies have been found. One theory is that the Small Blue may not take to some alien sub-species of the foodplant (Wood) although one large colony was established on a new roadside bank in Sussex that was sown with a tall continental variety (Heath et al.). The subspecies polyphilla was used on the verges off the Baldock bypass as well as at the former Stevenage Environmental Centre at Lonsdale Road (James). John Tomkins informs me that the Kidney Vetch A. vulneraria seeds planted on the chalky bank near St. Nicholas Church are native.

Life History

There are two generations of this butterfly but the second brood is usually much smaller. It is on the wing in late May and early June, and again in August. Eggs are laid on the florets of the foodplant and there may be several on the same flowerhead. The larvae eat the developing seeds until they are ready to pupate on the ground either in mid summer or in the following spring.
More details on the UK Butterflies website

Behaviour/Observation notes

It spends most of its time close to the ground with short flights. Males hold territories on shrubs or grass tussocks often at the bottom of south-facing slopes and mating usually takes place here. After mating, females spend the rest of their lives basking, feeding or laying eggs amongst the foodplants. In dull weather, early morning or late afternoon, the butterflies can be seen roosting about 50cm above the ground on the stems of taller grasses (Riley).

Variations/Aberrations

One aberration of the species was reported by David Miller at Totternhoe Knolls in Bedfordshire on 2 June 2010: ab. obsolete where the spots of the undersides are almost obsolete.
Find out more on the UK Butterflies website