Chalkhill Blues mating. Photo: Ken Dolbear
The Chalkhill is one of the blue butterflies which has a relationship of mutual benefit with ants. Its caterpillars exude a honeydew consisting of sucrose, glucose and 14 amino acids; these are ‘milked’ by the ants, which can receive over a quarter of their total energy needs this way. In return, the caterpillars are given protection by the ants, and the chrysalises are protected in earthen cells especially created for them by the ants. The chrysalis itself can even attract ants by exuding amino acids, and has a sound organ which can chirrup to them, though humans cannot hear it.
We are concerned about the Chalkhill Blue in Dorset. It seems to be disappearing from the west of the county, though it is still doing well on Portland.
Where to see
- Habitat: Chalk and limestone grassland where its caterpillar foodplant is present, with a preference for south and west facing slopes and shorter turf. Steep downs, ancient earthworks and abandoned quarries are often used.
- Caterpillar foodplants: Horseshoe Vetch
- Best places: Clubmens Down, Fontmell Down, Portland Perrryfields Butterfly Reserve, Portland Tout, Bindon Hill
Distribution map
Chalkhill Blue was recorded from these 1km squares in Dorset (2020-24).
- Key:
- 1 record
- 2-9 records
- 10+ records
When to see
A single-brooded butterfly, peaking in August.
Sightings by month (last 5 years)*
This year and last*
What to look for
While this butterfly is characterised as ‘small’ it is the largest of our blues.
The male Chalkhill is probably the easiest blue to recognise. It is one of the larger blues and the male has a very distinctive milky-blue appearance, though the female is mainly brown.
The dark veins continue through the white fringe to the edge of the wing on both male and female, similarly to the Adonis Blue, though they are brown rather than black. The male Chalkhill can be distinguished by the black circles on the inside of the white fringe at the foot of the hindwings.
The Adonis might be out at the same time towards the end of August, making telling the females apart rather challenging. If you can get a good look at the underwing, the Chalkhill has white rather than blue outer circles to the eyespots, though this may not be very clear.
Photo gallery
Click thumbnails to view full-size images.
Chalkhill Blue male: some males have slightly thinner black edges to the borders of their wings and a brighter more silvery colour. Photo: Catriona Miller
Chalkhill Blue male. Note the white fringes are crossed by brown lines and the wing edges are bordered by thick black lines. Photo: Mel Bray
Chalkhill Blue female. Note the brown females have the same pale brown lines crossing the fringes as the males. Photo: Mark Pike
Chalkhill Blue female. Note the orange spots along the hindwing margins have pale lower edges, unlike the female Adonis, which edges them with blue. Photo: Ken Dolbear
ADONIS BLUE female. Orange spots edged with blue. Photo: Ken Dolbear
Chalkhill Blue male. A grey background to the spots. Photo: Andrew Cooper
Chalkhill Blue female. A brown background to the spots.Photo: Andrew Cooper
Chalkhill Blue (to left) and SILVER-STUDDED BLUE to show the size difference. Photo: James Gould.
Chalkhill Blues. Males frequently gather in 'parties' on animal dung to gather minerals. Photo: David Carter