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
When to see
A single-brooded butterfly, peaking in August
Sightings by month (last 5 years)*:
Sightings this year*:
While this butterfly is characterised as ‘small’ it is the largest of our Blues.
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.
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 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.
*Note: The charts shown on this page are drawn only from casual sightings submitted to this website. Records from this website will be added to a lot more data collected throughout the year and used to compile the five-yearly Butterfly Atlases for Dorset and the UK.