Chalk grassland butterflies project

The Isle of Wight National Landscape is internationally important for its chalk grasslands – species-rich, flower-filled, insect-loving fields and downs – which are a haven for many rare and beautiful plants, insects and birds. These grasslands include the largest populations of early gentian in the world as well as orchids, medicinal plants and culinary herbs.

The chalk grasslands also hold nationally important populations of five blue butterflies which depend on these habitats as they lay their eggs and caterpillars feed on plants found almost exclusively on the chalk.

These butterflies also have a strange relationship with ants which take their caterpillars and chrysalis’ underground. The caterpillar provides a sugary secretion that the ants love and by hiding them they protect the caterpillar from predators. The foodplants and ants on which the caterpillars depend are only found on these short chalky grasslands.

With the exception of the common blue, all these butterflies are only found on south-facing chalk downs of southern Britain below a line from the Wash in the east to the Severn in the west.

Small Blue

Flying as adults in May/June, these tiny blue/black butterflies lay their eggs on Kidney Vetch. Once the caterpillar has fed it lies overwinter as a caterpillar in the seed head of the vetch. The best plants are found in chalk pits, embankments and broken cliffs where grass is sparse and the bare chalk is warned by early spring sunshine. Found on east Afton, Mottistone and Ventnor Downs.

The Brown Argus

The least ‘blue’ of the five this little brown butterfly appears in May/June and again in August when the adults lay their eggs on rockrose. Once it has fed the caterpillar overwinters in the ground, attended by ants. The Brown Argus prefers open chalk grassland on south-facing slopes with an abundant supply of rockrose for its caterpillars to feed upon. Found on Mottistone and Ventnor Downs as well as Carisbrooke Castle, Brading and Arreton Down.

Adonis Blue

Flying in May / June and again in Aug / Sep this small butterfly is a distinctive bright blue colour as it looks for plants of horseshoe vetch to lay its eggs. Again the caterpillar, once it has fed, is taken underground by ants that protect it through the winter. The Adonis blue can be found on open south-facing chalk slopes with plenty of horseshoe vetch. Found across the chalk from east to west and on Ventnor Downs.

Common Blue

Flying in May to July and again through August and September the Common Blue can be found in areas off the chalk but still needs habitat with an abundance of its food plant – bird’s-foot-trefoil. Finding its foodplant the eggs are laid and they stay overwinter before the caterpillar emerges and feeds before being taken underground by ants. The common blue is the most common of these butterflies and can be found in grasslands, heathlands and waster ground where bird’s-foot-trefoil is abundant. Found in many grassland habitats and even woodland rides across the Island.

Chalk Hill Blue

Our final of the five blue butterflies, the Chalk Hill Blue. Flying from July to early September the Chalk Hill blue is a distinctive milky-blue with black edges to its wings. The adults seek out horseshoe vetch where they lay their eggs. The caterpillar feeds and is taken underground by ants to overwinter. The Chalk Hill blue requires very short grasslands with an abundance of horseshoe vetch and is becoming increasingly scarce in southern Britain. Found in sites across the chalk from east to west but in some years found in exceptional numbers on Brook and Arreton Downs. The Island is the best place to see this butterfly in large numbers in the south of England.

Saving the Blues – project starts in autumn 2024

In partnership with the National Trust, Hampshire and Isle of Wight Wildlife Trust, Gift to Nature and local landowners, projects will be delivered across the chalk grassland landscape of the east Wight to help the blue butterflies thrive. At Arreton, Knighton, Mersley, Brading and Bonchurch work will include building cattle corrals to help grazing, fencing, enhancing water supplies and scrub removal. These works will provide the mixture of warm, south-facing, short-grass habitats with plenty of the foodplants needed for the caterpillars to feed and a wide range of nectar sources for the adults. Chalk grasslands are a landscape feature across southern Britain. More information about this important habitat and the ‘Big Chalk project’ can be read on the Cotswolds National Landscape website