Mole Valley’s Natural History

Butterflies

White-letter Hairstreak butterfly Mole Valley is nationally important for its butterflies. These are cold-blooded insects that need warmth, shelter and the right plants to be able to fly, feed and breed. The warm, south-facing slopes of the North Downs with their variety of plants, support the highest numbers of butterflies locally. Most adult butterflies need nectar to feed on, and will take it from a wide range of flowers. A few feed on ‘honeydew’, a sweet sticky substance produced by aphids, which coats the leaves of trees, and the Purple and White-letter Hairstreaks which use it spend most of their time in the treetops. Caterpillars are choosy about what they will eat, and female butterflies lay eggs on the plants that the caterpillar will feed on as soon as it hatches. Adonis Blue and Chalkhill Blue caterpillars feed on Horseshoe Vetch, which only grows on chalk grassland, so they are restricted to this habitat. Caterpillars of the Red Admiral and Peacock feed on nettles, and these butterflies are found in many more places, including parks and gardens.

Many kinds of butterfly have declined in numbers in recent decades, and some need conservation work, such as grazing by cattle or sheep, to provide the right conditions for them to survive. These conditions are suitable for many other insects too, most of which are less conspicuous than butterflies. Thus butterflies are good ‘indicator species’; if a site supports a good number of butterflies of different kinds, it is likely to have a good range of other insects too, which provide food needed by other wildlife, such as birds and bats.

Here we illustrate five butterflies found in Mole Valley that are of conservation importance as well as being attractive and interesting. While a visit to a good chalk grassland or woodland site is necessary to find these five, many others can be seen visiting gardens, parks and roadside verges. Up to 20 different kinds of butterfly, including the Brimstone, Peacock, Red Admiral and Speckled Wood, can be attracted to gardens to feed by providing a supply of nectar from spring through to autumn (Buddleia, Michaelmas Daisy and Aubrieta are among the best flowers for this).

Silver-spotted Skipper butterfly Silver-spotted Skipper: This little butterfly flies in August on the downs. Box Hill is a good place to see it. A few decades ago it was so rare nationally that it was feared it might become extinct in Britain, and the our local chalk grassland held important populations. Its caterpillars feed only on a grass called Sheep’s Fescue, and it used to breed only on south-facing chalk slopes, in the hottest spots with very short grass and lots of bare ground. Lack of grazing and loss of chalk grassland to agriculture and housing led to the extinction of many populations. In recent years however, conservation grazing has helped the Silver-spotted Skipper to survive, warmer summers have enabled it to breed in slightly longer grass, and it has recovered in numbers and spread to new sites. It flies close to the ground on hot August days, visiting Stemless Thistles for nectar, but if disturbed it flies away very fast.

Adonis Blue butterfly on eyebright The Adonis Blue is a vivid blue butterfly that is a favourite of butterfly enthusiasts. It has two generations a year, and can be seen in May and again in August. Denbies Hillside, to the west of Dorking is the best place to see it in Mole Valley. Like the Silver-spotted Skipper it is a chalk grassland butterfly that became very rare in the 1970s and ‘80s, due to loss of the habitat it needs. Because Horseshoe Vetch, which the caterpillars feed on, is itself a scarce plant that has disappeared from many places on the downs and does not easily re-colonise, it has not recovered its lost populations here, though it has done better in Wiltshire and Dorset. Grazing is carried out on the few sites in Surrey where it survives, and must be carefully monitored to provide the right conditions. The caterpillar produces a sweet liquid which attracts ants that form an excited cluster around it. This helps to protect it from predators and parasites, and both the caterpillar and the ant benefit from the association.

Brown hairstreak butterfly Brown Hairstreak: This is the last British butterfly to emerge in the summer, not flying until August and September. It is nationally scarce, with scattered populations in a few counties. The weald of Surrey and Sussex is one of its strongholds, and in the last 20 years it has extended its range, and now occurs across Mole Valley and up to the southern edge of London. Males stay up in the treetops, but females fly out from these to lay their eggs singly on young shoots of Blackthorn in hedges, woodland rides and even gardens. The adults are rarely seen, which is a shame because this is a beautiful butterfly. The eggs stay on the Blackthorn all winter till they hatch in April, and volunteers map the distribution of the butterfly by searching for them. It gives enthusiasts a chance to spot butterflies even in the coldest months!

Purple Emperor butterfly Purple Emperor: People come from all over Britain to Mole Valley to see this spectacular butterfly! Bookham Common is one of the top places in the country for visitors to get good views, and many join special guided walks run by the National Trust and by Butterfly Conservation. The Purple Emperor has a limited distribution, occurring in woodland in a few southern counties of England. It flies from the end of June till the beginning of August. Only the males have the stunning purple sheen on the wings, but the females are still striking butterflies. This butterfly never visits flowers, but males may land on woodland rides and can be approached – with care – to obtain good views. Eggs are laid on Sallow bushes and the caterpillars feed on the leaves in late summer and the following spring, spending the winter camouflaged on the stems.

Silver-washed Fritillary butterfly Silver-washed Fritillary: This is another large, spectacular woodland butterfly. It occurs in several woods in Mole Valley and can be seen from late June till mid-August in sunny rides and clearings, where it feeds on flowers, especially bramble. Unusually, the eggs are not laid on the plants that the caterpillars eat, but on mossy tree-trunks close to patches of Common Dog-violet. The eggs hatch after a few weeks but the caterpillars stay on the tree-trunk until spring, when they drop or crawl down to the ground to feed on the violet leaves, basking in the sun on dead leaves to raise their temperature. At Box Hill this butterfly occurs along with the very similar Dark Green Fritillary, which is very scarce in Surrey and Box Hill is now the only place where it is regularly seen. It also needs violets for its caterpillars to feed on, but unlike the Silver-washed, this Fritillary is a grassland butterfly, feeding on Hairy Violet in longer grass and around the edge of patches of scrub.