Elusive butterfly arrives unseen at the Welsh Harp

Female Brown Hairstreak (image by Hectonichus, CC0, via Wikimedia Commons)

An elusive and nationally scarce butterfly, the Brown Hairstreak, has been recorded at the Welsh Harp for the first time — but without anyone having reported seeing the adult insect or even its caterpillar.

Instead, it is the butterfly’s tiny but distinctive eggs that have been found. They were discovered by Liz Andrew, who chairs the Hampstead Heath Survey section of the London Natural History Society. She found them on young shoots of blackthorn (sloe) in the Shrike Field while at the Canal & River Trust’s Welsh Harp Open Day on 3 February 2024.

Brown Hairstreak egg

Brown Hairstreak eggs are round white blobs about 1mm in diameter and intricately patterned — like minuscule sea-urchin shells. Despite their small size, they are easy for the trained eye to detect because their bright colour stands out on the blackish twigs, and midwinter sunshine can almost make them sparkle. 

In contrast to the eggs, the adults and caterpillars are hard to spot. Although the adults are on the wing from late July until November, they spend most of their time high in the tops of large ash trees, feeding on aphid honeydew. The caterpillars, which feed exclusively on blackthorn, are well camouflaged, resembling blackthorn leaves, and they feed only at night.

The Brown Hairstreak declined in Britain during the 20th century but is beginning to thrive again in scattered locations across southern England. In recent years it has begun spreading into North London from hotspots south of the Thames. 

The insect’s upperwings are brown and its underwings are bright orange, with two white lines streaked across them. Females have an orange patch in the top corner of each forewing. Small “tails” protrude from the hindwings.

The Welsh Harp is already home to two other elusive hairstreaks. The Purple Hairstreak spends most of its time in the canopy of oak trees and the White-letter Hairstreak flutters around the tops of elms. Both may occasionally drop to ground level to nectar on flowers.