A Butterfly Enjoying the High Life

The Brown Hairstreak is, unsurprisingly, a brown butterfly, with beautiful golden-brown wings—especially the females. This makes both the males and females incredibly difficult to spot in the hedges.

Unlike many butterflies that flutter around garden flowers, Brown Hairstreaks spend most of their lives high up in the canopies of trees and hedgerows. They tend stay in the master trees (often Ash), because they eat honeydew from aphids and use the more open space to display and find mates. Because they rarely descend, getting an accurate head count of the adults is nearly impossible.

Why We Look for Eggs, Not Wings

Because the adults are so elusive, we’ve had to get creative. Almost uniquely among British butterflies, the Brown Hairstreak is much easier to find as an egg than as a flying adult. During the winter months, our dedicated teams of volunteers help search the Blackthorn scrub for tiny, pinhead-sized white eggs. 

This is where the red wool comes in, serving two purposes. The wool marks a found egg so we don't count it twice, ensuring our data is accurate. The wool also signals to our site managers and volunteers that the specific Blackthorn plant must not be trimmed or cut until at least the following autumn.

A Wiltshire Success Story

Numbers of the Brown Hairstreak in Wiltshire are increasing. In the North of the county we see colony density numbers are on the rise and they disperse, growing into other locations. In the South, the numbers are less dense, but we are seeing a wider dispersal. We are even seeing them appear in new locations, such as Cockey Down, where they haven't been recorded before.

This success is down to careful and sympathetic management of hedgerows in the North and scrub in the South.

The Goldilocks of Butterflies

Brown Hairstreaks are picky, not only laying their eggs only on just one type of plant, but also only laying their eggs on the first year of growth on Blackthorn. With this in mind, we are very mindful about our cutting regimes. By only trimming our hedgerows once every four years and scrub on rotation, we ensure there is always a fresh supply of new growth for the butterflies to use. This careful management of scrub across the south of the county is helping these butterflies disperse and claim new territory.

If you see the red wool now, here is what is happening behind the scenes.

In April or May the eggs will hatch into tiny caterpillars and begin feeding on Blackthorn leaves. By early summer they will descend to the ground to pupate, and by late summer the adults emerge to fly until September, starting the cycle all over again.

So if you spot some red wool during your next visit, please do not disturb it. Each piece of wool represents a future butterfly and helps us keep Wiltshire’s hedgerows and scrub full of life.