The threatened brown hairstreak butterfly’s range in south west Wales has more than halved in the last decade, a new study by Butterfly Conservation South Wales branch reveals.

To help reverse the decline it is appealing to farmers and landowners that blackthorn hedges, where the butterfly lays its eggs, should be not be flailed every year but should be left uncut at least every alternate year.

The study from Butterfly Conservation South Wales branch shows that smallholdings are now the main refuges of the rare brown hairstreak butterfly, which is just about hanging on in south west Wales.

The brown hairstreak is listed under the Environment (Wales) Act 2016 as a species of principal importance for the purpose of maintaining and enhancing biodiversity in Wales.

Branch member Richard Smith said: “The adult butterflies live mainly in the tree canopy (typically ash) but lay eggs on young shoots of low-growing blackthorn in the hedges below.

"The eggs remain on the blackthorn shoots through the winter, with the caterpillars hatching the following spring. Mechanical flailing of a hedge or scrub during the autumn and winter period has been shown to remove 80-90 per cent of eggs. If this continues year-on-year, the ever-decreasing population vanishes within two or three years.”

Hedging experts advise that once every three or four years is a suitable interval for trimming hedges. Despite this, mechanical flailing of most farm hedges every autumn/winter seems to be common practice.

This annual flailing is far more intensive than traditional labour-based management, in which hedges were laid every few years on rotation across the farm.

The study is based on intensive regular surveys by volunteers over 20 years. It shows how brown hairstreak butterflies breeding on hedgerows on flatter, although often wetter, low-lying pasture (particularly in the Taf and lower Tywi valleys in the St Clears, Carmarthen and Pontargothi areas), have been especially hard hit.

In contrast, those using hedgerows on the slopes of the mid and lower Teifi valley, which are less easily workable with machines and often on smallholdings, are more able to survive.

The study also indicates that even where irregular or traditional hedgerow management rotations do occur, the butterfly is still being lost if the surrounding landscape is dominated by more intensive hedgerow management.

Ash disease and climate change may also play a part in the recent decline of the brown hairstreak in Wales, but finding direct evidence of this is difficult. However, Mr Smith says the apparent increase in annual hedge flailing is more likely a major contributor to the decline of brown hairstreak and is something which land managers can easily do something about by changing practices.

"To help the brown hairstreak thrive, this study suggests that, in south west Wales, fields below 150m above sea level with blackthorn hedges or blackthorn scrub, should, as a minimum be left uncut every other year, and ideally cut on a three to four year cycle," he said.

"We would also expect Welsh Government to encourage less intensive farming practices, given its commitment to tackling both climate change and the biodiversity crisis.

"Not only should this benefit brown hairstreak, but also enhance wildflowers for pollinators along the base of hedges and provide berries for birds in the autumn. Currently too many hedges are over-managed and inhospitable to wildlife.

"The reduction in the brown hairstreak’s range of more than 50 per cent in the last decade should be a warning sign of the need to change."

Butterfly Conservation in South Wales has an advice leaflet and a gatepost badge scheme for appropriate 'Butterfly Friendly Hedgerows' – find out more by searching online for 'Butterfly conservation in south Wales'.