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 this decline it recommends that blackthorn hedges suitable for this butterfly should be not be flailed every year but should be left uncut at least every alternate year.

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.

Richard Smith of the South Wales Branch of Butterfly Conservation said: “Mechanical flailing of a hedge or scrub during the autumn and winter period has been shown to remove 80 to 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. 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 small holdings, are more able to persist in the landscape.

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, 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.

Recommendations to help the species are:

  • 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.
  • Encourage less intensive farming practices. 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.