Growing Our Reserves

Brown Hairstreak Butterflies – why are we so excited about having them at Coldwell?

About the Brown Hairstreak

The brown hairstreak is the largest hairstreak found in the British Isles. The female has striking orange patches on its wings and naturalists originally thought it was a separate species, and named it the golden hairstreak. It is one of the latest species to emerge, with adults appearing in late July or early August. It can be a hard butterfly to see – as both males and females spend most of their life in and around the tops of ash trees, basking in the sun and feeding on honeydew. If honeydew is scarce they will come down to feed on nectar from a variety of flowers (hemp agrimony, bramble, devil’s-bit scabious, hogweed, ragwort and thistle). The larval food plant is blackthorn.  When the female is ready to lay eggs, she comes down from the tree-tops to select suitable sites, laying a few eggs at a time on new-growth twigs on the ‘sunny’ side of the plants. At this time she feeds almost entirely on nectar.

Brown hairstreaks live in self-contained colonies that breed in the same area year after year. So, whilst colonies can easily be lost, it is rare for a new colony to be established. Its ideal territory contains tall trees, preferably ash, but it must also have suitable flowering plants, and scrub areas or hedges of blackthorn that catch the sun, in order to provide food for the larvae. This butterfly is becoming increasingly rare and it is now a priority species for conservation efforts. Its habitats have decreased since the 1970s, as has been the case for so many species but, because the eggs stay on the new-growth blackthorn over the winter, it is particularly vulnerable to hedge-trimming and flailing. For it to survive, it needs more hedgerows to be planted and managed for conservation.

Where are they found?

In the UK, brown hairstreaks are found almost exclusively in the southern half of England and Wales where the climate is warmer. Until very recently its most northerly site was in Lincolnshire – but there are now colonies at Gait Barrows and Coldwell! This a source of much pleasure and excitement to butterfly enthusiasts eager to photograph them here on our doorstep. There are already monitoring schemes in place, counting adults as part of the regular butterfly transects and searching for and recording egg placements. 

How did they get here? And what should we do about them?

It is unclear how the hairstreaks got here. It is probable that they came as eggs on blackthorn stems, either as a deliberate introduction or as a chance consequence of hedge-planting work (because people could get funding for planting hedges so long as they bought in the plants!). But however they came, the environment here is ideal – our estuary climate is mild, and we have all the right plant species in the AONB.  With the sensitive development and management of the land that the Landscape Trust is committed to, they could continue to thrive.

So we are in an interesting situation. There is a school of thought, particularly within long-established conservation organisations, that says that artificially introduced species are ‘unnatural’ and should be eradicated, whether or not they are invasive or pose a threat to the survival of endemic species. There is another school of thought that says that if a severely endangered species can be nurtured, and numbers built up for the future, the world will be a better place for us all. Should we be discouraging the spread of these butterflies in our reserves? Or simply tolerating them? Or should we be encouraging these beauties by careful management of the trees, hedges and flowering meadow plants at Coldwell?

The Landscape Trust is a membership organisation dedicated to the conservation and improvement of the landscape of the AONB for the enjoyment of us all – the trustees are always interested in members’ thoughts.

Information on the brown hairstreaks sourced from UK Butterflies.

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