Butterfly Conservation
Hertfordshire & Middlesex Branch
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Millhopper's Reserve

Brown Hairstreak and Blackthorn (Sloe) Survey (Lockdown 2021)

With lockdown continuing, the branch has had to stop most of its winter Brown Hairstreak (Thecla betulae) egg searches; especially as for some it involves unnecessary travel. However, members and volunteers can still help us!

We are despite lockdown allowed to take daily exercise and although daily exercise doesn't necessarily mean you can stop and search blackthorn for eggs, it does allow you on your permitted daily walk to look for blackthorn (sloe) (Prunus spinosa) or other prunus species such as cherry plum (Prunus cerasifera), especially in Middlesex and the London area as well as south Hertfordshire.

The first Brown Hairstreak eggs in Middlesex were found at the end of 2016, in a park just north of the River Thames in the London Borough of Richmond. However, it soon became apparent that this wasn't just a one-off find with more eggs being found at a nearby site in 2017 (London Borough of Hounslow). However, the highlight came in September 2018 when the branch had a report (with photo) of an adult Brown Hairstreak in an Ickenham garden (London Borough of Hillingdon). This lead the branch to discover a previously unknown but thriving colony at Stafford Road Open Space and in the hedge lines of the adjacent meadows at Ickenham. The hedgerows and scrub belt are blackthorn rich and winter egg searches have found numerous eggs with adults being reported in the summer. Since then several very determined surveyors have found even more sites and at the end of 2020, a single egg was found at Totteridge Fields in the London Borough of Barnet close to the M25 corridor.

There are several theories behind the discovery of all these previously unknown sites. What is acknowledged is that colonies of Brown Hairstreak in south London and north Surrey are expanding northwards. What is also thought is that they are using the river corridors so that the River Crane/Yeading Brook and River Pinn are being favoured and acting as conduits. However, whether the butterfly is expanding from Ickenham southwards as well can't be discounted. The majority of sites found have blackthorn and or prunus species but not necessarily in large quantities but they do tend to prefer sites with plenty of ash. The males use the ash as a master tree - somewhere to gather, meet up with females and feed on aphid honeydew. The geology is also relevant but this gets more complicated but will probably explain why one site is thriving but a large swathe of blackthorn/prunus a short distance away is ignored.

The adult is very elusive and hard to find - the colony found at Ickenham is an illustration of this because there was an historic report from the 1980s of Brown Hairstreak close to the A40, but the butterfly was only rediscovered in 2018 but present in good numbers! The eggs although very small are much easier to find and like other Butterfly Conservation branches most sites are found through egg hunting and not relying on adult sightings. The eggs become even easier to find once the leaves have dropped off the blackthorn or prunus species but before they start to flower. Trees covered in lichen are best avoided and relatively new growth is preferred. Much literature discusses suckering growth but also younger stems on uncut hedges with no suckering growth can be searched with equal success in our area.

So this is what we hope our members will do - find more blackthorn and/or prunus species before or whilst flowering.

Since 2017 a lot of sites have been visited and a Google Map is linked to show where the butterfly or its eggs have been recorded and where we know there is blackthorn and/or prunus. However, there are still many places waiting to be visited. Maybe just a footpath by a railway line or stream-side path might just hold a few bushes but could have been found by the butterfly. If the hedge looks to be regularly cut back unnecessarily, we still would like to know where it is because we might be able influence the managers to change the management? If you can identify ash trees, please tell us if there are any nearby as well.

Key to markers on the Google Map
  • Brown Tick - Brown Hairstreak (mostly eggs) have been recorded at this site in Middlesex
  • Orange Tick - Brown Hairstreak have been recorded in this area in one of our neighbouring branches
  • Dark Green Tick - Blackthorn has been noted here and in most cases searched for eggs
  • Pale Green Tick - This area has been visited and either no blackthorn or very little found
  • Grey Cross - an area considered unlikely to support Brown Hairstreak and has been visited
  • Pale Green Teardrop - areas noted from the Google satellite map that might have some suitable habitat and needs visiting and surveyed for blackthorn/Brown Hairstreak

It is known that Brown Hairstreak will lay their eggs on a variety of prunus species but blackthorn is the recognised favourite. However, when confronted with a hedge line of prunus it isn't always that easy to tell them apart. Identifying prunus species? That's really hard unless you are a tree/flower expert! One key factor with regard to cherry plum is that it flowers earlier than blackthorn. For an example it was already starting to flower at the beginning of March in 2020, whereas blackthorn was nearly a month later. However, we know they use cherry plum so examples of this are also welcome. Photos of stands of blackthorn/prunus species will also be welcome as it will give us a better understanding of the quality and quantity. A reminder, despite its thorns, hawthorn (Crataegus monogyna) is not in the same family and not used by Brown Hairstreak. It the berries are red then it is hawthorn!

Below are a few examples of a fairly typical blackthorn or prunus hedgerow that you might encounter on a local walk.

blackthorn 29 Dec 2018 blackthorn 5 Jan 2019 blackthorn 24 Jan 2021

This YouTube video from BC Surrey Branch is very informative and gives lots of additional information, on looking for blackthorn and finding eggs of Brown Hairstreak plus the occasional moth egg! If a walk is local and you feel able to search for eggs do have a go but remember several species of moth also lay eggs on blackthorn and these are shown in the video as well.

Brown Hairstreaks in Surrey

Some of the sites in Surrey are a slightly different habitat type especially in the more rural settings so don't compare everything. Many of our sites are just small areas of blackthorn tucked away along a footpath or on the edge of a recreational area. One site was even found under the flight path of the north runway of Heathrow Airport.

blackthorn 3 Mar 2020

So please take advantage of lockdown to walk some new footpaths in your local area and report back what blackthorn or prunus you can find, ideally supported with a photo to give us an idea of extent and quality and an Ordnance Survey grid reference (or postcode) to Liz Goodyear. Because there will be other eggs on the blackthorn or prunus do remember to take a hand lens. The moth egg records are also valuable so do report them to the Herts & Middlesex Moth Group.

BC Upper Thames Branch have published a very helpful guide to the eggs found on prunus at this link on Twitter:  Images of eggs found on prunus © Dave Wilton

Having found new areas, why not return in the summer to record your local butterflies? Just remember you could enjoy the satisfaction of finding a new site for Brown Hairstreak!

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