A Summary of the Monitoring Carried Out by the Branch's Black Hairstreak Recording Group

It was important to check these sites, and try and establish which colonies still existed, ownership of many of the sites had to be found, to get permission to visit, once on site we have not only looked for the butterfly but also tried to assess the suitability of the habitat, 60 sites have now been visited during the flight period, and several others we know have insufficient Blackthorn to support a colony.

Quite a large group of people have become involved in the monitoring, some sending in casual records others spending many hours searching for the B.H, to all of these I am very grateful, it has considerably increased our knowledge of the butterfly.

We have used Dr Jeremy Thomas's thesis on the B.H. dated 1974 to gain information on the butterfly's life cycle and behaviour, and also its distribution, at that time there was an estimated thirty colonies countrywide.

Discoveries

We have found 5 new colonies, 20 colonies had been discovered in Bucks and Oxon between 1975 and 1998, 16 of these of these new ones have been at hedgerow and copse sites which are unlikely to have been checked before, many are far from woodland which had always been regarded as the only habitat suitable for the butterfly.

Only in 2003 two new hedgerow colonies have been discovered by three people on a walkabout in the general vicinity of several woodland colonies but at least 250 metres from these sites, further confirmation that the B.H. find hedgerow habitats suitable.

This gives the impression that the B.H. is thriving and expanding its range but our research indicates many colonies have been lost, even in the six years of our monitoring some of the colonies seen 1998 / 2000 we cannot confirm still exist.

The 15th of June had been regarded as the likely date for the start of the flight period, depending a little on the weather, for the last two years we have seen them on the 13th, with records from the visitor's book at one popular nature reserve as early as the 9th, peek numbers used to be reached by the 26th / 27th of the month, this has now come forward to 20th / 21st and the flight period all but over by the end of the month with only one or two seen in July.

Black Hairstreak Yearly Comparisons

The figures for 2003 are only provisional as there may be more records still to come.
Figures in italics are estimates, in 1998 we had two weeks of poor weather during the flight period.

Colony Information

*Many of our colonies appear to have been in isolation for over 25 years and yet are still surviving so the exchange of gene's between one colony and another (a metapopulation) may not be essential but is probably beneficial, but with a butterfly that is so elusive one cannot ignore the possibility that small satellite colonies may become established around a larger colony and provide the exchange of gene's that is usually necessary for a species to survive.

Threats

Since our monitoring began 40 colonies have been recorded in Bucks and Oxon this probably represents 70% of the national total of an estimated 50+ colonies, most are vulnerable due to lack of management especially those in private ownership.

So loss of suitable habitat for the butterfly is the major threat, one can understand those who consider if the B.H. is thriving now why start coppicing or laying the Blackthorn, but the female only oviposits at a height of two metre's and above so it will probably take 8-10 years before work undertaken will be beneficial for the butterfly, much of the Blackthorn on which our colonies are surviving is over 25 year's old once it has past 40 it will start to collapse and become moribund, if there is no regenerating Blackthorn to move onto the colony will be lost, so encouraging management has to be one of our main objectives.

At two sites where management has been undertaken with a known colony in excess of 100metre's away only after about 10 years has the butterfly been seen in sufficient numbers to indicate a colony has become established.

We have also witnessed collecting taking place at one site on three occasions, we have discussed this with English Nature and the police, who advise us that in this case a crime is being committed, so we have started to give them information on the cars seen parked in the locality, unfortunately this has meant that we cannot be more specific about the location of the colonies.

Managing Blackthorn For The Black Hairstreak

The most important point to remember in managing the Blackthorn is that the butterfly emerges around the middle of June, mating takes place quite quickly and ovipositing starts within one or two days on the Blackthorn twigs. These hatch the following late March early April the caterpillars feed on first the flowers then the leaves. Pupation takes place from the middle of May onwards on the Blackthorn leaves or twigs. This means cutting and removing Blackthorn at any time will remove part of the butterfly's life cycle.

According to Dr Jeremy Thomas's thesis on the Black Hairstreak the adult female lays her eggs throughout her life cycle, usually mid June to early July, individuals will only live about 14 days. They will not oviposit when it is cool or windy. The early summer period of many recent years has been wet cool and often windy, this will mean the butterfly will have died before laying all her eggs.

Providing a microclimate within the Blackthorn structure, with small sunny sheltered hollows where the temperature is a few degrees higher, would be most beneficial.

Shelter has always been important, where you have a colony on a Blackthorn thicket sheltered by a tall hedge remove or lay the hedge and the exposed colony will disappear. The eggs are usually laid on 2-3 year old twigs at a preferred height of 2 metres? and above. The Black Hairstreak is our most sedentary butterfly it takes many years for it to colonies ideal Blackthorn habitat only 50 metres away, so any changes must be slow.

Any method of management must only be small areas at one time around 10-15 square metres or if linear like hedgerows only a 10 --15 metre length so 1-3 days work for one man a year. The following year work in a different area well away from the first one and possibly use an alternative method. Aim to produce a mosaic of Blackthorn of differing heights and plan a 25-30 year rotation. Large areas of Blackthorn over half an acre may need two small areas worked on in a year.

Another aspect that has become apparent when looking at some of the woodland sites is the extent that maturing trees are shading out the Blackthorn, what had been regarded as a woodland or woodland edge butterfly has already become predominantly woodland edge, and in the last twenty years many hedgerow colonies have been found. It is very important at most of the remaining woodland sites to remove the mature trees that have shaded out much of the Blackthorn, only tall, week plants remain that are not adequate to retain a colony, so it will be lost.
 

Methods To Use

Cutting and removing small areas of Blackthorn has been the method used in the past.

Cutting at the pupal stage usually the first two weeks of June leaving the cut Blackthorn close by so that when the butterfly emerges it can fly on to adjacent live Blackthorn (problem --disturbing nesting birds).

Cutting at 1.5-2 metres high is a good method to use where Deer are common as they eat the regenerating Blackthorn, try and leave one or two side shoots or small branches of younger growth to help regeneration. This method should in theory mean the butterfly will use it much quicker for egg laying.

The method of management promoted in recent years is to lay the Blackthorn much as you would a hedge this will ensure that when the caterpillar emerges in the spring it has live growth to feed on. Try and lay it to the North to let in the maximum amount of light and sun. To encourage regeneration from the ground leave one or two open areas. Laying tall old Blackthorn without it snapping of is difficult, if working on a hedgerow live stakes could be used, but stakes and binders are not really necessary as a wide hedge is unlikely to blow out of alignment, this should leave an area impenetrable to Deer. Any work is best carried out in the winter months.

Where possible leaving the Blackthorn to sucker out and spread into an open area, or planting with quicks is a good alternative. The best areas for this are those with a Southerly aspect sheltered but not shaded by trees and close to a colony. If Blackthorn quicks are used make sure it is English stock to ensure flowering and leaf emergence is at the correct time, and only plant half the available area do not leave strait paths (wind tunnels) but hollows and scallops. Consider how it will look in 10 years time.

Where old Blackthorn (40 cm and over in girth) has been cut down regeneration will be slow therefore planting quicks is probably the best option.

Fencing to keep out Deer may be necessary as their numbers are increasing in many woodland areas, and you can see a browse line with all fresh Blackthorn growth eaten as high as they can reach. Unfortunately fencing will greatly increase the costs.

The best nectar source to bring the Black Hairstreak down from the canopy is Privet which usually flowers when the butterfly is on the wing giving an opportunity to approach quite close for identification and photographs so this could also be planted, once again make sure it is native stock it may need to be cut back when it gets too tall.

One last point ideal Black Hairstreak habitat is also good for Nightingales, Brown Hairstreak butterflies and many moths Beautiful Hook Tip, Sloe Carpet, Sloe Pug, Green Brindled Crescent, Orange Moth and Magpie Moth to name a few.

Initially when looking for management advice little was available, this is a combination of this advice, and some ideas of my own, as yet limited management has been undertaken so we have no firm indication of which is the best practice.

If you need further information please contact Stuart Hodges Black Hairstreak co-ordinator Upper Thames Branch of Butterfly Conservation : stuart@thegreen40.fsnet.co.uk