A Proposal for the Introduction of the Silver-studded Blue Butterfly
(Plebejus argus) at Thurstaston Common, Wirral

(1) The status of Plebejus argus in North West England and North Wales.
(2) Habitat requirements on Heathland Sites.
(3) Prees Heath, shropshire as a Donar Site.
(4) Possible Introduction at Thurstaston Common (SJ 248 846).
(5) Outline of Required Management.
(6) Monitorinq the Success of the Introduction and Targets for the Future.
(7) Monitoring the Effects of Removal of Stock from Prees Heath and Safeguarding Against a Population Crash.
(8) Removal of Stock from Prees Heath.
(9) Remarks on Bulking Up the Thurstaston Population in the Future.
(10) Possible Expansion of the Thurstaston Population.
(11) Meeting the Financial Costs.
(12) Reporting on Progress.

(1) The status of Plebejus argus in North West England and North Wales.

Plebejus argus is one of the classic butterfly species of English lowland heath and its decline has followed the destruction of this habitat. On the Wirral the species was present on Bidston Hill (SJ 288894) near Birkenhead until the end of the 19th Century. It appears that this colony was destroyed by the establishment of woodland on its former breeding grounds. The very poor powers of dispersal for this species meant that it did not move onto other possibly suitable habitat close by.

The nearest existing heath land colonies of Plebejus argus are twofold. Firstly, a strong. colony on the heathland of Holy Island's South Stack (SH 208 827) and secondly on Shropshire's Prees Heath (SJ 565375). The survival of the colony at South Stack is thought to be dependant upon the sea breezes that are a strong feature of the site. These cause the heather species to grow dwarf and sparse and provide ideal conditions for egg-laying and larval development. In addition, the R.S.P.B. have engaged in periodic heather cutting to ensure the suitability of this habitat.

Colonies of Plebejus argus are not totally confined to heathland. For example, very large colonies exist on the limestone cliffs of the Great Orme (SH 766 834) and llanddulas (SH 915 775). Here the larval foodplants are rock-rose (Helianthemum chamaecistus), bird's-foot trefoil (Lotus corniculatus) and wild thyme (Thymus serpyllum). The poor powers of dispersal for Plebejus argus are again highlighted by the success of the species at Llanddulas. The species was absent from this area, despite its close proximity to the Great Orme, until 90 adults were released by A.J. Marchant in 1942. They enjoyed a great success, slowly spreading to 16 sites in 40 years, these containing in total perhaps 90,000 adults by the early 1980's (1). Go back to the top

(2) Habitat requirements on Heathland Sites.

On heathland sites the most usual larval foodplants for Plebejus argus are heathers (Calluna and Erica species) and sometimes gorse (Ulex species). For example, Dr Jenny Joy (2) & (3) studying the Prees Heath site quotes that 69% of ova found were laid on ling (Calluna vulgaris). It is well documented (1), (2), (3) & (4) that egg-laying females concentrate on sparse patches of ground where young heather or gorse sprouts through bare ground in the first years after clearance or burning. The preference for fresh clearings is particularly obvious here in the North, reflecting the females requirement to lay in ground that is warmer than the surrounding vegetation. This obviously provides suitable conditions for the overwintering ova and young larvae the following Spring.

It has become clear that Plebejus argus also has a very close relationship with ants, particularly the common black ant (Lasius niger). It is possible that the egg-laying females can detect the chemical trails left by foraging ants and lay eggs close to suitable foodplants exactly where ant activity is at a maximum. It is clear that the black ants tend the larvae of Plebejus argus, milking them of the sweet secretions exuded from a gland on the larval abdomen. The larvae also possess microscopic pores which relea.se chemicals that appear to have a calming effect on attendant ants. The pupae of the species have been found lining the corridors of black ant nests. Whether the larvae crawl there to pupate or whether the larvae or pupae are carried there by ants is uncertain. It is certain that emerging adults are attended by ants and protected during the dangerous period of wing inflation, as observed by Dr Joy (2) & (3). Go back to the top

(3) Prees Heath, shropshire as a Donar Site.

In considering a possible donar site for an introduction of Plebejus argus on the Wirral, one must obviously assume that 'local' heathland stock would be most likely to survive the climatic conditions prevailing in the area. From a conservation viewpoint the Prees Heath site would be preferable to South Stack. The Holy Island colony is very large and stable at this point in time, whilst Prees Heath is much smaller and the future of the site is still uncertain (5). Furthermore, Dr Christopher Thomas, who has studied Plebejus argus in some detail, feels that the Prees Heath race may be a distinct sub-species making its conservation all the more important. Dr Jenny Joy has also studied the colony at Prees Heath and outlines the habitat requirements on this site in her report (2). She found that egg-Iaying females concentrate on sparse areas of heather where Lasius niger is in high density. The adults use more mature heather for roosting and bird's-foot trefoil as a nectar source. In the event of an introduction going ahead Dr Joy would be pleased to apply for permission to remove a number of adult butterflies from the Prees Heath site. Go back to the top

(4) Possible Introduction at Thurstaston Common (SJ 248 846).

In an attempt to assess the potential for a possible introduction of Plebejus argus on the Wirral, Merseyside Branch of Butterfly Conservation has visited most of the heathland sites in the area. Our studies showed that areas such as Bidston Hill, Caldy Hill (SJ 225860), Heswall Dales (SJ 260820) and Thurstaston Common still had areas of heather with potential, but which would require suitable management regimes. Because of the potential for management and the expertise of Andrew Brockbank (Senior Area Ranger), we decided to look more closely at Thurstaston Common. Here an area of heathland of approximately 6 acres was identified at SJ 248846 lying between the Cottage loaf public house and the Heatherlands restaurant. This plot has a good, warm, southerly aspect and is sheltered by silver birch scrub and gorse to the south, east and west. The elevation towards the north should shelter the area from this direction. Of particular note on this area is the high proportion of bell heather (Erica cinerea) which could act as a nectar source for adult butterflies. Some management has already been carried out with the removal of birch scrub intruding on the heather rich areas. Also, four acres of the area was subjected to burning in 1989 to eradicate heather beetle. In the bare, sparse areas remaining the black ant Lasius niger is prevalent. We feel that with suitable management this site would be most suitable for Plebejus argus. Go back to the top

(5) Outline of Required Management.

For a successful introduction of Plebejus argus on the area outlined in (4) and for its future survival, one would envisage a management plan based on the following outline:

(i) Keeping areas of heather free of scrub, particularly birch.

(ii) Splitting the six acre site into thirds of two acres each.

(iii) Carrying out management on one third to produce a mosaic of heather forms. Mature heather, particularly Erica species to act as roosting sites and nectar source plants for adults. Flailing between mature heather to produce pioneer heath. Grubbing out flailed areas to produce sparse, bare patches.

(iv) Introduction of Plebejus argus at a time when heather regeneration and ant numbers were at an optimum (as agreed with Dr Jenny Joy).

(v) Carry out (iii) on the next third at a stage when the pioneer heath is becoming less suitable, allowing the butterflies to cross-over to the new, more suitable growth forms.

(iv) Continue the rotation to the third area etc., etc.. The time period between flailing on one area and the same area becoming too mature for Plebejus argus may vary between 3 & 5 years and would have to be monitored closely to gauge the rotation cycle (see (6)). Go back to the top

(6) Monitorinq the Success of the Introduction and Targets for the Future.

Members of the Merseyside Branch of Butterfly Conservation will monitor the introduction site at Thurstaston Common on a twice weekly basis over the adult flight period (last week in June, all July and August). This will consist of a count of male and female adults over the whole introduction area, noting weather conditions, time of day, etc.. The counts will be compared with the management of each of the three areas of two acres. Thus the effects of flailing, heather regeneration, etc. on the abundance of adults in a particular area will be gauged. In addition, once a week during the flight period, mated females will be followed in their search for egg-laying sites. The micro-habitats chosen for egg-laying will be described i.e. amount of bare ground, age of plants used for egg-laying or in the close vicinity, egg-laying substrate, abundance of ants, etc.. Thus a detailed picture of the egg-laying requirements of females should appear and will be useful in deciding the flailing regime. By this method and by studying the fluctuations in population we hope to be able to time the flailing to allow suitable movement of stock onto. fresh ground. The area of heather on Thurstaston Common is very much larger than our proposed introduction site and flailing is carried out routinely on areas where considerable die-back is occuring. Thus we will also monitor other suitable areas on the common once a week to get an appreciation of any expansion of the colony.

We would consider the introduction a success if in the first year after transfer of stock we observed at least as many adults as were released. Over five years we would hope to have at least doubled the numbers. After ten years we would hope to be running a strong colony of many hundreds of adults and would hope to have built up sufficient information to provide a simple and efficient management regime that can be carried many years into the future. Go back to the top

(7) Monitoring the Effects of Removal of Stock from Prees Heath and Safeguarding Against a Population Crash.

Dr. Joy has been monitoring the Prees Heath stock of Silver-studded Blues by a transect walk over 1991 and 1992. Should the introduction go ahead at Thurstaston she is happy to continue the monitoring of the Prees Heath site and to raise the alarm should the population significantly fall in the years after the removal of adults to Thurstaston.

I should also like to add that Dr. Joy feels that the time for introduction is now while the Prees Heath colony is still thriving (she estimates the population as several thousand adults). She feels that removing fifty females (see (8)) from the site now will be less significant than the damage done to the site by continuing scrub and tree invasion and an increase in nutrient levels from the fertilisers drifting from adjacent farmland. In all I think it is her opinion that Prees Heath can only deteriorate in the long run and there are no plans to carry out any significant management on the site in the foreseeable future.

In the worst case scenario of a crash in numbers at Prees Heath we would clearly consider removing stock from Thurstaston back to Prees. If numbers were low at Thurstaston then we have the option of captive breeding at Ivy Farm, Arrowe Park, Wirral. Here we already have heather growing in the butterfly house with a view to any disaster occuring after the introduction. It is possible to breed the silver-studded Blue in captivity although some losses are to be expected due to the lack of attention from ants and the build up of sweet secretions over the larval body making the larvae prone to infestations. Go back to the top

(8) Removal of Stock from Prees Heath.

Dr. Joy feels that the removal of fifty female, adult Silver-studded Blues should give the introduction at Thurstaston a fair chance of success. Several successful introductions of the species have involved females only. It is thought that multiple matings in the species increases the number of eggs laid by an individual by 10-15%. We therefore feel that removal of males will not be necessary if as many as fifty females are released at Thurstaston (that is to say that the effect of transfering males could be equaled by simply removing a few more females). Successful introductions are known where the numbers of females released have varied between six and ninety individuals. Go back to the top

(9) Remarks on Bulking Up the Thurstaston Population in the Future.

We would consider bulking up the Thurstaston population of Silver-studded Blues if the weather conditions over the introduction period were particularly unfavourable i.e. the transferred females had little opportunity to egg-lay after introduction. In this case we would consider removing a further twenty to twenty five females from Prees Heath in the following season. It is now believed that in the case of such sedentary species as Plebejus argus the genetic pool does not have to be large for a colony to enjoy success and we believe that we will not have to bulk up the stock to improve genetic viability. Go back to the top

(10) Possible Expansion of the Thurstaston Population.

As mentioned in (6) the area of heather at Thurstaston is extensive and periodic heather flailing is a fundermental part of the overall management plan for the site. Suitable dispersal routes exist at present and the chances for expansion of the colony may improve on this in the near future since a plan for major scrub removal is under way. We fully intend to monitor any expansion of the population. Other heathland sites also exist on the Wirral and although it is unlikely that the butterfly will reach these unaided, we may consider introductions on these sites in the future obviously depending on the progress at Thurstaston. Go back to the top

(11) Meeting the Financial Costs.

The cost of management work on the Thurstaston site will be met by the land-managers, Wirral Council. The work will be costed as part of the overall management plan for the site. Other costs such as stationery, postage of reports etc., photocopying & time will be met by Merseyside Branch of Butterfly Conservation. Equipment necessary for the transfer of adults is already owned by the branch. Go back to the top

(12) Reporting on Progress.

An annual report outlining the progress of the Thurstaston population will be sent to the National Trust (land-owners),Wirral Council (land-managers), English Nature, JCCBI, Butterfly Conservation (Nationally, West Midlands Branch & Merseyside Branch). We will outline in this the general health of the population, the results of monitoring, the significance of management (flailing etc.) and any interesting lessons learnt during our studies. Go back to the top

REFERENCES

(1) Thomas, J. The Butterflies of Britain & Ireland. Dorling Kindersley Ltd..
London (1991).

(2) Joy, J. Shropshire's Silver-studded Blues. The status and conservation of the Silver-studded Blue Butterfly Plebejus argus in Shropshire. Report for the World Wide Fund for Nature UK (in prep.) (1992).

(3) Joy, J. Shropshirets Silver-studded Blues. Butterfly Conservation News, 51:40-43.

(4) Thomas, C.D. The Ecology and Status of Plebejus argus L. in North West Britain. M.Sc. Thesis, Univ. of Wales, Bangor (1983).

(5) Joy, J. The threat to Shropshire's Silver-studded Blues. British Butterfly Conservation Society Newsletter, West Midlands Branch, 26:27-28.

D.C. Hinde
Hon. Secretary
Butterfly Conservation (Merseyside Branch) Go back to the top

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