Report on Dorset's Butterflies 2002

by Bill Shreeves

My grateful thanks to all those walkers, surveyors, data inputers and coordinators who continue to send in valuable records and process them. I only wish there were time and space to refer to more of them. Altogether 47 walks completed enough weeks in both 2001 and 2002 to provide comparable data. Another 5 walks provided sufficient data to give abundance codes for some species and 2 new walks entered the system for the first time at Langton Westwood and Winspit Valley, both organised by the National Trust. Unfortunately no data was available from Brackets, Higher Hyde, Corfe Castle Mound and West Hill and Mude Valley arrived just too late to get into the official report. Our hard working Regional Action Plan Coordinators and their surveyors have now completed the second year of our five year plan and have managed to visit an amazing 829 species sites for our rarer species, 587 with positive records and 242 with zeros. These together with the garden and casual records all play their part in assessing the year 2002 for butterflies.

Migrants had a mixed year in Dorset. Table 1 shows that Clouded Yellows and Painted Ladies, as measured by Walk records, did better than in 2001 but not nearly as well as in 2000. On the other hand Red Admirals had their worst year for some time and Monarchs, had only 2 sightings. Records of Clouded Yellow in April on Portland and on Southbourne Cliffs were almost certainly natives surviving from autumn larvae.

A small migration may have occurred in May between week 6 (May 6^th –12^th) when the first was counted on a transect walk at Ballard Down, and June 30th when another 10 were counted spreading north to Duncliffe and Deadmoor and west to Cerne and Powerstock.. In week 18 (July 29^th-Aug. 4^th) a count of 23 on Ballard Down and another 3 in the Winspit Valley may suggest a second migration. Numbers built up to a peak of 56 on all walks in week 22 (Aug. 26^th-Sept. 1^st) of which just under half were recorded in the north of the county. In the last walking week at the end of September the species was still on the wing with 25 recorded on all walks. A late record came from Lodmoor on 18^th November (J.Reeve). The Painted Lady also seems to have had a migration in May, with the first record on a walk in Hethfelton Wood in week 7 (May 13^th–19^th), which reached a peak of 44 in week 11 (June 10^th-16^th). The offspring of the early migrants produced a second peak of 149 in week 21 (Aug. 19^th-25^th). The fact that no less than 6 walks recorded Red Admirals in week 1 (April 1^st-6^th) and that nearly all the early April records came from walks in the north of the county suggests that many may have survived the British winter in some form.

Table 2 shows the best and worst performers for 2002 measured in the traditional way, according to the percentage of walks, which recorded species well above 2001 or the average counts since the walks began. The 2002 weather, at least as measured by the Fontmell Magna weather station, was not very promising. May-July was above average in rainfall and sunshine, and June-July were also exceptionally cool.

August and September were much better but that was too late for 2002 and should be more useful for helping late summer egg laying species to provide better foundations for 2003. The one saving grace was that the May-June weather of 2001 had been warm and dry and so was the March and April, which followed in 2002. In theory this should have meant that single brooded species emerging early in the year ought to have done well in 2002. Generally speaking the list of better performers in parts 1 and 2 of Table 2 does support the hypothesis. Dingy and Grizzled Skippers, Orange Tip and Wood White are all single brooded early season species, which did well in 2002. The Dingy Skipper, with 63% of its 19 sites showing big increases on their 2001 totals, should be nominated as Dorset’s butterfly of the year. At Clubmens’ Down and Townsend Quarry the counts were the highest ever and Fontmell, Cerne Giant and Cashmoor recorded their second highest totals. Table 3 provides a summary of the walk results for the Dingy Skipper since 1993. It should be noted that the exceptionally low count in 2001 might be due, in part, to Foot and Mouth which closed many walks during the flight period.

TABLE 3.

The good results on the transects were echoed by the discovery of no less than 15 new km square sites which did not appear on the 1995-99 Atlas. Of these, 3 came from North Chalk (B.Shreeves, J.Westgate and B.Raymond);and 5 from Central and South Chalk (S.Anderson, L.de Whalley and S.Phelp). Cranborne Chase yielded another 3 (D.Taylor and PandR.Farrant; Purbeck added 2 more (P.Grey and C.Nunn) and finally 2 others from West Dorset Coast (D.Thomas). However despite these excellent results a few words of caution should be added. The large number of zero counts on the walks, shown in table 3, suggests that many of the colonies are small and fragile. Indeed, although no fewer than 33 walks have recorded this species only 20 of them meet the basic criteria for a self supporting colony, laid down by Pollard and Yates, that the species must have been recorded in 4 consecutive years. Moreover the vast majority of the stable 20 colonies are on chalk or limestone sites (70%) or woodland (20%). Only one occurs on heathland (West Moors) where it was ‘missing’ for the whole period 1994-2001. The only neutral grassland colony, at Stones Common, Kingcombe, has been lost since 1996, and, by Pollard and Yates’ criteria, must be deemed extinct. We estimate that there are about 73 sites remaining to be checked by the regional action plan surveyors but it looks ominously as though the range of the Dingy Skipper may be contracting.

The Grizzled Skipper (one of our bad performers in 2001!) was another early single brooded species, which had a good year with record counts at Clubmen’s Down and Sovell. New km. squares were uncovered by the surveyors; 7 of them on the chalk regions (B.Raymond, S.Phelp, L.de Whalley, J.Harris), and 6 on Cranborne Chase (PandR.Farrant, D.Taylor and B.Shreeves). As with the Dingy Skipper, non calcareous sites seem to be getting scarcer. It was therefore pleasant not only to find a new site on the clays of the Blackmoor Vale but also one (Three Bridges on A30), which was on a roadside verge (Bill Shreeves). Emergence times for this species are now moving into April. The Orange Tip probably did much better than the statistics show because comparative figures with 2001 were in many cases made impossible to calculate because of foot and mouth. The Wood White had a good year at Powerstock and re-appeared for the first time on the Kingcombe Stones Common walk since 1998. The West Dorset Coast survey teams also found it doing well on the Lyme Regis undercliff and it was recorded for the second year running at Black Ven. The Lulworth Skipper was recorded again from Portland, had good counts on its Purbeck walks and was re-discovered at Acton and the Pinnacles (P.Grey and Rev.Pratt).

It must be admitted that the good performance of the Dark Green Fritillary on 60% of its walks was rather a statistical mirage. Table 4 shows no improvement indistribution and the increase in the numbers counted came mostly from one site, Melbury Down, with 112 counted. As always the behaviour of the Holly Blue continues to be complex.

Table 5 shows that the general recovery reported last year has continued with 87% of the walks now recording at least one. However if it is to regain the peaks of 1997-98 it still has some way to go.

It may be relevant that the 2^nd brood in Dorset was lower (234) than the first (318). It can also be seen from the distribution table 6, that the rapid increase of the Holly Blue in East Dorset has now been pegged back, presumably by the expansion of its parasites. In 2002 most of the increase has been taken over by the north and south regions, which recovered both in numbers counted and in their percentage of the total Dorset count. Will the parasites catch up again with their hosts in North and South Dorset by 2003 and thus stifle the developing boom? What will happen in the West? Can they regain the high numbers of 1998 and thereby help the boom to continue or will the parasites and/or the weather keep them pinned down to the absurdly low counts of 2000-2002? Watch this space!

Our other problem double brooded species, the Wall, appears in part 7 of table 2 in the category of ‘doing better than in 2001 but still below its best’. Unfortunately as table 7 shows, the improvement is not very real. As usual most of the recovery in numbers has come from the coastal walks. Only 5 have been added to the inland total and there is a long way to go before the more healthy 1995 position is regained.

The surveyors for Central and Southern Chalk have, however, uncovered a more optimistic picture. So far they have found 35 sites and it may be that our transect walks are just on locations which are no longer very suitable for the species.

The worst performing butterflies of 2002 are listed in sections 4,5 and 6 of table 2 and no fewer than 5 species were also in that category in last year. The Grayling, continued the slump reported in 2001. The only walk to go against the downward trend was Hams Common. The West Dorset Coast surveyors are making a determined effort to check their non heathland sites. So far out of the 8 possible locations where the species had been previously recorded, 3 have produced zero records, 2 have yet to be visited and the remaining 3 have not reported more than 2 sightings. Four other species in section 6 of table 2 which experienced serious decreases on their count averages also featured among the bad performers in the 2001 report. The Silver Spotted Skipper at Fontmell Down plunged to its lowest walk count ever. None of the walks, which record White Admiral got up into double figures; Fifehead Wood was the best with a count of 7. The Regional Action Plan surveyors have found great difficulty in locating the species; 18 out of the 30 sites checked so far (60%) have recorded zero counts. Better news was the discovery of colonies in many of the coppices in the Manswood area of Cranborne Chase, some in km squares where the species had not previously been recorded in any of the post 1970 Dorset Atlases (B.and.J. Shreeves). The Chalkhill Blue slump continues with 8 out of the 10 main walks recording totals well below their averages. Only Hod Hill and Badbury Rings went against the trend. The pessimism is supported by the work of the Regional surveyors who reported zero counts on 36 out of the 73 sites surveyed. At present only 3 sites each have been recorded positive from Purbeck and South Chalk. Better news was the discovery of 4 new sites – 2 from the Central Chalk (K.Blease) and 2 from Cranborne Chase (N.Butt and D.Taylor). The Duke of Burgundy fell to its lowest count ever of 2 at Fontmell Down, and did not show at Clubmens’ Down or reappear at Melbury Down after its dramatic first recording there in 2001. Outside the walks the surveyors found 8 sites but 7 others came up with zero counts.

Sections 6 and 4 of table 2 contain 4 species newly ‘promoted’ to the bad category. Although 32 of the walks record Small Heaths, 47% of them show the species to be well down on its average counts. As there has been a marked downward trend on the National Monitoring Scheme since 1976 this is a species, which we need to watch carefully. Six of Our Dorset walks in 2002 saw their Small Heaths drop to zero counts. Three of our Dorset walks have experienced extinction (At least 4 consecutive years of positive records followed by at least 4 of zero counts). The species was last seen in 1997 at Sovell Down (Highest ever count 112), Stubhampton Bottom (15) and Lydlinch (10). At Kingcombe, Stones Common, (264) it has been absent since 1999 and at Cashmoor this year’s count was 9 compared to 353 in 1997. The Small Pearl-bordered Fritillary, as shown in table 8, is now threatening to follow the Pearl-bordered Fritillary on the downward plunge to extinction. The only walk, which recorded the species in 2002, was Deadmoor. One was seen off the walks at Powerstock (D.Thomas) and there were possible sightings on Hardown Hill (C.Burnet and Carless) by West Dorset Coast surveyors. Apart from the small colony on Stonebarrow and possibles on Studland Golf course and at Povington there seem to be few sites remaining.

TABLE 8: SMALL PEARL-BORDERED FRITILLARY.

The Pearl-bordered Fritillary has not quite become extinct as one was seen in late May at Stubhampton Bottom (J.McGill) but hopes are not high. The Silver Studded Blue has still not recovered from its slump with 6 out of the 7 walks with useable results registering below their average counts. Only Studland, with its second highest count ever, went against the trend, thanks perhaps to some firebreaks providing better breeding conditions. The North and Central Heath surveyors found new sites for the species at Parley Common (BandE.Knowles) and Canford Heath (J.Argent). Finally the Brown Hairstreak, which is my choice for the worst performer of the year: this has nothing to do with its poor performance on the walks where only 1 was recorded (on Deadmoor). In 1998 none were recorded. As is well known the butterfly itself is not often seen let alone under the restrictive conditions of transect walking. The eggs, reasonably easy to find in the winter on blackthorn, are much better indicators. The bad news is that the usual winter searches (559 people minutes altogether!) only succeeded in finding eggs in 3 out of the 11 squares, which yielded results in 1995-99 and 21 in 1980-95. This may not be as bad as it sounds since 2002 does seem to have been a poor year everywhere for the species and the eggs that were spotted were in the Deadmoor heartland of the colony. What was of most concern though, was the way all the hedges had been drastically cut with no sign of a rotation. If this species is not to be lost from its last stronghold in Dorset there is an urgent need to get on with the project recommended in our RAP. This requires us to organise liaison with farmers, landowners and Countryside Stewardship officials in the Deadmoor/Rooksmoor/Lydlinch triangle to set up suitable hedgerow management rotations.

This report cannot be concluded without reference to Dorset’s most successful but most neglected species, which continues to expand across the county – the Essex Skipper. It was officially recorded for the first time on the Fontmell Walk (L. de Whalley) and began to turn up unexpectedly in newly planted woodland still dominated by tall grasses, for example at Brooklands Wood, Fontmell Magna and various coppices in the Manswood area (B.Shreeves). Another of its typical roadside verge colonies was uncovered near King's Stag (B.Shreeves) and DBC survey teams found that very small numbers had penetrated the sites at Badbury Rings and Pamphill. So far there is little evidence of any colonisation of west and south Dorset but this may be because people are not looking. We still need intrepid surveyors to reconnoitre the Wareham and Dorchester bye-passes! More volunteers are required to carry out surveys like those undertaken independently by members of the Kingcombe butterfly course and Lawrie de Whalley at Lankham Bottom which, after catching and peering at large numbers of ‘Small’ Skippers, proved fairly conclusively that the Essex invaders have not yet penetrated this new Butterfly Conservation reserve near Maiden Newton. It should be noted that visitors from outside Dorset (B. and S. Roberts), using close focus binoculars, reported the species at Broadcroft Quarry. This will need further investigation by the Portland survey team but, if confirmed, will mean that Broadcroft may become the first site in Dorset to experience the headache of having all four golden Skippers (Large, Small, Lulworth and Essex) to grapple with!