Observations on the life-history of the Silver-studded Blue, Plebejus argus L.

OBSERVATIONS ON THE LIFE-HISTORY OF THE SILVER-STUDDED BLUE, PLEBEJUS ARGUS L. H O W A R D M E N D E L a n d ERIC PARSONS

In spite of the very large number of naturalists interested in butterflies and the small number of British species to study, it is surprising just how little is known about the biology and ecology of many of them, and how much can be learnt by careful Observation. Between 1985-1986 observations were made, in the field, on two colonies of the Silver-studded Blue on the Ipswich 'Eastern Fringe', at Purdis Heath and Nacton Heath, and in 1986 the species was reared in captivity.

The Silver-studded Blue is a declining species in Suffolk as it is in B ritain as a whole. It has now gone from the Breckland, the former stronghold (Morley, 1937) and is confined to the Sandlings of East Suffolk (Mendel & Piotrowski, 1986). The first butterflies of the year usually appear at the end of June, and the species is on the wing throughout July and the early part of August. The eggs produced by this Single generation do not hatch until the following April and the larvae pass through four instars before pupating in June.

The Egg According to Thomas (1983), who was studying the Silver-studded Blue in western Britain, female butterflies lay their eggs on a wide variety of substrates including known and potential larval food plants, mosses, lichens, plant litter, soil and bare rock.

In the Sandling colonies under study, on the Ipswich 'Eastern Fringe', sparse marginal growth of encroaching Bracken, Pteridium aquilinum (L.) Kuhn., with an average height of 420mm, was the preferred site for oviposition. Eggs were normally placed singly on the underside of a pinnule and were well distributed among chosen plants. Prior to depositing an egg, the female butterfly would 'test' the plant with the tip of her abdomen in a series of 'dabs' as she walked. Both upper and underside of the fronds were explored in this way before an egg was finally attached to the underside of a basal pinnule. Not all fronds 'tested' were acceptable and what appeared to be oviposition often proved to be a 'dummy run'. Female butterflies feeding on Bell Heather, Erica cinerea L. frequently left to oviposit on Bracken, returning soon after to resume feeding.

The 48 eggs initially found on Bracken in 1985, and marked, were distributed between 42 Bracken plants. Only a Single plant held more than two eggs and the average distance between eggs in recognisable groupings was 750mm. It would be all too easy to dimiss egg-laying on Bracken as aberrant behaviour. However, literally hundreds of eggs have now been found on Bracken in the study areas, and large numbers have, since the initial report, been found at two further Suffolk colonies (Ravenscroft, 1986), and at a site in Norfolk (M. R. Hall,pers. comm.).

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The extrafloral nectaries of Bracken were first described by Darwin (1877) and the attraction these hold for ants is well known (Heads & Lawton, 1985).

The work of Pierce and Elgar (1985) on Jalmenus evagoras ( H b n . ) , a myrmecophilus lycaenid butterfly widespread in Australia, demonstrated that females of this species were far more likely to lay their eggs on plants with attendant ants than plants without. T h e authors catalogue every reference to ant induced oviposition and, in all, it has been suggested or specifically described in 46 species f r o m 29 genera and 5 subfamilies within the Lycaenidae.

T h e association between the Silver-studded Blue and ants, Lasius spp. is well known, and the preferred Bracken fronds are well attended by workers of the Black G a r d e n A n t , Lasius niger (L.). In view of how widespread ant induced oviposition is in the Lycaenidae it seems likely that this p h e n o m e n a may account for the large n u m b e r of eggs laid on Bracken.

T h e clearance of encroaching Bracken from Sandling heaths is a laudable conservation objective. H o w e v e r , the removal of sparse marginal growth on known colonies of the Silver-studded Blue is likely to remove large numbers of viable eggs. Initial clearance by hand, in J u n e , is r e c o m m e n d e d , prior to the season's emergence of butterflies. Ants are known to attack and remove insect eggs f r o m plants ( H e a d s & Lawton, 1985). Of the 48 eggs individually marked (above), 35 were still intact by D e c e m b e r . Only eight had been eaten or removed f r o m the b r a c k e n pinnules and five could not be found because of fragmentation of the dead fronds. Clearly predation of Silver-studded Blue eggs is at a low level and somehow predation by ants is avoided. U n d e r experimental conditions ants, L. niger ignored the eggs, even those near to hatching containing fully developed larvae. It has been demonstrated experimentally for several insect species that attendant ants protect the larvae and p u p a e f r o m predators and parasitoids (Pierce & M e a d , 1981). Perhaps this protection extends to the eggs of species such as the Silver-studded Blue.

Only 17 of the original 48 eggs could still be found the following Spring due to fragmentation of the B r a c k e n fronds. All but four of these hatched and none of the 17 were parasitised. It is assumed that the majority of those that could not be f o u n d remained viable.

Egg laying on sparse Bracken growth does confer certain benefits. It ensures that larvae hatch in an area where their ant associates are likely to be found the following year, possibly protecting the eggs f r o m predators and parasites; an important factor considering that the egg stäge lasts 8 months.

Eggs laid on B r a c k e n , a plant generally avoided or poisonous to herbivores, are also less likely to be accidentally consumed.

Larva: Ist. instar T h e Silver-studded Blue is known to overwinter in the egg stage but this is probably not strictly true. Eggs collected f r o m the wild and dissected 17th.

Feb. 1987 contained fully f o r m e d though torpid larvae. It is most unlikely that the larvae had developed in the cold weather immediately prior to collection and seems far m o r e probable they develop in the season the eggs are laid and overwinter within the egg shell.

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The Ist. instar larva emerges from a jagged hole eaten in the top of the egg and does not begin to feed immediately, even if presented with suitable food.

The tiny larvae move energetically and can sustain a speed of 130mm per hour over a period of 8 hrs. In captivity Ist. instar larvae could survive at least 6 days without feeding; a useful adaptation for a species whose larvae, in the wild, often need to search for a suitable foodplant. Captive larvae would not eat Bracken and Ist. instar larvae, in particular, were not keen to eat even the tenderest parts of Bell Heather or Ling, Calluna vulgaris (L.) Hull. A variety of leguminous plants such as Least Bird's-foot, Ornithopus perpusillus (L.), Bird's-foot Trefoil, Lotus corniculatus (L.), Common Vetch, Vicia sativa (L.) and the flowers of Gorse, Ulex sp., were eaten readily. The larvae in captivity fed by night and by day.

It is interesting that Bell (1915-1920) maintained that some lycaenids possessed an unusually wide variety of host plants because the females of the species laid eggs in response to attendant ants as well as using plant cues. In such butterfly species, legumes feature prominently amongst the foodplants (Downey, 1962).

A shiny, dark-olive near-black, arrow-shaped area of sclerotized cuticle is a prominant feature of the prothorax. This feature remains with the larvae through successive moults until pupation though becomes smaller in relation to size in later instars. Its function, if any, remains unknown. The larvae are able to produce silk and will hang from a thread if dislodged from the foodplant.

The Ist. instar larvae are attractive to the Black Garden Ant, L. niger - a very common species of the Sandling Heaths - even though a honeygland does not develop until the 2nd. instar. Ants stroke the larvae with their antennae and palps and then draw these appendages through the tarsi of their forelegs and their mandibles.

The Ist. instar lasted for about 21 days but was as short as 16 days in one case, and moulting is preceded by approximately 3 days inertia.

Larva: 2nd. and 3rd. instars In the 2nd. instar a honey gland on the 7th. abdominal segment is present and becomes the focus of attention for the L. niger ants, producing large globules of transparent liquid in response to their attentions. In addition to this honey gland the 3rd. and 4th. instar larvae have a pair of erectile tubercles on the dorsal surface of the 8th. abdominal segment.

The function of the erectile organs with their terminal 'brushes' is not absolutely clear, but their deployment invariably prompted a flurry of activity from ants in the immediate vicinity. In particular, the larvae appeared to dislike the attention of ants at their heads and would usually retract them beneath the pro-thorax and erect the retractile 'brushes' in response. The effect would be to displace and attract the ants thereby guiding them to the honey-gland.

Ants kept in captivity with larvae were invariably found attending them.

The first indication that L. niger workers were prepared to carry larvae occurred when a 3rd. instar larva was tipped from its Container with attend-

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OBSERVATIONS ON THE LIFE-HISTORY OF THE SILVER-STUDDED BLUE

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ant ant. The larva was immediately picked up in the mandibles of the ant and remained held whilst the pair were transferred to a potted plant. The larva was found to be alive and well on subsequent checks.

Similar behaviour was observed in the wild. L. niger workers were seen to pick up and carry larvae and would also transport captive larvae released into the foraging area of L. niger nests well away from Silver-studded Blue colonies. Wild larvae were very difficult to locate and the few observed in situ were 3rd. or 4th. instar and always attended by ants. Sometimes larvae were carried for short distances and released; at other times they were carried out of site into the herbage. The reason for the behaviour is still a mystery.

Perhaps the larvae were being moved nearer to the nests of the ants, but there was n o evidence of larvae actually being carried into the nests.

The 2nd. and 3rd. instar larvae last, on average, of 13 and 15 days respectively.

Larva: 4th. instar The 4th. instar larvae are very similar in appearance to the 3rd. and lasted a mere 11 days in one individual and up to 26 in another. As larvae approached pupation they stopped feeding and became restless. Larvae released into the wild at this stage would enter any holes in the ground of suitable diameter, including the entrance holes of L. niger nests. W o r m holes, the old nests of solitary wasps and even artificial holes made with a pencil were all treated similarly.

In controlled conditions, captive larvae readily made use of holes of about 6mm diameter bored at an angle of 45°, in sandy soil, to a depth of 70mm, with a crude Chamber at the bottom, about 10-15mm in diameter. This apparatus with the artificial holes was placed within the foraging ränge of a large L. m'gernest.

A larva with three attendant ants was watched as it descended a borehole to the Chamber at the bottom. The larva proceeded to line the Chamber with silk but at the same time the attendant ants were enlarging the cell and eventually removed all the silk. More ants introduced into the open apparatus joined in with the excavation of the floor of the cell - even though free to come and go at will - which was extended downwards by about 15mm overnight. For a period of at least five days after pupation an assembly of about 20 ants could be seen crowded against the walls of the cell, virtually motionless for much of the time.

The way the ants were seduced away from their nest and so readily adopted the pupating larva and newly formed pupa was quite remarkable. It goes a long way towards explaining the presence of mature larvae and pupae in ants nests. Whilst larvae with the 'urge' to pupate will most certainly descend into ant nests, larvae using other burrows are also likely to 'acquire' ants and appear to the uninformed to have pupated within a nest.

Pupa Captive larvae in glass Containers constructed silk mats on which to pupate and the pupae were attached to these by the cremaster. Pupae within the

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artificial bores gradually became less attractive to ants. After the complete departure of the ants a small quantity of earth was noted sealing the bores. It is not clear whether this resulted from a natural collapse of the tunnel or infilling by the ants. It presented no obstacle to the emerging butterfly. The pupal stage varied in length between 12 and 21 days.

Butterfly The first butterflies usually begin to emerge during the last week of June, the males about a week before the females. Although encounters between males and females were often witnessed during the study, the actual engagement of copulation was only seen on one occasion. This occurred at around midday when a female Aying down-wind was intercepted by a male. The pair quickly came to rest on the flower of some Rose Bay Willow Herb, Epilobium angustifolium L., after a short flight in close contact. The female landed and proceeded to walk on the plant with wings held vertically. The male followed with vibrating wings and coupled in seconds. They remained in cop. for 55 minutes during which time the female was seen to pull the male along as she walked.

Wing wear during copulation was often intense. A photographic record of one pair in cop., made by Mr. P. Hyre of Ipswich, initially shows a freshly emerged female with a lightly worn male. In the process of pulling the male into the herbage the female's wings were abraded and the final photograph of the series, only 20 minutes later shows the female with frayed wings.

Attempts to relate wing wear directly to age (Thomas, 1983) are of very limited value.

Recent studies based on 'mark-release-recapture' methods (Thomas, 1983; Ravenscroft, 1986 and Read, 1986) have emphasised the sedentary behaviour of individual Silver-studded Blues and the formation of 'closed' colonies. However, it must be remembered that 'mark-release-recapture' methods within a colony are not the best way of measuring the drift of individuals from a colony. The Silver-studded Blue is a pioneer species, and a pioneer species forming 'closed' colonies is a most unlikely product of evolution: 'The static butterfly species does not and cannot exist' (Dennis 1977).

Allen (1980) reported a male Silver-studded Blue on Golden R o d , Solidago canadensis L., in a Surrey garden in the first half of August 1978, and during the Suffolk Butterfly Survey (1983-1985) a Single male was found at least 2km f r o m any known colony. By good fortune, during the course of this study, a female marked with a red and a green spot appeared at the Nacton H e a t h site on 16th. July, 1986. Numbers of butterflies were marked with a combination of green and red spots at Purdis Heath on l l t h . July 1986, in a study organised by the Suffolk Trust for Nature Conservation. It is extremely unlikely that the butterfly originated elsewhere. Nacton Heath is about 1 -25km SSW of Purdis H e a t h .

Details of the weather between l l t h . and 16th. July, at nearby Levington Research Station, have kindly been supplied by Mr. D . A. Barnard (Norsk Hydro Fertilizers Ltd.).

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OBSERVATIONS ON T H E LIFE-HISTORY OF T H E S I L V E R - S T U D D E D B L U E

Date 11.07 12.07 13.07 14.07 15.07 16.07

Wind Direction

Strength

ENE S ESE NW N S

4 2 1 3 2 4

Temperature (°C) Max. Min. 17-9 16-3 19-7 23-4 24-6 26-4

12-4 11-0 11-8 14-2 14-3 16-6

7

Sunshine (hrs) 7-6 0-0 4-9 2-8 8-3 III

The 15th. would appear to provide the best conditions for such a movement with a wind of force 2 from the North, but this is sheer speculation.

The remaining Suffolk colonies of the Silver-studded Blue are usually regarded as relicts from the days when the species was found widely across the more extensive heathland. There is no evidence to support this. More iikely, populations build up on suitable areas of habitat and small numbers of individuals disperse from these centres to found satellite colonies. These may last only a few years or may expand to form the next generation of population centres.

Satellite populations are usually associated with the larger colonies and it is Iikely, for example, that the two Nacton Heath populations and even the Warren Heath population of recent years were established by individuals from the much larger colony on Purdis H e a t h , and of no great age.

The foundation of new colonies by a small number of strays, or even a Single gravid female, from an established colony not only helps to explain the present distribution of colonies. It also helps to explain the variability between different populations of the species. Some Suffolk colonies were once noted for their butterfiies' 'large size and tendency of the females to blueness' (Worms, 1952), possible results of natural in-breeding.

Acknowledgements We thank Ransomes Sims and Jefferies for unrestricted access to the Nacton Heath site, Mr. D . A. Barnard of Norsk Hydro Fertilizers for local weather information and the Suffolk Naturalists' Society for the Chipperfield Bursary, to one of us, helping to make this study possible.

References Allen, A . A . (1980). An unlikely find in a N. E . Surrey garden. Entomologist's Ree. J. Var., 92, 6.

Bell, T. R. (1915-1920). The common butterflies of the plains of India. J. Bombay Nat. Hist. Soc., 1915-1920.

Darwin, F. (1877). On the nectar glands of the common brake fern. Journal ofthe Linnean Society (Botany), 15, 398.

Dennis, R. L. H . (1977). The British butterfiies. Their origin and establishment. Classey, Oxford.

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D o w n e y , J. C . (1962). Host-plant relations as data f o r b u t t e r f l y Classification.

Syst. Zool., 11,150.

H e a d s , P. A . and L a w t o n , J. H . (1985). Bracken, ants and extrafloral nectaries, 111. H o w insect herbivores avoid ant predation. Ecological Entomology, 10, 29.

Mendel, H. and Piotrowski, S. H . (1986). The Butterflies of Suffolk. An atlas and history. Suffolk Naturalists' Society, Ipswich.

Morley, C. ( E d . ) , 1937. The Lepidoptera of Suffolk. Suffolk Naturalists' Society, Ipswich.

Pierce, N. E . and Elgar, M. A . (1985). T h e influence of ants on host plant selection by Jalmenus evagoras, a myrmecophilus lycaenid butterfly.

Behav. Ecol. Sociobiol., 16, 209.

Pierce, N. E . and M e a d , P. S. (1981). Parasitoids as selective agents in the symbiosis between lycaenid butterfly caterpillars and ants. Science, 211, 1185.

Ravenscroft, N . O . M. (1986). An investigation into the distribution and ecology ofthe Silver-studded Blue butterfly ( P l e b e j u s argus L.j in Suffolk an Interim report. U n p u b l i s h e d ms. Suffolk Trust for N a t u r e Conservation, S a x m u n d h a m .

R e a d , M. I. (1986). The Silver-studded Blue Conservation Report. U n p u b lished ms. Joint C o m m i t t e e for the Conservation of British Insects.

T h o m a s , C. D . (1983). The ecology and status o / P l e b e j u s argus L. in North West Britain. M.Sc. Thesis, University of Wales.

W o r m s , C. G . M . de (1952). Collecting Lepidoptera in the eastern counties; some reminiscences. Trans. Suffolk Nat. Soc., 8, 43.

H o w a r d M e n d e l , T h e M u s e u m , High Street, Ipswich IP1 3 Q H .

Eric Parsons, 41 Dover R d . , Ipswich IP3 8JQ.

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