Silver-studded Blue Butterfly (Plebejus argus): A heathland gem.
IT MUST HAVE BEEN QUITE A SIGHT, back in the day, the blooms of flickering iridescent blue hanging from yellow flowered gorse and broom bushes populating the dry, dusty heathland; a rising sun caressing the land as thousands of silver-studded blue butterflies warmed themselves in the warming morning rays. Only when approached too closely by a resident 'heathcropper' would they would launch themselves, as one, a scintillating, fluttering shoal flighting from the apparent human danger towards a neighbouring stand of vegetation. Reportedly, this spectacle was not uncommon on Norfolk's heathlands of centuries past, when those making a living out of the intensively managed environment would frequently encounter such breathtaking sights during the short weeks of high-summer. It is still possible to find such numbers in other parts of Britain; sadly, no longer in Norfolk.
Throughout the 1980s there were only three known sites to find the silver-studded blue: Horsford, Buxton and Marsham heaths. Happily, this paucity of range has been somewhat bolstered by translocations to suitable heaths within the county, including East Ruston Common and Kelling Heath, in a bid to repopulate locations of yore that may once have held significant numbers of the insect. The butterfly, Plebejus argus, is one of nine British resident blue butterflies of the family Lycaenidae -- the coppers, hairstreaks and blues -- that have spread themselves over many of the Southern Counties and even venture north into Scotland. However, the silver-studded blue is on the British Red List of species (recorded as vulnerable) and marked down as a 'priority' on the U.K. Biodiversity Action Plan, more through a loss of suitable habitat than lack of physical numbers.
So why should a moderately numerous species receive such attention from well-meaning organisations? The answer lies in the butterfly's inability to colonise new territories of satisfactory habitat within any sort of reasonable timescale (the adults seldom wandering more than 30-meters from their natal beginnings), exhibiting a near lethal ponderousness that sees entire colonies moving at no more than glacial pace (1/2 mile per decade) into unchartered districts, so-much-so that a habitat appropriate to their needs a mere 100-meters away may as well be on the moon. This fatal lethargy is exasperated threefold: i) the fragmented heathland habitat required by Norfolk's silver-studded blues has only recently reversed its decline, needing regular management work and grazing to prevent its degradation and thus unsuitability for recolonisation: ii) the adult butterfly only lives for a maximum of 4 - 5 days; an incredibly short window of opportunity to colonise and mate: iii) it's a pathetically weak flyer, typically flapping low and without ambition amongst the gorse, heather and furze in the open and breezy heathland landscapes. In essence, it is not a creature suited to the capriciousness of 21st Century living. All of this said, they have occasionally been found great distances away from the nearest known colony, presumably caught-up on a strong wind and dumped miles away from home.
The epithet of blue is somewhat of a falsehood for this family of butterflies, the males in particular (females are duller in appearance): they haven't a trace of blue on their tiny bodies.
It is quite easy to confuse the silver-studded blue with other members of the same clan, particularly where ranges can overlap, with the common blue being the most likely transgressor. Nevertheless, the silver-studded blue does have one distinguishing feature that no other blues share and the feature from which it is denominated: on the hind under-wing, each black marginal spot has in its centre a distinct, silvery-blue speckle, or stud, that is clearly viewed when the insect is at rest. Confusingly, the epithet of blue is somewhat of a falsehood for this family of butterflies, the males in particular (females are duller in appearance): they haven't a trace of blue pigment on their tiny bodies. Instead, their wing scales comprise transparent layers that alter the reflection of light; the spacing of these scales produces an effect that we humans see as a beguiling, iridescent blue.
As alluded to earlier, Plebejus argus is extremely vulnerable to slight changes in its immediate environment, but it has a very surprising ally that protects it not from human intervention; more specifically a foe that lurks predominantly unseen by the butterfly until it is too late: parasitic wasps. (There seem to be two families of wasp that particularly favour the lifestyle of the silver-studded blue, the Ichneumonidae and Brachonidae, each with their own ingenious way of procreating at the expense of another species.) The female butterfly will lay her eggs (usually mid-June to early August) immediately after mating, choosing areas of low, freshly growing gorse or heather -- whose young shoots contain necessary nutrients for the feeding caterpillar -- where the ground temperature is slightly warmer than the surrounding terrain, depositing her intricately patterned ovum, one at a time, onto a sturdy stalk or stem: the egg will overwinter here with a fully formed larvae inside. However, the female butterfly has one last trick up her sleeve; she has not only chosen a balmy location, but a spot where the nests of black ants frequent. This is no fluke or haphazard occurrence, for it is these ants that act as guardians to the young silver-studded blue caterpillars -- a relationship that is not fully understood at this moment in time.
The egg will hatch in March, releasing a tiny caterpillar that is immediately found by the Lasius ant (either L.niger or L.alienus), presumably able to locate the larvae by detecting pheromones released during hatching. The ant takes the larvae back to the outer chambers of its nest where it is ministered to by many worker ants, carefully nourishing themselves on minute excretions of a fluid (thought to be a mixture of sugars and amino acids) excreted by the caterpillar through special organs evolved for this purpose: incredibly, more complex secretary organs develop with each skin moult (instar) of the caterpillar. Once capable of movement, the larvae will take night time sojourns outside the nest chamber to feed on tender shoots of ling and gorse, shadowed by their attentive bodyguard ants. Although receiving round-the-clock protection, the caterpillars are susceptible to attacks from the Ichneumonid and Brachonid wasps that will each attempt to lay its own eggs in the soft, gelatinous body tissue of the larvae, with around half to one-third of attacks successful. Infected caterpillars carry on as normal, forming a pupae (chrysalis) inside the black ant's chambers; only at the last moment will the young parasitic wasp emerge from the body of its host, exuding chemicals to protect itself -- and ease its escape -- from the fury of the incumbent ants: these chemicals can be deterrents, or even intoxicants inducing a state of drunken animosity amongst the ants, who will consequently pick fights amongst themselves. The unaffected caterpillars emerge as adult butterflies, where, damp and immobile, they climb out of the subterranean chambers followed by their servile attendants until they are capable of flight.
Another blue butterfly, the large blue (Maculinea arion) also has an extremely complex relationship with ants, although in this instance the ant genus is Myrmica. Intriguingly, the Lasius ants and Myrmica ants seem to only attend to the silver-studded blue or large blue butterflies respectively, suggesting a complex array of pheromones unique to each ant species is deployed by the butterflies: other ants of the Formica family will simply treat the larvae as meat, carrying them off to be butchered and eaten.
Clumps of heather are a favourite vantage point for male silver-studded blue butterflies wanting to take stock of their surroundings. Back in the day when heaths were abundant, lore has it they used to explode from the scrub by the hundreds.
Clambering steadily but surely up a stem of vegetation on a breezy July morning, a male silver-studded blue butterfly stands conspicuous in his bright blue and orange garb. Once your eye is in, they are surprisingly easy to locate.
Tender young heather shoots are a chosen food plant of the silver-studded blue butterfly, where the female will lay her eggs after mating. Here, a male perches on a dried stem of grass, possibly waiting for a passing female.
Lasius ants act as guardians to the young silver-studded blue caterpillars -- a relationship that is not fully understood at this moment in time.