How to tell Britain’s blue butterflies apart: Differences, histories and trademarks of species

SIGNIFICANT numbers of holly blue butterflies (Celastrina argiolus) were out and about in spring 2022, flitting around evergreen shrubs in parks and gardens. Holly blue is a beautiful little butterfly with the added advantage of being there for everyone to see in our parks and gardens, and unlike the closely related, declining and now somewhat perversely named common blue butterfly (Polyommatus icarus). This similar-looking species is traditionally associated with unimproved grassland on chalk down land and the plant species which thrive in these calcareous soils and situations. There are some nine native ‘blues’ found in Britain and most are closely associated with chalk down land and lowland heath, leaving the holly blue as the only one likely to be seen by most arborists during their everyday work.

The holly blue relies on two of the most ubiquitous and common native evergreens (holly and ivy) to successfully complete two broods and life cycles each year. However, more noticeable than ever this year was the interest apparently being shown in other evergreen garden shrubbery, including the large shiny leaves of cherry laurel (Prunus laurocerasus) and the aromatic leaves of Laurus nobilis (bay laurel). Despite the commonality of their vernacular, these ‘laurels’ are not related. L. nobilis is a true laurel (Lauraceae) while P. laurocerasus belongs to the plant family Rosaceae.
Dr Roderick Robinson, a colleague who lives in North Yorkshire, recently reported holly blues basking on cherry laurel, though this interest shown in evergreens other than larval food plants (holly and ivy) is nothing new. Richard South (Butterflies of the British Isles) quoted the Rev. Gilbert Rayner (May 18, 1901) who observed a female holly blue butterfly deposit a single egg on an unopened flower bud of rhododendron in his garden. Holly blues appear to check out other evergreens in their search for holly and ivy.

HOLLY BLUE

Holly blue, wings open, basking on ivy foliage. (Image: eA)

The upper wing surface of the male holly blue butterfly is blue tinged with lilac, displaying narrow black edging on the outer margin of the forewings and a narrow black line on the outer margin of the hind wings. The female is of the same shade of blue, or sometimes much paler. Underwing surface is whitish with sparse black specks – key to separating the holly blue from the common blue. The underwing of the common blue is quite different; grey-beige with orange-centred lunules (crescent-shaped markings).

Throughout history, Celastrina argiolus has carried a range of names. It was originally called the ‘Blue Speckt’, which changed to ‘Azure Blue’ in 1775, only to be changed again to ‘Holly Blue’. The exact timing of the last name change is unclear. Mid-19th century lepidopterists were still using ‘azure blue’, but by the end of the century most texts were freely interchanging the two names.

Be that as it may, even the name holly blue is not entirely accurate because this butterfly has two broods per year, each with a different food plant. The first (spring) brood feeds on Ilex aquifolium (English holly), while the late summer/early autumn brood typically hatches from eggs laid on Hedera helix (common ivy), so that the hatching, neonate larvae have a ready source of food in common ivy’s highly unseasonal flower buds.

The spring generation of holly blue butterflies emerging from the overwintering pupa stage lays whitish or bluish green coloured eggs on the undersides of the calyx (collective name for sepals) of holly flower buds. Hatching larvae feed on the flower buds and later the young green berries of this native broadleaf evergreen tree. The fully grown and developed larva, with a blackish head and yellowish green body, metamorphoses into a pale brown-coloured pupa. Adult butterflies emerge in late summer and early autumn when most wild plants have done with sexual reproduction for another year, while common ivy is just starting to flower. Come September, common ivy is covered with young, compact flower heads, and having selected one of these the female butterfly affixes an egg to the underside of the flower-head stalk. Larvae hatch and feed through autumn and metamorphose into the pupal stage in which Celastrina argiolus passes the winter.

Holly blue butterflies favour tall, stand-alone holly trees in spring and ivy-bound hedges and ivy-clad walls during the summer months. Nectar plants include holly, ivy, bramble, dogwood, snowberry, marjoram and forget-me-not. Holly blue was traditionally regarded as a south of England species, but more recently moved northwards to colonise parts of the Midlands and northern England, including Yorkshire.

UPS AND DOWNS OF THE ‘BLUE’ BUTTERFLIES

Changes in the circumstances of butterflies including climate, weather and land use are plain to see, but by no means new phenomena.

Despite its name, the common blue butterfly has generally struggled over the last 50 years, due to a depreciation of its favoured natural habitat. Chalk down land has become increasingly bereft of wild flowers, due to increasing intensity of livestock grazing.

Unlike the holly blue, the common blue is not generally seen in gardens, preferring unimproved flower-rich grassland, including that on southern chalk down land, lowland heath, woodland clearings and even sand dunes. A key requirement is bird’s foot trefoil (Lotus corniculatus), the main food plant, though larvae do feed on other leguminous plants including white clover.

You only have to read 19th-century texts to see that British butterflies have been experiencing ups and downs for centuries, which not infrequently end in a species being lost.

Such was the fate of another ‘blue’, the large blue butterfly (Phengaris arion), before it became extinct in 1979.  Surprisingly, this largest of the British blues was only discovered in 1795, though from the very start it was regarded as a very rare species. It was apparently rarer still 100 years later, though lepidopterists of the late 19th century were still expressing optimism after some findings in England’s West Country. 

Richard South (1906) described a ‘fine series of specimens’ found in West Cornwall, subsequently visited by an increasing number of lepidopterists with some ‘big bags’ made. Of course these guys were not clicking cameras but taking butterflies with nets, killing them in jars with chemicals and pinning them to a display board. It’s little wonder the large blue butterfly – and some other native species – became extinct in Britain.

Given historical pressures on butterflies from specimen hunters it is surprising the large blue lasted until 1979.

PARASITISM PLAYS A BIG PART

All that said, it was two related ecological shocks which delivered the final blow to this largest of Britain’s native blue butterflies. The first was decreasing frequency of wild thyme, its main food plant. The large blue butterfly had clearly co-evolved with wild thyme, which according to older texts provided an uncompromising level of camouflage for the larvae which fed upon it. However, more intriguingly still was the large blue’s peculiar-though-vital relationship with a particular species of red ant on which it relied as a source of food.

Having fed for a period of three weeks on the flower buds of wild thyme or marjoram, the larva produced ‘scents and songs’ which tricked the red ants into believing it was one of their own grubs and so was carried underground into the ants’ nest and placed with the brood. The larvae subsequently spent the following 10 months feeding on the ant grubs before pupating inside the nest the following year. When fully developed, the adult imago (butterfly) emerged from the pupa and crawled above ground.

Butterfly enthusiasts only had to wait until 1984 to see the large blue back in Britain, following the sterling efforts of conservation organisations who brought the species back from the dead using stock sourced from Sweden. Some two decades later, in 2006, the estimated number of adults flying in Britain was 10,000 across 11 sites, mostly in southwest England, and the largest number recorded in Britain for more than 60 years.

It was discovery in the 1970s of the details of the relationship between the large blue butterfly and a single species of red ant called Myrmica sabuleti which enabled conservation organisations to quickly re-establish the butterfly in Britain.

Common blue nectaring on ragwort flowers. Wings closed to show the intricate pattern on the underwing which distinguishes this ‘blue’ from the holly blue.

Parasitism on a red ant is essentially what determines success or failure of the large blue – perversely so, because it is generally the other way round. So it is with the holly blue in its perpetual duel with a tiny hymenopteran (wasp) parasite, which sends the holly blue into a cycle of ‘boom and bust’. An ichneumon wasp called Listrodromus nycthemerus inserts a single egg into the holly blue larvae using its long, syringe-like sting. Wasp grubs hatch inside the larva, feed on the body tissues and eventually kill the host. 

Lepidopterists had known about this boom-and-bust cycle for a long time without realising what was behind it. 19th-century butterfly texts frequently referred to sudden falls in population, but put it down to climate and weather. An increase in number followed by a rapid and significant fall is the classic effect of natural enemies, which are by definition density-dependent factors responding to changes in population size of the host insect. Natural enemies rise with an increase in host insect numbers and crescendo when the host insect population reaches a peak, only to fall away sharply as the host population falls due to the high level of parasitism.

A prime example is the period between 2010 and 2015. The population of the holly blue fell away sharply after 2010 as the butterfly became locked into its perpetual evolutionary struggle with the wasp parasite, but showed a 151 per cent recovery in 2015, compared with 2014, having escaped the shackles and restraints imposed by the wasp parasite.