The Holly Blue is a small to medium sized blue butterfly.  Its shade of lilac-blue is extremely pretty and eye catching.  It looks quite similar to the Common Blue from a distance but whereas the Common blue stays close to the ground on open grassland areas, the Holly blue generally flies at bush height  in more shrubby areas such as by trees in gardens.  The underside of the Holly Blue is very different to the Common Blue because it is silvery blue with black dots, unlike the Common Blue’s Beige with many orange lunules.  It is sometimes confused witH teh small blue because of the small black pots on its underside but the Small blue is much smaller and much much darker with only a few blue scales

 The Holly Blue usually perches with wings closed or partially open to about 90°.  It is never seen with its wings fully opened.  Male Holly Blues have wings which are plain blue on their upper side but females have noticeable black margins to their upper forewings and to a lesser degree, to their upper hindwings.  First brood females are less strongly marked than the second brood the forewing dark band being much narrower.  The adults nectar on flowers like buddleia and thistles and will also use nutrients from damp mud and animal dung.

While the butterfly will be found in the same locality year after year, it does not form discrete colonies and will wander across the landscape. This is a species which roams around looking for its foodplants.  The main hostplants are Holly Ilex aquifolium and Ivy Hedera helix.  Therefore the butterfly can be found in gardens, parks and woodland clearings.  It is thought that the species mainly uses Holly for the spring generation and Ivy for the summer but some individuals use other foodplants as well, including dogwoods, gorse, brambles and possibly both alder and  both common and alder  buckthorns.

Overall the Holly Blue is increasing its range north and  is doing really well.  In the past we have experienced invasions from the south in some warmer years, establishing semi-permanent populations, which then often disappear in colder years. The first breeding colonies appeared in Yorkshire in 1978 in Nidderdale and around York and survived but were single brooded but then larger invasions happened  in the 1990's which were double brooded.  The Holly Blue is the first of our ‘blues’ to show itself during the year, with the spring brood emerging from overwintering pupae at the start of April in good years, peaking in early May, and giving rise to a summer brood that emerges in the second half of July and that flies until early September. While the butterfly is normally doublebrooded, this does vary by region in Britain and Ireland - the most northern populations produce a single brood and, conversely, some southern regions manage to produce a third brood in favourable years, with sightings in October and, on the rare occasion, in November. 

 Yorkshire sightings are typically in ones and twos with occasional concentrations on prime sites like York Cemetery  and Haw Park near Wakefield.

 The Holly Blue has a range from North Africa to northern England and south Scandinavia and eastward across central Asia to Japan, and North and Central America.

In the north this species tends to only have one brood but in the south it can have up to three.  Adults emerge around the beginning of April to May and if there is a summer brood it emerges from the end of July and lasts until September.

 As its name suggests, Holly is a foodplant of the spring brood of the Holly Blue butterfly, whose larvae feed on developing flower buds. Larvae will also feed on developing berries on female plants, although eggs are occasionally laid on male trees that do not produce fruit and, in this case, larvae reach maturity by feeding on the terminal and tenderest leaves of the plant. An egg-laying Holly Blue will flutter around a plant that is growing in a sunny and sheltered situation, before settling on a flowerhead, curving her abdomen underneath an unopened bud and depositing a single white egg. She then flies off to repeat the process, often on the same bush. Eggs are quite easy to find on favoured plants, especially in good years when the number of eggs laid is relatively high. The larva starts to feed by burying its head deep into the bud on which the egg was laid, before scooping out the contents. There are four instars in total and the last instar is the easiest to find in the field, not simply because it is larger than previous instars, but because the feeding damage on developing flower buds is quite noticeable—the flower buds that have been eaten become discoloured and stand out from the others, making it very easy to scan a patch of foodplant in a short amount of time (and there may also be frass in the vicinity of the feeding damage). The buds that have been eaten also have a distinctive hole in one side where the larva has bored into the bud before scooping out the contents. Prior to pupation, the larva gradually turns a dull purple and starts to wander away from the flower buds to find a location to pupate, when its new colouring possibly offers better camouflage as it crawls over woody stems and branches. 

 The eggs hatch after about two weeks.  The larvae then develop through four instars and can appear in at least three forms; most are plain green but some have either purple dorsal and lateral stripes or pink dorsal and lateral stripes.  Larvae first eat the flower buds and later the insides of the developing fruit in which they leave a trail of holes.

One unusual feature of the species is that no-one has ever reported seeing the pupal stage in the wild; perhaps because the pupa are high up in trees or shrubs, or low down, possibly underground, where they may be looked after by ants.

The parasitic ichneumon wasp Listrodomus nycthemerus appears to play a major part in the butterfly's life cycle, causing the cyclical boom and bust pattern in its populations.  The wasp injects an egg into a first instar larva, which then develops inside the larva without killing its host until it emerges as a wasp from the pupal shell.  The Holly Blue spends winter as a pupa and it is assumed that wasp eggs laid on second or third brood larvae also overwinter inside the pupa, which is not killed until just before the wasp emerges the following year.  A few days before this event the pupa becomes discoloured in patches.

Since the butterfly arrived in the early 90's it has gone through the typical population cycles that are known in the south  The wasp builds up its population with that of the butterfly until the wasp is prolific enough to kill lots of the butterflies and reduce the wasp’s own numbers as a result.  The butterfly then breeds again in peace for a few years and build its numbers up again until the wasp catches up and the cycle starts again.

Photo above right  of a female laying an egg by Catherine Jones

Butterfly Conservation priority: Low

European status: Not threatened

Fully Protected under the Northern Ireland 1985 Wildlife Order

At least 35 aberrations have been described but it is unusual to see an aberration.

With thanks to The Butterflies of Yorkshire edited by Howard M Frost and Peter Eeles