The Holly Blue, Celastrina argiolus ssp. britanna,
is a small butterfly often overlooked, but that can be found regularly in most gardens and woodland areas of Hadlow Parish, usually seen singly as fleeting glimpses as it passes through, searching shrubs and trees for suitable habitat. Related to all the other “Blues”, it is however the only Blue with a caterpillar that feeds on shrubs, rather than herbaceous plants, and so tends to fly rather higher than most of the others. A small blue(-ish) butterfly with very pale blue specked undersides, flying mainly above waist height, almost always turns out to be a Holly Blue. The eggs are laid next to the flower buds of mainly Holly in the late spring, and then later in the year, Ivy, and the caterpillar feeds mainly on the developing fruit that follow.
The butterflies are roughly an inch, or only a little more, across, and as they fly past you get flashes of mid-blue from the upper surfaces of the wings, alternating with pale grey-blue from the lower surfaces, giving you an overall impression of a light blue butterfly. As you track the blue flashing across your garden the butterfly is fairly easy to follow despite its erratic flight patterns – but if the butterfly settles you may lose sight of it altogether as the mid-blue vanishes, and the pale grey of the underside of the wings is all that is left. You then have to look for a butterfly that hardly looks blue at all, and it is often impossible to pick up again until you startle it into flight.
In flight they may easily be mistaken for Common Blues at all times of year, and I am so often doing this, that I am more often initially mis-identifying than correctly identifying! However the only time you are likely to get close enough to photograph them is when they are at rest, when they look nothing like the Common Blue, as seen below!
In the photo above, the butterfly is nectaring from a Clematis vitis-alba flower, Old Man’s Beard. In my experience it is relatively unusual for these butterflies to feed from any flowers, although the UK Butterflies website mentions flower feeding from quite a wide range of various bushes and herbaceous plants.
If the butterflies ever stop long enough to open their wings while at rest at all, which may happen in hot sunny conditions, females may be seen to have broad dark, more or less black, borders to the upper surface of their front wings, even more so in the summer brood, while males only have chequered edges to theirs.
The Holly Blue does come down to ground level from time to time, as seen in this photo from Queensdown Warren (16.05.2017) where this particular butterfly is exploring the chalk downland vegetation. This butterfly is well-marked, and is showing the pattern of spots on the underside of both wings very well. The chequering on the edge of the fore-wings is just visible.
Found primarily in the southern half of the UK, the Holly Blue is common in gardens in Hadlow and the rest of Kent in many years. However populations do fluctuate massively due to parasitism by tiny wasps and in some years they may be in very small numbers generally, while at other times they seem almost to vanish from particular areas. I have seen them in Dene Park and other sections of woodland, along the larger hedgerows, and very regularly in my own garden, flitting across the back shrub border, particularly near to the dark and evergreen Viburnum tinus, which looks rather like Holly or Ivy foliage to me (and the Holly Blue?) – that same dark green and gloomy appearance,
The small white eggs are laid on the sides of the flower buds, on female plants of Holly in the spring and on Ivy plants in the summer. The butterfly is unique in the UK in using two different main caterpillar food plants sequentially over the year.
After the first caterpillar stage on Holly they pupate on or near the ground for 2 – 3 weeks and then the second generation, that will lay its eggs mainly on Ivy flowers appears, to fly through July and August. After the second caterpillar stage on Ivy they change into chrysalises that overwinter (again on, or close to, the ground mainly I think) into the spring when the first generation of adults appears in late March flying to the end of May, laying their eggs on Holly again.
Well, it is the only UK butterfly that switches from one caterpillar food plant from one season to another, and it is the only Blue that feeds off woody shrubs rather than herbaceous plants. Definitely unusual!
Predators and parasites
The parasitic wasp Listrodomus nycthemerus kills many caterpillars and can cause populations of the Holly Blue to fluctuate wildly year on year.