By James Duncan
Learning and Engagement Officer
The Holly Blue (Celastrina argiolus) is well adapted for life amongst us and is the one member of its family you're most likely to spot in your garden, often flitting around at head height. The reason for its visibility amongst our urban sprawl quite simply relates to its diet. Rather than a reliance on the vetches, clovers, heathers and trefoils that feed the other members of the Blue (Lycaenidae) family, the diminutive Holly Blue has a strong penchant for Holly and Ivy, amongst others such as Bramble, Dogwood, Gorse and Spindle. Depending on climatic conditions, they're typically a double (occasionally triple) brooded species, the first generation of adults usually timing their emergence with the advent of spring. They're one of the first butterflies you may spot that has overwintered in their pupal form and their appearance is always well ahead of the other Blues. In early spring this makes them hard to confuse, though as a point of recognition the females exhibit a striking broad black band on the upper forewing. This may not be particularly easy to spot for they have a habit of keeping the wings firmly closed when perched, gingerly opening them in bouts of weak sunshine. Fortunately both sexes have underwings distinctively punctuated with black dots, though the most frequent sight is that of a small silvery shape dancing high in the canopy. If anything, their behaviour seems more consistent with that of the Hairstreaks than the more typically low-flying Blues.
Upon hatching, from mid-spring, the first brood of caterpillars will generally be found munching vigorously on the flowering buds of Holly (Ilex aquifolium), leaving a mini trail of destruction in their wake as they scoop out the contents of each bud. After weeks of ceaseless eating these will eventually pupate and emerge as a fresh second adult brood from mid-summer. In an unusual departure the adults won't lay their eggs on Holly once more, but change their selection to Ivy (Hedera helix), where the caterpillars can take advantage of its later flowering time. In this sense, it really could be named the 'Holly & Ivy Blue', though admittedly this isn't quite as catchy. The adults err towards a number of food sources, displaying a real taste for honeydew over nectar. They do have a more unsavoury habit and may well be spotted on the ground, lapping up salts from carrion and animal waste, though they usually find the equivalent from damp muddy ground.
They're a widely distributed butterfly, predominantly in the lower half of Britain, though in some years they may appear to disappear altogether. This isn't as far-fetched as it sounds, for fundamentally they have. The reason for this is no fault of their own and is in fact due to another species entirely, for they have a symbiotic relationship with a parasitic ichneumon wasp known as Listrodomus nycthemerus. We tend to think of these relationships as having mutual benefit for both species, but the result may in fact be positive, negative or neutral for one of them. The entire life-cycle of the wasp is decisively intertwined with that of the Holly Blue. The female wasp will inject an egg directly into a Holly Blue larva using a modified sting known as an ovipositor (a tubular egg-laying structure). The caterpillars are then kept alive to provide a constant food source all the way through to pupation. Quite unsurprisingly, this will prove fatal and with few surviving adults their numbers will crash. With nothing to feed on, Listrodomus will eventually follow suit and Holly Blue numbers can begins to replenish. As a result, populations have a cyclical nature over a number of years, though surprisingly the general trend indicates not just a growing population, but increasing expansion northwards for the Holly Blue.
Holly Blue © Nigel Symington