On the Wing: the Holly Blue

Holly Blue Male

Of the ten resident species of “Blue” butterflies to be found in Britain only one is likely to be seen in suburban gardens, the Holly Blue Celastrina argiolus. The spring brood is just now emerging, and will be on the wing until early June. A second brood begins to appear in late July and will be seen until September.

Holly Blue male underside

Holly Blues are unlike all our other blues in that they are woodland specialists, whereas our other species are all grassland butterflies. Holly Blues get their name from the food plant selected by the spring brood, which is usually the developing fruits of holly Ilex spp. The summer brood targets flowers and developing fruits of Ivy. These are not the only food plants however, and gorse, Spindle, Dogwood, Snowberry, Bramble and even heathers may also be used by one or both broods.

Holly Blue egg

Eggs are laid singly at the base of flowers or young shoots, and hatch after two weeks into a typical green, slug-like lycaenid caterpillar. As with most of the blues the caterpillar has attractant glands which draw ants to tend and protect it from parasitic wasps and flies, although in Britain there may be a lack of arboreal ant species compared to other parts of its range.

When mature the caterpillar descends to the ground and pupates in the leaf litter. The pupa is attractive to ants as with many other lycaenids, and probably produces attractive pheromones or even stridulates. Although too faint for human ears, ants will use sound to communicate with each other and some other lycaenid caterpillars and pupae are known to co-opt this as part of their interaction with ants.

Holly Blue female. Note black wingtips

When they emerge adult Holly Blues fly high, and tend to feed on aphid honeydew at least as much as they do on nectar. Unlike many of Britain’s blues both male and female are similarly coloured other than the female having black edges and tips to her wings.

In Britain Holly Blues are mostly absent from Scotland except in the south, but they are spreading northwards in response to a warming climate. Populations fluctuate considerably however in response to weather and the boom and bust cycle of their main parasites.

This is the species of Blue most likely to be found in a typical suburban garden. Growing a female Holly Bush and having a flowering and fruiting Ivy is all that is required to have them breed in your garden, especially if there are hedges nearby.

Cherry Gall Azure, C. serotina, N. America

Outside Britain Holly Blues have a massive range, extending across Eurasia and into North America, as well as south into India. Other species of Celastrina are mostly in temperate zones, but one species Celastrina philippina extends as the specific name suggests into the Philippines, and others extend into Sri Lanka and Papua New Guinea.

Plain Hedge Blue C.lavendularis, Sri Lanka

(images from Wikipedia)