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Holly Blue
Behaviour and Ecology The general habits of this species perhaps more closely resemble those of the hairstreaks than of the other blues. Adult Holly Blues fly high in the tree canopy, where they feed on aphid honeydew, seemingly in preference to flowers. Individuals will also descend to the ground, to extract salts from patches of damp ground or mud. The inter-annual fluctuations in the numbers of this butterfly appear to have a roughly cyclical nature, with a periodicity of 6-7 years. This suggests that it may well be the abundance of the parasitic wasp Listrodomus nychemerus (which incidentally appears only to parasitise the Holly Blue) that drives the cycles in abundance. Identification This is the only species of blue likely to be encountered in most gardens, which is fortunate since the Holly Blue can superficially resemble a Common Blue in appearance. Females have dark black wingtips (lacking in Common Blue) and pale blue undersides. Second brood females are darker in appearance than those of the first brood. Males also have pale blue undersides but only have a very small amount of black on the upper wing tips. Fortunately, both sexes have narrow black bands that cut across the white edging of the forewings and this feature is very useful for identification purposes. The tendency to fly high around shrubs and tress is another useful feature, only rarely shown by other blues.
The use of gardens - results from Garden BirdWatch Holly Blues typically have two generations a year, though in common with some other butterflies northern populations may be single-brooded. The first adults emerge from the pupae in which they overwintered and are on the wing from the end of March (well before any other blues). The females lay their eggs below the flower buds of holly and these hatch within a week or two to produce small green caterpillars. The second brood is on the wing from July until late September and emerges from eggs laid on ivy. A number of other plants may also be used including Pyracantha, spindle and dogwood. It is the young from this second brood that overwinter as pupae. The caterpillars feed on the buds, leaves and berries of their favoured food plants and the switch from holly to ivy probably reflects the availability of buds and leaves at a suitable stage.
Populations in rural areas may have declined in recent decades with the loss of hedgerow trees and shrubs, something that may have been partially compensated for by the widespread establishment of holly within parks and gardens. This may explain the higher reporting rates for urban and suburban gardens than for rural ones.
Although the range of the Holly Blue has expanded in recent decades, its distribution remains quite scattered across northern England and into Ireland, with the core population in southeastern and central England. In Ireland, the species is most often found in deciduous woodland but it will enter rural gardens. Click here to see a map showing the distribution of Holly Blues within gardens at the national level, as recorded by BTO/CJ Garden BirdWatchers during 2003.
Click here to see what the region codes on the above graph mean. |
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