By Digby Wood
The late Jack Green tells us in "The Butterflies of Worcestershire" that a blue butterfly seen before 20th May is usually a Holly Blue. The other candidates are the Common Blue and the Small Blue which appear at the end of May, but both are creatures of open grassland whereas the Holly Blue is essentially a woodland species invariably seen scudding over the contours of small trees and shrubs in woods and gardens. The females are searching for egg-laying sites and the males are searching for females!
Holy Blues are particularly attracted to the shiny surfaces of evergreens, especially Holly, the food plant of the spring brood caterpillars. When settled on holly leaves with their wings closed, their light silvery-blue underside exactly matches the reflection of the sky on the shiny holly leaves. The underside of the wing has a scattering of tiny black dots but none of the orange crescents that edge the underside of the Common Blue's wing. The upper side of the male's wings are bright blue, thinly edged with black and with an outer edging of white, whilst the female looks very becoming with a broad black edge, particularly on the tips of the forewings.
The males of many species of butterfly set up individual territories which they vigorously defend against all other males, but the Holly Blue males appear to be "patrollers", visiting all the likely-looking small trees and shrubs over a sizeable area. One summer a male with a badly damaged wing appeared in my garden; it was easy to spot in flight and to note that it visited us up to half a dozen times each day as it flew round its patrol circuit.
As the name implies, holly is the food plant of the spring brood caterpillar. The eggs are laid singly on the underside of the holly flower buds. They hatch in a couple of weeks and the caterpillars start to eat the contents of the bud through a neat round hole. As they grow they undergo several moults. One odd thing about the caterpillars of all the blue butterfly family is their special relationship with ants. The caterpillars have a variety of scent-emitting devices which attract the ants, which then collect a sweet, honeydew-like substance of secreted by the caterpillars. The ants return the favour by guarding the caterpillar against the many predatory insects which enjoy eating juicy young caterpillars. In my garden this duty is performed by the common black garden ant Lasius niger which swarms all over the holly trees.
Holly Blue males emerged early this year (30th March) and one week later, when the first females were on the wing, I was lucky enough to see the full courtship ritual of the species. The pair met in mid-air, there was a quick twirl (two seconds), down onto a leaf (one second), a second quick, mid-air twirl (two seconds), down on to a different leaf to couple (two seconds), a total of seven seconds in all! However they remained coupled for about 1½ hours before going their separate ways. The eggs are laid on holly to produce a second brood of butterflies in late July, and these in turn lay on ivy flower buds forming in late summer. The caterpillars pupate in the autumn and pass the winter as chrysalids to emerge as the bright blue butterflies of April.
One further curiosity about the Holly Blue is the way the numbers fluctuate from year to year. It appears that 1997 is the start of a period of plenty which may continue for three, four or even five years, but eventually the numbers will suddenly drop to near zero and very few will be seen for several years, only for the cycle of glut and famine to be repeated. The reasons for this behaviour are not really known, but it is believed to be largely caused by a build-up in the population of parasitic ichneumon wasps which sting and lay their eggs in the caterpillars. Of course, in killing the entire local population of Holly Blues the parasites also bring about their own demise, enabling re-colonisation by butterflies from the surrounding countryside to begin again.
If you have a nice summary spot in your garden, big enough to plant a Holly tree, there is a very good chance that it will attract Holly Blues into your garden. Try to grow female trees: hollies have their male and female flowers on separate trees. Eggs laid on male flowers have less chance of producing butterflies as the caterpillars have to find their way from the shrivelling male flowers up to the tender leaves growing on the tips of branches if they are to survive. Eggs laid on female flowers have no such problem as the developing berries provide ample food. Even the garden varieties of holly with variegated leaves will attract the butterflies, but beware of the varietal names: Silver Queen is male, and Golden King is female!
Emmet, AM and Heath, J (1990) The moths and butterflies of Great Britain & Ireland, Vol 7, part 1, The Butterflies. Harley Books.
Thomas J and Lewington, R (1991) The Butterflies of Britain & Ireland. Dorling Kindersley and The National Trust.
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