Thursday, 19 August 2010

A silvery tree-top dance

A friend recently told me of a White-letter Hairstreak butterfly that had appeared in her Thetford garden. The butterfly, which spent a few minutes on some climbing beans, soon disappeared but the proximity of the garden to a site where the butterfly had once occurred hinted at its origins. A trip to the site in near perfect weather revealed the presence of at least one other individual and provided welcome news that the colony is still in existence.

The chances are that you may never of heard of the White-letter Hairstreak. It is one of our more enigmatic butterfly species, spending most of its life in and around the canopy of an elm tree. Given the tremendous damage that Dutch Elm Disease did to our hedgerow elms, together with the butterfly’s lifestyle, it is easy to see why this species is so rarely seen. It seems certain that there are many White-letter Hairstreak colonies that go unrecorded and even butterfly enthusiasts tend to just visit the best-known local colonies.

This butterfly derives its name from the white hairline markings set against the pale brown underwings. The upperside of the wings is almost never seen, not least because the butterfly sits with its wings folded. Watching the White-letter Hairstreak is best done with a pair of binoculars and, when in active flight, the silvery form of the butterfly can be seen to tumble and circle its way around the treetops. Rarely moving any distance from the elms with which it associates, your chances of getting a close view are not good. There are some places and times where White-letter Hairstreaks descend from the treetops to nectar on thistles or other flowers but they only seem to do this when honeydew is in short supply. Honeydew is the sugary substance produced by feeding aphids; it collects on the surface of leaves and this means that butterfly does not normally have to leave the tree in order to feed. In fact, the entire lifecycle is acted out at canopy level. Courtship, mating and egg-laying all take place high off the ground, with the resulting caterpillars feeding, growing and then pupating within the elm’s luxuriant growth.

Despite the huge impact that disease had on the elm population, the hairstreak has survived well in many of its former haunts. One reason for this may be that the species of elm preferred by this butterfly (the Wych Elm) was less severely affected by Dutch Elm Disease than other species of elm. An additional factor may well be the changing climate, the warmer conditions allowing this butterfly to expand its range northwards. East Anglia was one of the former strongholds so it is worth checking out your local elms.

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