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.

A white-letter day

It has not been much of a summer for our resident butterflies and the cool, often wet, conditions will have undoubtedly caused problems for many species. While a single poor summer, or even a short run of poor summers, may not prove catastrophic, it can exacerbate the impact of long-term declines brought about by other factors. The poor conditions tend to have an impact on butterfly survey work as well, limiting opportunities to get out into the countryside to locate populations of some of our more under-recorded or localised species. Three such species are on the wing now; the White Admiral, the Purple Hairstreak and the White-letter Hairstreak. Of these, it is the White-letter Hairstreak that we should, perhaps, be most concerned about.

This is a species that has undergone a significant decline since the 1970s, in part a reflection of its association with the various species of elm. The widespread outbreak of Dutch Elm Disease in the 1970s resulted in the loss of some 10 million elms, and with them, the removal of the favoured larval foodplants.  The spores of this fungal disease were carried by bark beetles and transferred from tree to tree as the beetles moved about to seek food or lay eggs. At the time, experts predicted that the loss of the elms would be extremely damaging, suggesting that the future of the White-letter Hairstreak hung in the balance. However, the predicted sudden and catastrophic loss of the butterfly did not happen, largely because the butterflies took to feeding on the regrowth brought about by the elm’s ability to sucker readily; they also exploited some of the more disease-resistant varieties of elm. As a result, colonies still cling on across a large part of central and eastern England albeit at a greatly reduced level.

Our understanding of how the White-letter Hairstreak population has changed comes from regular monitoring work, carried out annually across a series of sites through the UK Butterfly Monitoring Scheme. What the scheme doesn’t give us, however, is a clear picture of where the remaining colonies are located. This is where we, as amateur naturalists, come in. Over the next few weeks, the hairstreaks will be on the wing and can be spotted making their erratic spiralling flights around the tops of elm trees. Here they will feed on aphid honeydew. They will also descend to nectar on bramble or thistles, so it is worth staking out a group of elms with nectar sources below. You won’t see the dark uppersides of the wings because the butterfly perches wings closed, but you might just see the orange stripe and white ‘W’ on the underwing. If you do, make sure you let Butterfly Conservation know.