Disease resistant elm

We have precious elm saplings, resistant to Dutch elm disease, that will need to be planted out in the park soon.

Elm is the sole foodplant of the park’s rarest and most elusive butterfly, the white letter hairstreak, Satyrium w-album, a species which spends every stage of its whole life high in the top of elm trees.

The adult stage of white letter hairstreak butterflies

Dutch elm disease, in the second half of the 20th Century, killed millions of elm trees in Britain. As a result the white letter hairstreak’s population plummeted and there were fears that both species, tree and butterfly, would become extinct in Britain.

However, both the elm and the white letter hairstreak clung on. Elm saplings, their trunks still too narrow to win the attention of the elm bark beetles that spread the disease, lived for as long as a couple of decades in hedgerows before they succumbed. They were replaced by new seedlings and suckers, equally short-lived. The butterflies established their colonies in these young trees.

In the park, white letter hairstreaks have been seen in the hedge between Cornfield and Sleeper Field. There are young elm trees in this hedge, with vigorous new elm suckers as well as the skeletons of trees that have died from the disease. White letter hairstreaks are not great wanderers, they will reuse the same site year after year, so  we hope that there is a thriving colony high up in this corner of Sleeper Field. This is where we plan to plant the new disease resistant saplings.

The egg and caterpillar of the white letter hairstreak butterfly.

White letter hairstreaks overwinter at the egg stage of their life cycle. During the late summer, in their colony’s elm trees, the adult butterflies laid their eggs on twigs and small branches, on the scars between this year’s and  last year’s growth. The caterpillars are fully developed by now but will remain in the egg until next spring; they will hatch in time to feed on the tree’s flower buds.

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