Satyrium w-album (Knoch, 1782)

Host plants:
The larva feeds on Ulmus species, very often Ulmus glabra.

Habitat:
The habitat is diverse and determined by the larval host plant. It ranges from individual trees in the landscape (also within human settlements) over bushy grasslands on the Swabian Alb with Ulmus glabra to woodland edges, ravine forests and other woodlands.

Life cycle:
The egg or better the caterpillar overwinters in the egg shell. It is hatching between late February and early April. The young larva drills into a growing flower bud as it is the case in similar species like S. pruni. The larva changes to leaves usually not before the final (L4) instar, but facultative also earlier. The caterpillar is mature in May and pupates between leaves, on bark or trunks, presumably also in the litter under the trees. The butterflies fly from mid-June to mid-July, more rarely until mid-August. They sometimes visit flowers like Mentha or Sambucus ebulus in woodland edges etc.

Endangerment factors:
Satyrium w-album was originally one of the most common and widespread hairstreak species. But it has extremely declined due to the elm disease. In areas where the elms are still represented better, it is still quite common because just a few trees or even a single tree suffice for reasonably stable, small populations. In addition, Satyrium w-album is threatened by spruce monocultures.

Due to the recovery of elms e.g. in southern Germany in the last years, Satyrium w-album is spreading again.

Remarks:
Butterfly observations are rare, for example, on nectar plants like Sambucus ebulus. Satyrium w-album can more easily be detected by searching for the eggs in winter, as it is the case with Satyrium spini, S. ilicis or Quercusia quercus. Eggs of Satyrium w-album can be found by scanning the topside of the lowest (they are reachable, the higher branches - usually unreachable - also host eggs) branches of trees with flower buds. Most eggs are observed at the base of terminal leaf buds (hardly of flower buds) on twigs with flower buds. Alternatively, the fully-grown caterpillars are quite conspicuous on the leaf undersides of the lowest branches in May.

The overall distribution ranges from northern Spain across much of Europe and the belt of the deciduous forests in temperate Asia to Japan.