For the latest Langford ID challange....can anyone tell me what made this hole in the base of a goat willow tree, Salix caprea? The hole is about 10cm above ground level and is about 8mm in diameter. They can appear singly, but often there will be a few in the base of each tree trunk.
The hole is made by a moth called the lunar hornet clearwing, or Sesia bembeciformis to give it it's scientific name. As it's name suggests, it has evolved to resemble a hornet as a form of defence and is bright yellow and black in colour, with a wingspan of c.30mm. The name clearwing refers to it's transparent wings and indeed it is a member of the clearwing family, of which there are 14 resident species in the UK.
The female lays her eggs singly on willow trunks, usually around a wound or area of damaged bark. On hatching, the larva bores into the trunk and feeds inside the trunk and upper roots, usually for two years before pupation. Just before the larva pupates, it bores a hole for the imago (adult) insect to exit once emerged from the pupa. It is this hole that you see in the picture. The damage to the bark in the picture below the hole is possibly woodpecker damage, where a bird has tried to get to the larva inside the trunk.
The imago insect is rarely seen, but look around the base of willow trees for freshly emerged moths early in the morning during their flight period in July and early August. Many willow tree species are utilised, including goat willow, grey willow and crack willow and they have also been reported from poplar species too. Their preferred habitat is damp or wet areas, with mature specimens of the foodplant, but they can also be found in drier areas too in presence of the foodplant. They are the most common and widespread of the UK clearwings, being found through most of England, Wales and Scotland north to Caithness.