Photo © Ilia Ustyantsev
The triangle (Heterogenea asella) is a species of moth of the family Limacodidae. It is found in most of the Palearctic realm. The wingspan is 15–20 mm. Adults are on wing from the end of May to mid July in one generation per year.
The larvae feed on Acer, Corylus, Fagus and Quercus species.
Source: Wikipedia
The primary larval foodplants are Beech (Fagus sylvatica), Pedunculate Oak (Quercus robur), poplars (Populus spp.), Silver Birch (Betula pendula), Small-leaved Lime (Tilia cordata) and Sweet Chestnut (Castanea sativa).