Oak Processionary Thaumetopoea processionea
Wingspan 25-35 mm.
A central and southern European species, adults males of this moth occasionally appear as vagrants, usually in August and on the south coast. Since 2006, adventive populations have been discovered in west London (now considered to be established in several boroughs) and in a small area of Berkshire. It is also considered to be resident in the Channel Islands.
Both the English and scientific names refer to the behaviour of the larvae, whose habit of forming a long nose-to-tail procession is quite peculiar. It is a pest species, in Europe feeding in large numbers on oak (Quercus), causing severe defoliation of oak and creating a health hazard. Minute, severely urticating hairs on the dorsum of the larger larvae, can cause persistent or severe (occasionally life-threatening) symptoms, and are readily dispersed on local air currents.
The adults appear from late July to mid September and can be distinguished from related species by the pale basal area of the forewing.
More information can be found at http://www.forestresearch.gov.uk/fr/INFD-6URJCF. This species is covered by UK Plant Health legislation, under which it is illegal to knowingly keep, store or sell it.