Autumn Webworm Hyphantria cunea
An example of this species, a male, was attracted to light in a Hertfordshire garden in September 2014 and constitutes the first British record, though another found near Cambridge in May of the same year wasn't identified until October.
The species is native to America but has been introduced to Europe where it can be frequent in places. The Hertfordshire record may be an immigrant or could have been accidentally imported.
The larvae feed in communal webs (hence the name 'Webworm') on a range of deciduous trees and can be pests in orchards where they occur.
In their native haunts, the adults are variable, with some having blackish or brown markings, others almost completely white.