A pest of Hawthorn
This photo (and there are a few more on the Flickr stream) is of a Hawthorn bush that was infested with caterpillars of a moth from the family Yponeumetidae. Commonly called Ermine moths there are hundreds of species, and many of them form communal webs. When this photo was taken the caterpillars had already pupated.
The best guess at species is Yponomeuta padella.