The baby sawfly larvae spotted recently on the goat willow (Salix caprea) have turned out to be Nematus pavidus, confirmed by John Grearson of Symphata: The Sawfly Forum (a very useful Internet discussion group for those interested in British sawflies). The picture above shows two mature examples on the window box.
N. pavidus does not appear to be at all common. There are no records on the Sussex Biodiversity Record Centre's database and none for Sussex on the National Biodiversity Network. Indeed there are only six 10km squares in Britain from which the species has been reported, including one that covers Lundy Island.
Although the caterpillars are very obvious, working out what they are was not easy and, as with many sawflies, the adults may not be much in evidence, so it is probably under-recorded rather than rare.
Whatever the case, they have turned many of the goat willow leaves into fretwork, though I am sure this will do no long term damage.