The caterpillar-like larvae of hazel sawfly (Craesus septentrionalis) feed on the leaves of various species of deciduous tree, such as alder, ash, birch and hazel. The larvae work there way inwards from the edges of leaves and when they are disturbed they cling to the leaf with their front legs and curve their bodies in an “S” shape, either to appear threatening or to emulate the leaf shape.
Mature larvae drop to the ground and pupate over winter in the soil. The adult sawflies have a black head/thorax and a brown abdomen.
A few years ago we had an infestation of hundreds of larvae on our hazel hedge in the garden. The hedge was completely defoliated but recovered completely the following year.
The three in the picture were nibbling on hazel at the south-west corner of the woods, where it fronts on to the water meadow. There were a few dozen of the larvae in this area but nothing like the hordes that stripped our garden hedge.