The mountain ash sawfly attacks trees in   late June or early July.  Both  European and American mountain ash (Sorbus aucuparia and S. americana) are hosts.   The larvae feed on the leaves and devour all but the mid-ribs and larger veins.    The larval stage  resembles caterpillar and are greenish with black dots down the sides.  The head and legs become yellow-orange at maturity.  The adult sawfly is yellow with black spots and wasp-like in appearance.  They normally go through only one cycle per season so a healthy tree will recover and put out a new set of leaves.

If alarmed, the caterpillars stop feeding immediately and adopt a rigid, flexuous pose sticking out from the edges of the leaflets of the tree (click here for wider field)