Individual leaves can be totally striped and whole plants can be left virtually leafless.
The caterpillar-like larvae of sawflies which are voracious feeders. There are also several generations produced in a single season from spring to autumn, so the damage can be devastating.
Plants can be seriously weakened or even killed.
The most crucial time is early spring, when sawfly damage on newly emerging plants will be the mosts severe. More mature plants are more able to withstand an attack. It is important to watch for signs of attack and take action at once.
Organic - Remove any larvae you see by hand. Check underneath the leaves as this is where sawflies usually lay their eggs.
Chemical - Spray with a contact insectiside like Scotts Bug Clear Gun.