Strip the willow

Today I found a sapling willow being systematically eaten by these Sawfly larvae. I'm fairly sure they are Croesus septentrionalis - nearby there were more mature larvae that matched all the photos I've seen of this species and were doing the 'waving thing' as a defense mechanism. However, the books are slightly confusing. C. septentrionalis is sometimes given the common name of Birch Sawfly although there is another species so named, Cimbex femoratus. Sometimes it is called the Hazel Sawfly though it is said to feed on a range of deciduous shrubs. In other descriptions it is said to feed on willow. Anyway...I am left confident that I've found Sawfly larvae, but will continue to explore the literature.