I think that would be over-reacting. Birch (like oak, hornbeam &c) are often attacked
en masse by sawflies, moths, bugs
but I have never seen even a single tree killed by them. It could happen, perhaps, with an isolated (young?) tree such as yours but I doubt it. I have often seen trees totally denuded early in the year - they simply make more growth later in the year (and probably more seed). I really wouldn't apply
any biocides - they might kill the sawflies but so what? You would also certainly be killing a lot of other insects, some of which might, eventually, have controlled the sawfly!
