The Stuff of Nightmares

Not this dried leaf but the extra photo - not for the squeamish.  The sawfly larva have struck big time.  Had bought some nematodes but am not convinced.  How can they possibly work quickly enough (if at all) to kill the caterpillars before they completely destroy all the leaves.  So am doing an experiment - cut off all (well probably didn't get them all) the leaves that had the larva on them and put them in a big bowl to see if the nematodes did their work before they could escape/eat the rest of the leaves.