I have a 5yr tree that drops almost all its fruit when they are only the size of a pea and everyone has a black spot on it at the bottom the size of a pin head. The ones left of which there arnt many dont have this spot. Ive read everywhere and it seems its the plum sawfly - am I right? If so the solution seems to be Derris but that is no longer available so what wld be the alternative? Ive also gone thro the soil at the base and cant find any of the grubs mentioned. Would it be any good putting a grease band round it?