White Fly sucks up all the nutrients from ficus hedge. The bad thing is that White Fly injects a die back into the tree killing the leaf and branch. This is why a systemic insecticide application is the best way to deal with them. Systemic will go into the vascular system of the tree and kill anything that feeds off the tree it self. You must use this type of chemical with caution, if you have trees that are blooming and bees or other pollinators are around you must not use this chemical. Bees get the pollen from the tree and if the tree has a systemic insecticide it can also kill them.
I recommend that this type of work be done by professional pest control operators. For your safety as well as the environments.