Every year since I planted my beech hedge, woolly aphids (the white things on the leaf) have been sucking the sap out of it:
They don't do the plants a lot of good. On full grown trees there's sometimes so many aphids that their excreta falls off (honeydew) staining whatever happens to be underneath. The excreta is often eaten by a black mould.
Now chemical warfare against aphids is a fool's game. You spray, the aphids die. But so do the predators of the aphids (how would you fare on poisoned food?). So as there's nothing eating the aphids, as soon as the spraying stops their numbers explode. The chemical companies have you hooked
The alternative is ecological management. The main predator of aphids in my garden is the ladybird:
Unfortunately all but a tiny proportion die off in the winter, so there's not many to eat the woolly aphids the next spring.
Supposedly the ladybirds overwinter in the box, ready to feast on the first woolly aphids that emerge.
Well I'm glad to say since installing the box, woolly aphid numbers on my hedge have reduced enormously. There may be a factor totally unconnected to my ladybird box at play, only time will tell.