A photograph would be helpful, but are you talking about cuckoo spit? This looks like white, frothy spit and is caused by the nymph stage of froghoppers - the insect inside looks yellowish-green. If it is cuckoo spit, it's only a temporary nuisance and picking off by hand can control most cases of infestation.

No it is not cuckoo spit, a photo would be no good as it is so fine you would not see it, it looks just like pollen that you could blow away but on touching it, it is very sticky.

Then it might be box sucker - these attack new shoots in the spring as nymphs with flattened, pale green bodies up to 2mm long and sometimes cause a white waxy coating on the tips. They do only attack the new growth, so you can either clip it backonce they've hatched and gone (by midsummer) or have a look at pesticide treatments to see if there's a spray you can use. Usually though, this pest causes the new shoots to look stunted, with more closely spaced cupped leaves that resemble cabbages.