No idea, notsocapable, without a lot more information, or a photograph of the problem. Info required would be things like how long you've had the plant, in a pot or in the ground, in sun or shade or half and half, what precisely is happening to the leaves and when did it start, where you are in the UK, etc.

To continue with Bamboos comment I am not aware of any bug that normally eats the leaves of Japanese acers... as Bamboo says much more info needed. Bulba and I recently lost all of the leaves off ones of our acers due to windburn.

Thanks for the replies,someone with good eyesight has pointed out the aphids!! Any recommendations on what is best to use,I planted it a couple of months ago,looking forward to the next few years with it hopefully!

Sorry did you post a photo elsewhere? Aphids do not eat the foliage, they can and do suck the sap but they do not eat the leaves... You say you planted the acers a couple of months ago - where do you live in the UK, are the acers planted in full sun, part sun, shade, windy area, have you kept them well watered since you planted them? Sorry lots of questions but this is the only way we can give you a proper answer! Oh and what type of soil do you have?

Hi notsocapablebrown,
If possible, it would still be beneficial to send in a photo (even just to help relieve our curiosity). Then we'd be able to see any particular pattern or shape to the damage on the leaves - along with any discolouration.
All the best, Tim

The only problem I have had over the years with acers is slug and snail damage BUT only early in the season the only time I had aphids is when I was over feeding them.

Gg