Why are wasps eating my fence ?

is your fence wood because different species use different materials to build their nests (e.g. mud, wood...)

teh g0d; Weigh it up; How much damage are they Really doing / likely to do to that fence? Will they destroy the fence to the point where ye have to replace it? If so? How much will that cost ye? Only, a local, professional Pest Controller could probably drop round and, if he deemed it necessary and worth while, spray some heavy duty, residual insecticide onto the fence and end the problem there and then. I'm a pro'. Without seeing the situation for myself? I reckon I'd cost ye under £50. Unfortunately, I'm in Eire and don't do call outs beyond my own locale. Ask a few small, local outfits for an all in quote. See what they say and you think.

They might have made a nest inside the wooden pole. If its brick cement they can find holes in that too. If its metal then no they found something else.

They might have also found some huge grubs inside your fence and they are digging it out. The reasons go on and on.

They could be carpenter bees. which make holes in wood.