flies

You haven't said what sort of flies, but if you mean bluebottles or the housefly kind, work out what's attracting them. because something in the garden will be - cat poo, for instance?

Or did you mean fleas?

or mosquitos or midges?

It sounds more like you have a dead bird or animal somewhere, probably in a bush. They will lay their eggs in the carcass, the maggots will feed on the rotting flesh and then... Thousands of flies will emerge.

If this is the case, then unless you find the source of this breeding ground the whole cycle will start over and over again as a fly's lifespan is only a matter of days.