[Sephiroth]

I think you'd need to shine a torch into the drop point and see if water's lying there or if the ants have indeed chewed through/into the cable. Aren't ants filled with formic acid or something?

In the first instance, though, you're looking for corrosion, IMO.

Also kill them with that ant spray you get in Sainsbury's. They can't be doing any good!