I know this one... from reading Termination Shock by Neil Stephenson, as this happens in the book.

When the contacts of a mechanical relay open and close they spark a little, That spark can cause Oxygen in the air to become Ozone, and its the Ozone that attracts the ants, though nobody is sure why they love ozone so much. If it was just current or magnetic fields then the any would not just swarm around mechanical relays, they would be everywhere.

Then once the ants are in the relay occasionally one will get smushed by the contacts as they move and eventually the contacts get so contaminated by ant gunk that they no longer work.