I have 40 years of gardening experience, but I have never had this problem before. In fact, I have postponed dealing with it (try to be organic) because I couldn't believe that ants were really killing my 4 spirea over 5+ year period.

It started with rabbits. In the winter, rabbits chewed my spirea to the ground and then kept at the green shoots all summer. This stressed out the westernmost 45 year old spirea and part of its roots died. Carpenter ants then set up shop in the dead roots and a huge colony formed. Then the ants moved on the next spirea, and decimated that one. Today there are just a few sprigs of growth on these two spirea.

Now the third spirea is swarming with ants and I finally put it all together: the ants were killing the shrubs. The chemicals are now in action and the ants are dying. Looking back I can't believe I didn't put together what was happening.