Last year, several apple trees that I’m growing as a hedge got infested with farmer ants. I suddenly discovered some trees that were filled to the brim with aphids. I sprayed with some aphid spray - which seemed to work killing the aphids, and I used an ant killer (comes in powder form) around the trunk. I also hosed off the trees. It was late in the year, and suddenly I didn’t have to worry about the pests again.
That is, until now! 
… Only one tree has been attacked notably yet. But, I know where this is going 
…
Now I’ve treated the aphids, hosed off the trees and I’ve mixed the antkiller in water and sprayed all the trees. It works like a charm to kill the ants. But, it’s just a matter of days, if not hours, until the guys show up again.
What can I do?
Also, to not having to pull weed and mow around the trees, the ground is covered with fiber sheet/mat/tarp (or what its called ). Could this be worsening the problem? Should I remove the fiber sheet? And if yes, what should I do to the ground to get as little of maintenance work as possible? 
…
PS: added an image of the little beasts, just to set the mood
