The GARDENA gardening expert
Dear Ms. R.-W., these are lawn ants (Tetramorium caespitum), and can become extremely annoying, in view of the fact that you can have as many as 80,000 ants in one colony. They can be effectively combatted with a combination of insect spray and insectcide bait such as Blattanex Ameisenmittel (Anti-ant agent), David Ameisenfrei (Ant-free agent) or Loxiran® -S- AmeisenMittel; these are all so-called “sprinkle and pouring agents”.
Purely ingestive poisons (without the use of sprays) would be preparations such as Aeroxon Ameisen-Köderdose (Ant bait tin), Blattanex Ameisenköderdose, Delicia Ameisen-Köder (Ant bait), Detia® Ameisen Köder, Detia Ungeziefer Köderdose (Vermin bait) NEW, Fourmidor®, and Loxiran® AmeisenKöderdose.
It is best if you obtain advice on the abovementioned preparations from a plant protection consultant in a garden centre. According to my knowledge, all of these are currently authorised for use in home gardens and small gardens.