Adult ants and their eggs in the soil under flagstone.
Everywhere I look there's ants. Under every rock and climbing up and down every tree. I try to leave them alone if they are in the yard. And if I can't because they are driving us nuts on the patio or invading the house, I pour boiling water into the nest or cracks. You know I won't do toxins. The boiling water will kill the good fauna in the soil but it will re-populate very quickly and it won't add anything harmful to the soil or water table. You can't use it near your plant roots of course.