If you have ants hills the cool weather is the time to put uncooked grits, cream of wheat or cornmeal on top of the mounds, they can not digest it.
someone else suggested 1/2 cup vinegar & lemon juice mixed in a bottle & poured on mound kills them in 30 minutes.
for spraying trees.............OMRI suggested
1 1/2 oz medina orange oil, 3 oz dawn liquid soap and 1 gallon of water and mix & spray
Fire ants are dangerous to pets & humans. If none of the above work for you, you need to call a professional. Best time to control them is in the cooler weather.