A. Fire ants are a pest problem from Texas to North Carolina to Tennessee and Oklahoma. In Texas they are the Red Imported Fire Ant. There are many publications devoted to this subject and the overwhelming suggested method of control is a two-step process.
The first step is using an insecticide formulated bait. There are many different types of bait. They are used for particular problem areas such as lawns, turf grass, orchards, poultry areas, corn, sod farms, nurseries, etc. The chemical used is what determines where the product may be used. In lawns, some of the trade names are Extinguish, Amdro Insecticide Bait, Logic, Award, Siege Fire Ant Bait and Pinpoint. These baits are spread around the mound, retrieved by worker ants, taken back into the mound and eaten by the Queen. They are 80-90% effective and take 2-6 weeks to accomplish their mission.
After the bait has had a chance to work, 2-3 weeks, proceed with step two. The second step is to drench the mound with a 2-3 gallon dilution mixture of an approved insecticide, either Diazinon or Pyrethrins. This step by itself is usually 60% effective.
I have had very good results reversing the steps. I drench the mound. The following day I knock the rest of the mound down with a water spray and clean the area. If the mound is not dead, they will rebuild and then I bait where the fresh dirt is showing. Always bait in the dusk as ants are more active at night.
As to the depression, take a sharp straight tool, edger, shovel or hatchet and outline the depression on 3 sides. Leave 3-4 inches outside the depression. Take a straight edged shovel and gently dig under the turf. Lay the flap back on the side that is not dug. Loosen the soil inside the flap and water thoroughly. Tamp the soil down moderately and add new soil to bring the level slightly higher than the surrounding turf. Water again and lay the flap back down. Tamp the flap and pin at the two corners with a nail. Keep the flaps watered and in week or 10 days it will be as good as new.