If you see ants in your lawn, it's possible that a colony is being established. It's much more likely, though, that they are members of an established colony, out and about foraging for food.
Different species of ants have different dietary requirements. Some feed upon sweet stuff, some like meats and grease, and still others will eat just about any food they find. All ants carry whatever food they find back to the nest to feed the rest of the colony.
Be a gracious host and offer your unwelcome guests a smorgasbord. That way, you won't need to ask them to identify themselves. You'll have something to please ant palates without resorting to harsh or dangerous chemicals.
These simple non-toxic foods are probably already in your kitchen cabinet, and will kill most ants in your grass quite effectively.
Spread out used coffee grounds on a cookie sheet. Cover them with a piece of wax paper and spread out the layer even thinner with your hands. Once finished, remove the wax paper. Now, set out the coffee grounds in the sun to dry, or air dry them indoors.
Scatter the dried used coffee grounds in the grass wherever you see ant activity. Not only is this good for killing the ants, but it's also good ground fertilizer. Earthworms will enjoy whatever's left over.
Collect and dry all of your used coffee grounds every day, and repeat the application at the end of the week.
Used coffee grounds aren't your only tool to help rid your lawn of ants. Combine about a handful each of dry cornmeal, wheat flour, instant grits, dried rice and instant cream of wheat together in a large mixing bowl. If you don't have all of them, use whichever that you have on hand. Any combination will work well.
Sprinkle the grain mixture anywhere in the grass where you've seen ants. This combination offers foods that appeal to all ants, no matter their normal diets. All of these foods work the same way -- the ants eat them, but can't digest them, meaning they die from severe internal compaction.
Reapply the grain mixture weekly as long as ant activity continues in your grass. Ants are highly successful insects and eradicating them can be extremely difficult. You may have to treat your grass multiple times to complete the job.