Some people are finding field ants nesting in their lawns. Field ants are about 1/4 inch long and black (some species are red and black). Their nests are slightly raised and mound-like and can cover a fairly large area in the ground, up to two feet in diameter. It is not uncommon for people to confuse field ants with carpenter ants. Although these two ants are somewhat similar in size and color, carpenter ants do not nest in the soil. The prefer to nest in cavities in rotting wood or in voids found in buildings. Field ants are most active during the day while carpenter ants are most active at dusk, dawn, and during the night. Because of the mounds created by field ants, some people assume these ants are fire ants. Fire ants are about 1/8th inch long or a little larger. They only occur in the southeastern area of the U.S.; the closest fire ants to Minnesota are in southern Tennessee. Minnesota winters are too cold for fire ants to survive.
Unlike cornfield ants which are found in bare or thinning areas of lawns, field ant nests are found in healthy, vigorously growing turf. Despite the appearance of the excavated soil on top of the grass, field ant nests typically do not cause any lasting damage to the grass (just rake the soil periodically to prevent it from mounding up). Field ant activity may also make the turf look like it is being undermined from all of the tunneling but this is not likely – a very large nest would need to be present to cause that kind of damage. However, these nests can cause problems when mowing. The blades can become dulled when they strike the mound. Field ants can also be an issue because they can bite, especially when their nest is disturbed.
It may not always be necessary to treat field ants in your lawn. If the nest is not intrusive in its appearance or cause problems in the maintenance or enjoyment of your grass, then just ignore it. If the nest is troublesome, then your best bet is treat it with a granular insecticide labeled for ants found in turf (follow all label direction carefully). Some people want to try a non-chemical method and will pour hot water on the nest to kill it. However, this is not effective; at best you might get the ants to move to another area of the lawn. Others will try much more dangerous tactics like trying to burn and destroy the nest with gasoline or lighter fluid. Not only does this not destroy the nest but you risk causing injury to yourself and damage to your property in the process.