Those ants in your lawn – let them be

Carl F. Hoffman
Many homeowners have been reporting large numbers of ant colonies in their lawns again this summer. Although there is considerable concern regarding their presence, they are probably causing no damage to the lawn and can be ignored.

Cornfield ants are commonly found in lawns where they construct a small mound that is found in exposed areas in the soil as well as in the cracks of sidewalks, patio bricks and driveways. Many mounds may be present in the lawn, especially in places where the grass is thinning or the soil is bare. Although their appearance can be unsightly, these ants are not harming the turf. They are merely taking advantage of favorable nesting sites provided by the bare soil. A thick, healthy lawn will discourage ants.

It is not practical to control cornfield ants in a lawn, and the homeowners should tolerate nests that are present. If control is attempted, try to eliminate ants only from small areas in the lawn, sidewalk, patio or other areas where they are most conspicuous. Seed or otherwise encourage grass to grow in areas where it is thinning to help discourage ants from establishing nests as well as to make nests less visible. Cornfield ant mounds found in the gardens can be ignored. These ants do not harm garden plants.

Field ants, sometimes called thatching ants, are a second species that may be found in the lawns. Field ant workers are up to 1/4 inch in size and are either black or black and red in color. Unlike the small mounds constructed by cornfield ants, the field ants build a crater-like nest that may be a foot wide or larger. Field ants do not need bare areas to establish their nests, and they are often found where grass is growing normally and even in tall unmown grass. Although they certainly are more of a nuisance than cornfield ants when found in the lawn, the field ant nests are not common in turf.

Jeff Hahn, University of Minnesota assistant extension entomologist, recommends that if it is necessary to control an ant nest in the lawn, first try pouring soapy water into the nest. Mix about a half-cup of any type of laundry or dish soap with a gallon of water to make a sudsy mixture, and treat the nests as needed. If soapy water does not give the desired control, try an insecticide with active ingredients of acephate (Orthene), carbaryl (Sevin) or diazinon. The chemical can be applied as either a granule or a liquid mixture with water. Be sure the product is labeled for turf or ground application. Avoid broadcasting fertilizer and weed and feed products that contain diazinon over the entire lawn as this type of application will kill desirable earthworms and other invertebrates in the soil.