Ant Mounds on Lawns in Wisconsin

Identifying Field Ants

• Field ants range in size between 1⁄5 and 3/8 inches long, depending on species. Their color also varies from black to black and red. You may not notice the presence of field ants until you encounter their mounds, which are the above-ground portions of their colonies. Field ant mounds can be as large as 3 to 4 feet wide and more than 2 feet tall. Some homeowners mistake these mounds for imported fire ant (Solenopsis invicta) mounds, but fire ants are not commonly found in Wisconsin landscapes.

Damage Caused

• Not only are they ugly, but also field ant mounds prevent grass growth and can damage your lawn mower if you accidentally mow over them. Some species of field ants also inject formic acid into the roots of herbaceous and woody plants that shade their mounds to kill them. If you disturb field ants, they will bite. However, they will not come indoors. Field ants feed outdoor on live and dead insects and on honeydew secreted by sucking pests, such as aphids and mealybugs.

Cultural Control

• Many species of field ants are attracted to honeydew, a sweet substance produced by sucking pests such as aphids and mealybugs that can lead to the growth of sooty mold on a variety of plants. They're attracted by honeydew on trees and shrubs infested with these pests, so you may achieve some control by controlling aphids and mealybugs to eliminate honeydew. Rinse the foliage of your landscape plants in the mornings with a strong spray of water to remove the pests and their honeydew. In addition to removing food sources, look for and remove materials in your landscape that field ants might use for their nests, such as leave litter and plant debris on your lawn.

Chemical Control

• If field ants have mounds in your lawn, the only way to eradicate the colony is to kill the queen. In large mounds, though, the queen may be 2 to 3 feet underground, making surface insecticide treatments ineffective. Borax and hot water are also ineffective at controlling field ants. To kill the queen, rake away excess debris from the surface of the mounds and push a metal rod or wooden dowel as deep into the mound as possible. Make 4 to 6 channels and pour a liquid insecticide containing bifenthrin, carbaryl, cyfluthrin, deltamethrin, or permethrin into the channels. Cover the treated mound with the debris you removed.