There are dozens of species of ants that nest in the soil of lawns, pastures, gardens, and fields in Iowa. Most species are beneficial and do not require control. However, if left undisturbed, and with good soil and cooperative weather, ants may bring enough soil from their expanding, underground colony to construct a small hill in the lawn or a truly impressive mound in the garden or pasture. These ant mounds may become a nuisance.
Types of Ants Found in Lawns & Pastures
The ants that nest in the ground are called field ants. Many of these are “the other” big black ants. The public often assumes that “big black ants” are carpenter ants. However, carpenter ants do not nest in the soil. Instead, they nest in moist, decayed wood such as hollow trees and rotted window sills. Not all field ants are large and black; there are many small, lighter-colored field ants as well.
The soil-nesting ants found in Iowa are not biting or stinging pests (though some will bite in defense if handled carelessly). The fire ants of the southern U.S., well known for their aggressive behavior and painful stings, are not present in Iowa.
How to Manage Ant Mounds
In Lawns & Landscapes
When anthills in the lawn appear above the grass tops the mound can be raked or “washed” flat as needed (use a forceful stream of water from the garden hose to disperse the soil on a regular basis). The need for such maintenance will be greatest during periods of prolific ant nesting activity such as during periods of wet spring weather. If necessary, as a last resort, you can spot-treat anthills in the lawn with an insecticide such as Sevin, bifenthrin, permethrin, cyfluthrin, or esfenvalerate. Rake the anthill flat and sprinkle granules onto the soil surface or drench the mound area with diluted solution. Read and carefully follow the instructions on the insecticide label.
In Vegetable Gardens
The other time ants seem problematic is when a colony digs and tunnels around vegetable transplants. Ants do not feed on the plant roots, but they may excavate so much soil from next to the plant that the roots are exposed to air and dry out, thus stunting or killing the transplant. If ants threaten your transplants and the plant is still healthy enough to recover, you can drench the soil around the transplant with a garden insecticide solution. There is no need to treat the entire garden.
In Pastures, Fields, & Priaires
Ant mounds in the prairie or pasture can be ignored if possible. In pastures and CRP fields, tillage can be used to disperse the soil. For larger mounds, a front-end loader can be used to flatten the mounds ahead of tillage. Insecticides for ant control in pastures and prairies are not recommended.