It has been a busy spring for phone calls and messages inquiring about large ant mounds in lawns, landscapes, prairies, pastures, CRP fields and roadsides. It has been that kind of year and the fall, winter and spring weather have created conditions to help ants prosper.
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. 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.
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). If necessary, 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 instructions on the insecticide label.
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.