Among the usual plethora of early spring phone calls and messages this year have been several inquiring about large ant mounds in prairies, pastures, CRP fields and roadsides. The ant mounds are not that unusual, though some callers have reported "hundreds" of mounds per acre and some startlingly-large mounds 2 feet high and up to 6 feet in diameter have been reported. The strangeness is in the number of calls and messages received, which has been more than the past few years put together.
I don't make anything special of this observation. Maybe there are more ant mounds this year, or maybe people who couldn't be in the fields planting because of the lousy weather were just spending more time looking. Several callers reported "discovering" the ant mounds after the existing vegetative debris was burned. For an example, see this article and photo from the Eastern Iowa Outdoors website.
There are dozens of species of ants that nest in the soil of lawns, pastures, prairies and fields in Iowa. Most are ecologically beneficial and do not require control. However, given lots of time, little disturbance, 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 prairie. 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 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 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.
Finally, I'm happy to pass along this link (and the link within the link) about ants in the prairie. See "The Density of Ants in Prairies" by Chris Helzer in The Prairie Ecologist blog, January 3, 2011.