The term "field ant" can refer to several species of ants from the Formica genus that inhabit grasslands and sunny clearings in woodland areas. They are sometimes known as wood ants or mound ants. This is the most common outdoor ant, and it rarely needs to be considered a pest. More often, field ants are considered beneficial insects because they feed on other insects, including many true pests. Occasionally, though, large colonies of field ants can cause landscape damage.
Because several species are included in this category, the appearance of a field ant can vary considerably. It can be as small as 1/5 inch, or as long as 3/8 inch in length. Colors can include red, brown, black, or tan—and some species are two-toned. The larger species of field ants are sometimes confused with carpenter ants.
At worst, field ants are nuisance pests, such as when they spoil a smooth lawn by building mounds or when they disrupt picnics or outdoor events when the ants swarm in the fall to mate. If they get indoors at all, it's usually because they have been brought indoors in firewood that harbors ants. Once indoors, though, field ants will take the opportunity to feed on any food they find.
These ants can cause problems in a landscape if the mounds they build while excavating underground galleries become disfiguring to a lawn. In some regions, field ants have been known to build mounds that are several feet tall, though in most areas the mounds are no higher than grass level. This can create a lawn that is irregular underfoot and bumpy to mow, but for most people, this is really not much of a problem.
Slightly more serious is the fact that field ants will sometimes kill the plants immediately around the mounds by injecting them with an acid substance. This behavior aims to eliminate the shade over the mound, but the result can sometimes be a dead patch of lawn or a bare spot within a flower garden. It's at this point that you might want to consider some method of controlling the field ant colony.
2 Ways to Get Rid of Field Ants
In areas that are not landscaped, there is really no need to control field ants in any way, as they do no harm and are usually beneficial. But if large colonies cause landscape damage to a garden or lawn—or if large numbers of ants become a nuisance to your family—there are a couple of ways you can try to control them.
Saturate the Mound With Pesticide
The best method of control is direct nest application of pesticide. Complete saturation of the nest is necessary because the queen may be living 2 to 3 feet below ground, and it is critical that she is killed to eliminate the colony. If necessary, increase the amount of water in the mixed ratio so the insecticide can completely penetrate the mound.
The liquid insecticides effective against field ants include cyfluthrin, deltamethrin, bifenthrin, permethrin, or carbaryl (Sevin). Of these, carbaryl is the most toxic. The others (and any products ending in "thrin") are from a class of pesticides known as pyrethrins, derived from chrysanthemum flowers. These are comparatively safe pesticides, but be aware that they are still toxic to fish and aquatic life. Don't use pyrethrins around ponds or streams, or in a location where they can flow into storm drains.
To treat the ant colony, first, rake away all excess surface soil and debris from the mound. Use a metal rod or dowel to create four to six vertical channels as deep as possible into the colony around the entry point. After mixing the pesticide, pour it slowly into the channels, then cover the area with the debris you raked away.
Allow a few days for complete elimination, as some of the ants may have been out foraging and will return to the nest for several days.
Drench the Nest With Insecticidal Soap
For homeowners who prefer not to use chemicals of any kind, you may be able to disrupt the colony and force them to move elsewhere by repeatedly drenching the colony with a water/insecticidal soap mixture. Field ants are tough insects that are not killed by this treatment, but you may harass them into abandoning the nest and seeking a friendlier location. Don't be surprised, though, if that new colony is established just a few feet away.
What Causes Field Ants?
Most (but not all) species of field ants seem to prefer areas with lots of direct sun, especially in grassy areas. Dead plant material is often used as building material to thatch the above-ground mounds. Field ants feed primarily on aphid honeydew and other insects, so they are common in areas in and around agricultural fields, meadows, and sunny gardens where insects thrive. Most species of field ants avoid shady areas, though some species are forest dwellers.
How to Prevent Field Ants
Most field ants don't much care for shady areas, so landscaping in a manner that provides plenty of shade may help discourage them. Keeping your grass mowed to a long length that shades the ground may help. At the same time, field ants make use of dead plant material, so keeping your landscape clean and avoiding firewood stacks, may also help.
Finally, many species of field ants use the honeydew secretion of aphids as a primary food source. Keeping your garden free of aphids eliminates one of the main attractions for field ants.
Though it's rare for field ants to deliberately enter your home, it does help to seal and repair any gaps, cracks, or other entry points, including openings where pipes and wires enter the home, door and window frames, and tears or holes in screening.
Although usually not necessary for field ants, a residual insecticide labeled for ants also can be applied around the perimeter and up and along the foundation wall of the home. Also, treat around doorways and windows, and underneath the siding. Such treatments will prevent all types of ants and many other insects from gaining entry into your home.
Field Ants vs. Carpenter Ants
Larger species of field ants are sometimes mistaken for carpenter ants. Carpenter ants are the same size and color as some of the larger species of field ants (up to 3/8 inch long) and they have the same single waist segment. Both types will fly in swarms during their mating season. However, with field ants, you will notice two distinct curves to the thorax segment of the body. The thorax will appear uneven and bumpy. If the profile is straight and continuous, then you are seeing carpenter ants.
Field ants are exclusively outdoor dwellers, so if you find a colony in the ground that features the characteristic mound, you can be assured you seeing field ants—carpenter ants never nest in this manner.
Carpenter ants will always be found nesting in soft decaying wood, often in the wood framing of a home's walls. This is not the behavior of the field ant, though they will sometimes nest in dead stumps. If you find a nest in a stump or log, closely examine the ant to make a firm identification.
Do field ants bite?
Field ants do not have stingers, but they do have jaws whose bite can feel like a sting as they inject formic acid into the bite. More than one gardener has experienced a surprise when sitting down or kneeling on an ant mound while weeding the flower garden. However, these insects bite humans only as a defensive maneuver and do not deliberately seek to bite. A colony of field ants that has been undisturbed for some time can get quite large and populous, posing an ongoing problem for families where small children play in the yard.
Can I flood a field ant nest with water?
The common method of flooding an ant nest with water does little except to temporarily slow down the excavation of the underground galleries. These are insects that routinely deal with heavy rains and even floods, and they aren't deterred at all by a simple garden hose.
What are the benefits of field ants?
In non-landscaped areas, field ants are a meaningful predator of many other insects, including harmful pests. In some regions, field ants have been deliberately introduced to help control serious pests such as pine sawflies and tent caterpillars.
In grasslands and prairies, the extensive underground colonies can aid in aerating and loosening the soil. Field ants sometimes nest in tree stumps and fallen logs, helping them break down naturally.
Marsden, Christy. “Field Ants.” Wisconsin Horticulture.