Despite the common name, the cornfield ant (Lasius americanus, formerly Lasius alienus), also known as the corn root ant, is more often found nesting in lawns; under stones, bricks, and sidewalks; and in or under rotting wood, bark, or logs. It is a very common outdoor ant, and you'll be hard-pressed to distinguish it from some of the other common species. It is called the cornfield ant because of its habit of fostering the corn root aphid and transferring it from grasses to growing corn.
Beyond this, the species resembles many other types of ant. It is 1/10 to 1/8 inch long and is usually light brown or reddish in color. It tends to seek out moisture, including that of damaged and decaying wood; it is found across much of North America, except for the extreme South and the Southwest.
Cornfield Ant Behavior and Biology
Cornfield ants, like many other species of ants, have a symbiotic relationship with a variety of garden aphids. The ants protect and even nurture aphid eggs, then feed on the honeydew excretions that the adult aphids produce as they feed on the sap of plants. The ants essentially "farm" the aphids for their own benefit. This fostering of plant-eating aphids is the single danger to humans posed by these ants, as a garden overrun with aphids can become badly affected.
Although cornfield ants may venture into a home in search of sweet foods and moisture, they do little damage there, since they are not even inclined to nest indoors, except on rare occasions when they adopt empty spaces carved out by carpenter ants. You may want to take measures to control these ants if they are a problem indoors, or if aphids are badly damaging your garden, but otherwise, they can be left to exist in peace. Like the field ant, this ant feeds primarily on the honeydew of aphids, nectar, and seeds, as well as on live or dead insects.
Cornfield ants have the same basic biology and lifecycle as other ant species. They are colonizing insects in which individuals specialize for different functions—one or more egg-laying queens, and various classes of worker ants. The species follows the familiar breeding cycle common to all ants—egg, larva, pupa, adult. Its colonies are usually underground— cornfield ants generally build their nests in the exposed ground, identified by a crater-like ring of excavated earth surrounding the entry hole. A neglected lawn with bare spots may well show evidence of cornfield ant colonies.
3 Ways to Get Rid of Cornfield Ants
The methods for getting rid of cornfield ants are the same conventional strategies used to combat all ants.
Use Ant Bait
If cornfield ants are a recurring problem indoors, then it makes sense to try and attack the nest by setting out bait that will be carried by workers back to the colony, where the entire population will eat it and die. The use of a commercial bait, or borax plus peanut butter and honey, is often very effective. Other sugary baits can also be effective for this ant. It can take several weeks for the bait to do its work on the entire colony, so be patient.
Baits can also be the most effective strategy to address garden ants that are fostering aphid populations. A mixture of borax and powdered sugar sprinkled over the top of the outdoor ant mound will soon bring the ant population to an end.
Though borax is a natural substance rather than a manmade one, it should be kept away from children and pets. Dogs can be especially fond of the sweet bait, and eating borax can cause eye, skin, and respiratory system irritation. The danger is even more severe if you use boric acid, which is a refined form of borax.
Use a Contact Spray
Since indoor cornfield ants are rarely nesting indoors, you may want to simply attack the ants when you see them with an ordinary contact pesticide that kills ants on contact. Be careful with the use of these products, especially on countertops and around food preparation areas.
Pesticides containing permethrin are a common choice for ants. This is a relatively safe pesticide, also used in topical medications to treat head and body lice. Pyrethrin-based pesticides can also be good choices to kill ants on contact. Pyrethrin is an extract from chrysanthemum flowers, and it is effective on many insects.
Use a Residual Insecticide on the Perimeter
If the nest cannot be found, and the ants continue to come inside, a perimeter treatment of the home with a residual insecticide, labeled for this type of ant, can help to keep them out. However, this is likely to provide only temporary control, with the correction of moisture problems as the key to long-term ant control and elimination from the home.
What Causes Cornfield Ants?
Cornfield ants generally come indoors because they sense moisture. Like all ants, cornfield ants are trailblazers, and once they find dependable sources of food—especially sweet substances—they will set scent trails to allow others to continue marching from the nest (usually outdoors) through cracks and crevices to the indoor food source.
Exterminators report that the damp walls behind the bathroom and kitchen are often where cornfield ants will enter a home. Fully waterproofing these walls is a good idea.
How to Prevent Cornfield Ants
Eliminating sources of moisture, keeping floors and countertops spotlessly clean, and blocking all cracks, holes, and crevices in your foundation and exterior wall, is the best way to keep cornfield ants (and all other ants species) out of your home. Once expelled, a perimeter application of residual pesticide will keep ants from successfully broaching your walls.
Outdoors, keep foundation plantings set well back from the outer walls, and keep the ground free of debris that can hide ground colonies. If you find colonies near the home foundation, use ant bait on them quickly before the ants discover a route indoors.
In the garden, frequently inspect for aphids and take measures to control them. Without the tasty honeydew that the aphids produce, cornfield ants are likely to go elsewhere.
Corn Ants vs. Carpenter Ants
The cornfield ant is sometimes confused with the carpenter ant because both species are sometimes found in moist wood within walls. But while the carpenter ant is actively chewing the wood to create galleries for nesting, the cornfield ant is there just to take advantage of the moisture. If you identify the ants in your walls as cornfield ants, it's a less serious problem.
Cornfield ants are generally light to dark brown, with eyes that are relatively large for the head. Worker ants will be 1/10 to 1/8 inch in size. Carpenter ants are usually dark in color, with workers ranging from 1/8 to 1/2 inch in length.
If cornfield ants are found in the walls, it is likely to be indicative of a moisture problem, and the source should be found and corrected. Carpenter ants, on the other hand, signify you have a structural problem with rotting wood.
Do Cornfield Ants Bite or Sting?
This ant may bite, but generally these bites cause very mild reactions.
Do Cornfield Ants Fly?
In August and September, reproductive adults emerge from burrows and take flight to seek mates. This mating period lasts for a few weeks, then the males die as winter approaches while the females return underground to lay eggs. In late summer, you may well see flying cornfield ants around your garden.
What Damage Is Caused by Cornfield Ants?
There's very little actual damage caused by cornfield ants, especially when compared to the active destruction caused by carpenter ants. The nesting of cornfield ants sometimes causes small "craters" in the lawn, garden, or planter boxes. Brick paver patios or driveways can sag if ants colonize below and excavate soil to create a nest. And occasionally, cornfield ants can exaggerate the damage to wood that has already been infiltrated by carpenter ants. They do not, however, eat wood, but instead only inhabit galleries already carved by other insects.
"Borates, Tetra, Sodium Salts (Anhydrous)." The National Institute For Occupational Safety And Health (NIOSH), Centers For Disease Control And Protection, 2019.
Toynton, K.; Luukinen, B.; Buhl, K.; Stone, D. "Permethrin General Fact Sheet." National Pesticide Information Center, Oregon State University Extension Services, 2009.
Bond, C.; Buhl, K.; Stone, D. "Pyrethrins General Fact Sheet." National Pesticide Information Center, Oregon State University Extension Services, 2014.