How to Get Rid of Grease Ants (Thief Ants)

The grease ant (Solenopsis molesta), also called the thief ant, is a very common insect in homes across much of the U.S.—especially in kitchens. It is considerably smaller than some other common ants, at only 1/32 to 1/20 inch long. The body is yellow, bronze, or light brown and very shiny and smooth. Its eyes are very small in relation to the size of the head.

Like all ants, grease ants are social animals that establish colonies, either indoors or outdoors. Its tiny size allows grease ants to easily slip into homes through cracks and make nests in tiny crevices. These insects prefer to eat greasy substances (hence the common name), but will also eat meat, cheese, seeds, and dairy products. Outdoors, these ants often eat larvae and pupae of other ant species or even steal the foods collected by other ant species (hence the other common name thief ant).

Grease ants are not the only very tiny ants you might find in or around your house. There are many species that are of similar size, but most are dark brown or black in color. Grease ants are one of the few species that is yellow or very light brown in color. It is also one of the few that is not particularly fond of sweet substances, which creates some challenges for getting rid of them.

Grease Ant Biology and Behavior

Grease ant colonies are generally small with several hundred to a couple thousand workers. Even so, a single colony can have a number of queens. Because both the colony and the ants themselves are so small, they can establish themselves almost anywhere—even inside the colonies of other ants. The grease ant nest may be connected by tunnels to the nearby nests of other ants, providing access by which they are able to steal their food.

Outdoor nests may be under rocks, around foundations, in bare soil or decaying wood. Indoors, these ants may nest anywhere there is a small crack, especially behind baseboards, in wall voids, beneath floors, in cupboards, and similar spaces. It's quite common for outdoor grease ants to migrate indoors, usually in mid to late summer.

These are trail-setting insects. The worker ants travel far distances for food, then establish scent trails to that food. Indoors, these trails may flow along baseboards and in cabinets; outdoors, the thief ants often trail along the branches of trees or shrubs or even along electrical wires. Because this ant is so small, it can easily get into food packages to eat and contaminate the food.

3 Ways to Get Rid of Grease Ants

Bait Indoor Ants With Borax

For an effective home-made ant bait, mix ordinary household borax with a cooling oil in a shallow dish and set it out wherever you have noticed ants traveling. The worker ants will carry the mixture back to the nest, where other ants eat the borax and perish due to the effects on the digestive system—unable to digest food, the ants effectively starve to death. Over a period of a few weeks, the entire colony can be eradicated.

For most ant species, the method for using borax is to mix it sugar or another sweet material, but grease ants are not fond of sweets, and this method will be more effective if you blend the borax with cooking oil, lard, peanut butter, or grease.

Use a Commercial Ant Bait on Indoor Colonies

Grease ants found to be nesting indoors can be eliminated through baiting with commercial ant bait. But for greatest success with grease ants, the Iowa State University Department of Entomology recommends the use of chemical ant baits mixed with grease or oil:

1. Mix a chemical ant bait, such as Terro or Drax Ant Bait, with grease or oil, such as vegetable oil and peanut butter. This mixing will require a bit of trial and error as exact proportions have not been documented, but you will need to add enough grease to the bait so it is attractive to the ants, but not so much that it will dilute the active ingredient below the level of effectiveness.
2. Add a single drop of grease or oil to five to 10 drops of bait on a piece of wax paper. Other options can be used instead of wax paper, such as the backside of masking tape, small squares of cardboard or paper, or directly on the floor or surface where the ants are trailing.
3. Place the baited wax paper in the area in which ants have been seen. If the ants are not attracted to the bait, add another drop or two of grease, or try another oil.

Use Chemical Ant Killer on Outdoor Colonies

Neither homemade borax bait nor commercial baits are very effective on outdoor grease ant colonies. Here, the best strategy is to try to locate the colony, then spray it directly with a contact pesticide that is toxic to ants. To locate a nest, follow the trail of ants backward from the food source. It may take a very good soaking—or repeated applications of contact pesticide—to fully eradicate an outdoor colony.

Treatment is really only needed if the colony happens to be close to the house, where indoor migration is a real likelihood. Isolated outdoor colonies of grease ants harm no one, so there is usually no need for treatment.

Use contact pesticides selectively, as they will also kill any beneficial insects they touch, such as ladybugs or honeybees. If you prefer not to use toxic chemicals, opt for a pyrethrin-based pesticide. Pyrethrin is derived from natural substances in chrysanthemum flowers; it will kill ants on contact, but is not significantly harmful to pets and humans.

What Causes Grease Ants?

Grease ants are drawn indoors in search of warm temperatures and greasy, oily foods. Food spills in kitchens are extremely attractive to grease ants, which is why this room is where you usually find them. Colonies will very often be located close to the kitchen—below floors, in walls, or behind door moldings.

How to Prevent Grease Ants

Keeping your kitchen countertops and floors spotless is the best way to prevent grease ants. Clean up spills immediately, and wash floors and countertops regularly with a disinfecting cleaner. Wiping down surfaces with a household vinegar solution seems to repel grease ants.

Do not leave open foods out on the counter—cover them in pest-proof containers or put them in the refrigerator. Do not leave pet foods out overnight, or even during the day once mealtime is over. Finally, wash dishes immediately after eating—do not let dirty dishes sit in the sink.

To minimize the chances of outdoor ants migrating indoors, caulk or otherwise seals all potential entry points that can be found. Remember, though, that these tiny ants can find the smallest of cracks through which to enter.

Grease Ants vs. Pharaoh Ants

Because of the similarity in size and color, the grease ant is often confused with the pharaoh ant (Monomorium pharaonis). Pharaoh ants however tend to be slightly more reddish in color, with black markings on the abdomen. While it may be hard to see without a magnifying glass, Pharaoh ants also have three clubs on the antennae, not two, like the grease ant.

Pharaoh ants are more likely to nest outdoors, and they are very fond of sweet foods, unlike the grease ant, which greatly prefers oily, greasy foods.