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 United States—especially in kitchens. These insects prefer to eat greasy substances, including fatty foods like meat, cheese, seeds, and dairy products. They also 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"). They do not bite humans, and their risk of spreading disease is low, although possible since they can be disease carriers and walk along food preparation surfaces.

Review these three methods to help you get rid of grease ants and keep them away for good.

What Does a Grease Ant Look Like?

Grease ants are considerably smaller than other common ants, at only 1/32 to 1/20 inch long. Its body is yellow, bronze, light brown, shiny, and smooth. Its eyes are tiny in relation to the size of the head. Their small size allows grease ants to easily slip into homes through cracks and make nests in tiny crevices.

Grease ants are not the only tiny ants you might find in or around your house. Other small species may be similarly sized but are usually dark brown or black. Grease ants are one of the few yellow or very light brown species.

Grease Ant Biology and Behavior

Like all ants, grease ants are social animals that establish indoor or outdoor colonies.

• Colony size: Grease ant colonies are generally small, with several hundred to a couple thousand workers. A single colony can have several queens. Because the colony and ants are small, they can establish themselves almost anywhere—even inside the colonies of other ants. Tunnels may connect the grease ant nest to the nearby nests of other ants to steal food.
• Food preference: Grease ants like greasy foods. They are not particularly fond of sweet substances, which creates some challenges in getting rid of them.
• Nest location: Outdoor nests may be under rocks, around foundations, 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 in similar spaces. It's common for outdoor grease ants to migrate indoors in mid to late summer.
• Trailing habit: 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 cabinets; outdoors, the thief ants often trail along the branches of trees, shrubs, or even along electrical wires. Because this ant is so tiny, it can quickly 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 homemade ant bait, mix ordinary household borax with cooking 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 few weeks, the entire colony can be eradicated.

For most ant species, the method for using borax is to mix it with 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 the greatest success with grease ants, the Iowa State University Department of Entomology recommends the use of chemicals and baits mixed with grease or oil:

1. Mix a chemical ant bait, such as Terro or Drax Ant Bait, with grease or oil like vegetable oil and peanut butter. This mixing will require a bit of trial and error as exact proportions have not been documented. 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 where 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 and 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 only needed if the colony is 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. Opt for a pyrethrin-based pesticide if you prefer not to use toxic chemicals. 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 for warmer temperatures and greasy, oily foods. Food spills in kitchens are extremely attractive to grease ants, so this room is where you usually find them. Colonies are often located close to the kitchen—below floors, in walls, or behind door moldings.

Grease ants may also gravitate toward garbage cans that aren't cleaned out frequently. Rotting wood and leaky pipes are other grease ant attractants, so address any leaks under sinks.

How to Prevent Grease Ants

Several simple steps can make your home unattractive to grease ants:

• Clean up: 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. Wipe down surfaces with a household vinegar solution to repel grease ants.
• Remove food sources: Do not leave open foods 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 after mealtime. Wash dishes immediately after eating; do not leave dirty dishes in the sink.
• Seal entrances: To minimize the chances of outdoor ants migrating indoors, caulk or otherwise seal all potential entry points that can be found. This might be difficult since these tiny ants can find the smallest cracks to enter, but it is worth trying.

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 are slightly more reddish, with black markings on the abdomen. It's hard to see without a magnifying glass, but pharaoh ants have three clubs on their antennae, not two, like the grease ant. Pharaoh ants are also more likely to nest outdoors. Pharoah ants are fond of sweet foods, unlike grease ants, which vastly prefer oily, greasy foods.

When to Call a Professional to Treat a Grease Ant Infestation

Consider contacting a pest control company when a grease ant infestation is overwhelming and you can't find the ant nest. Experts have access to poison or chemical controls that might be an effective option for your case. Using baits, especially if they are improperly placed or you have pets or children in the home, should be used carefully. A professional team can determine the safest option for your household.

When shopping for pest control, look for a local company specializing in integrated pest management (IPM).