Argentine Ants. Why They Keep Coming and How to Stop Them

Argentine ants (Linepithema humile) are 1/8 inch and dull brown. UC IPM calls them the most common pest ant in and around California homes and gardens.¹ Other small ants look close. Odorous house ants are 1/8 inch, dark brown to shiny black, and smell strongly when crushed. Pavement ants are 3/16 inch.¹

Argentine ants are known for scale. Colonies with several nests and queens can cover a large area because the ants show little aggression to each other.² Back to the ant hub for the rest.

Argentine ant colonies sit shallow, just below the soil surface, unless conditions are dry.¹ A substantial portion of colonies exist below ground.²

Colonies link into one supercolony with multiple queens. Newly mated queens leave with workers to start new nests.¹ Most other ants start from one queen.¹

This is why killing the ants you see does little. The queens sit in the nest, and workers share food with nestmates, which carries bait to the queens.³

• Trails with thousands of ants running from nests to food.¹
• Ants along wires and pipes, which they prefer to travel on.¹
• Ants swarming over sweets, pet food and spills indoors.¹
• Aphids, mealybugs or scales on plants with ants walking among them.³

Entry often means a crack, a window frame or a pipe gap. Wipe trails with soapy water to remove the scent trail and caulk the entry.¹

Argentine ants are mostly a household nuisance. They enter homes for food and water.² The sources we used list no sting or disease threat from them. UC IPM names the velvety tree ant as an aggressive biter, which is a different ant.¹

The bigger damage is outdoors. They disrupt native ants and food webs. Parasitism of California red scale was reduced two- to five-fold when Argentine ants were present.² They tend aphids, mealybugs and soft scales for honeydew and drive off the predators of those pests.³

Baits are the tool. UC IPM calls them the only type of insecticide recommended in most situations. They must be slow-acting so workers can carry them back.¹

For severe Argentine ant infestations, use refillable stations with 0.5 to 1 percent borate in sugar water. Prepackaged stations have about 5.4 percent borate and are weaker.¹ Stay at or under 1 percent, since stronger borate repels ants.³ Argentine ants take sweet baits year-round and protein baits mainly in spring. If the bait does not suit the ants, it provides no control.^1,3

Place one refillable dispenser on each side of the house in shady spots where ants trail, and space outdoor baits every 10 to 20 feet. Expect 5 to 10 days before ants drop and weeks for control.¹ Spraying indoors will not stop more from coming in.¹ Broadcast spraying has limited value because most colonies are underground.²

Fixr lists general ant extermination at $200 to $300. A barrier spray runs $80 to $120 and poison bait runs $120 to $200. The two-step method of bait plus barrier runs $600 to $900.⁴

Argentine ants often need follow-up visits because new workers arrive from linked nests. Ask for a plan that names the bait, the refill schedule and the outdoor treatment. DIY refillable stations cost far less if you keep them stocked.

• Caulk cracks and openings.¹
• Store food in sealed containers and clean up crumbs and spills.¹
• Fix leaky faucets.¹
• Keep plants, grass and mulch away from the foundation.¹
• Remove or manage honeydew insects like aphids and soft scales near buildings.¹
• Bait in late winter or early spring, when populations are lowest.³

For trees, band trunks with sticky material and check it every one or two weeks.¹

Start baiting in late winter or early spring, when Argentine ant populations are lowest. That is the most economical and effective time.³ Sweet liquid baits attract them year-round, and protein baits work when colonies produce young in spring.³

Plan ahead for annual invasions by caulking and baiting before the influx.¹

The Ohio State ant fact sheet lists carpenter, pavement, odorous house, Pharaoh and other ants for Ohio, and it does not list the Argentine ant.⁵ If you see tiny ant trails in Ohio, the likely suspects there are the odorous house ant and pavement ant.⁵

Ohio pest companies need a commercial applicator license from the Ohio Department of Agriculture. Category 10a covers general pest control.⁶ Back to the ant hub.

The Argentine ant was first recorded in California in 1907. It is now widely distributed in coastal and southern parts of the state, most common near the coast, along rivers and in cities.²

Avoid pyrethroid sprays on hard outdoor surfaces, since they pollute waterways.¹ California pest companies are licensed by the Structural Pest Control Board, and its site lets you verify a business.⁷