Ants in the home

Order: Hymenoptera
Family: Formicidae

Description

Almost anywhere in Colorado, ants will be the most common insect found in yards, gardens, fields and forests. Most ants found in homes nest outdoors and enter homes to forage for food or water.  Ants are social insects that live in a colony, usually consisting of thousands to tens of thousands of individuals. Within the colony, ants are divided into different castes: workers, males, and queens. Workers are wingless females that forage for food, construct, maintain and defend the nest, and tend the young. Each colony contains one or a few queens that are larger than workers and are dedicated to egg production. Upon egg hatch, the pale-colored, legless larvae are fed and protected by the workers. When full-grown, ant larvae produce a smooth silken cocoon within which they pupate, ultimately emerging as an adult ant. Ant pupae are often seen when turning over a rock that exposes a colony and are sometimes mistakenly called “ant eggs”.  

As colonies mature, winged ants are also produced. These include females that are potential queens and slightly smaller males. These reproductive forms periodically leave the colony during swarming events, when they fly away in large numbers and attempt to mate. Swarming ants can often attract attention and alarm, however, these ants never return to a colony once they have left on a mating flight. Winged ants are sometimes mistaken for winged termites. However, ants can be easily distinguished by having a narrow constriction between the thorax and, antennae that are elbowed, and hindwings that are smaller than the forewings. 

Ants commonly found in Colorado homes:

Pavement ants
Pavement ants (Tetramorium immigrans) arrived in Colorado only in the past few decades, but now are often the most common species that people notice. They produce small mounds of soils at the entrance of their nests and are often located under pavement or rocks. They are small ants that measure 2.5-4 mm (1/10-1/6 inch) long, are dark brown in color and have fine grooves that line their head. Pavement ants forage a wide variety of foods, usually consistently preferring food high in fats.

Problems with indoor ants usually occur in spring and early summer and can be managed with baits, habitat modification, cleaning, proper food storage and exclusion. Outside, pavement ants can be managed with habitat modification, exclusion, and residual insecticides.

For detailed pavement ant management strategies see this factsheet.

Field ants
Field ants (Formica spp.) are commonly found in yards and gardens and are observed in homes most commonly in the spring. They are black or reddish brown and 4-8 mm (1/6-1/3 inch) long. They nest outdoors in loose soil and some produce mounds that incorporate twigs, dried leaves and other plant materials. Field ants feed on a variety of foods but most often are observed visiting sweet honeydew excreted by aphids and or other food sources high in sugars.

Carpenter ants
Carpenter ants (Camponotus spp.) are the largest ants that occur in Colorado at 0.76-2.5 cm (1/3-1 inch) long. And are particularly abundant in forested areas. They may be either black or black with a reddish-brown thorax. Carpenter ants nest in wood and will rarely establish a colony in a building. Nests of carpenter ants may be located by the dust (grass) they deposit while chewing through wood. Carpenter ants feed on a mixture of dead insects and honeydew.

Infestations of carpenter ants usually indicate there is a moisture problem in the house. Carpenter ants prefer nesting in wood with high moisture content, especially when the wood is infested with fungi. Cultural controls of this ant species are centered around reducing moisture in ant-prone areas. This can be achieved by increasing ventilation in crawlspaces, basements and attics, fixing all leaks in the house, using a dehumidifier, and sealing possible entry points in the house.

For detailed management strategies of carpenter ants see this factsheet.

Cornfield ants
Cornfield ants (Lasius spp.) are small 2.5-4 mm (1/10-1/6 inch) long ants that are usually light to medium-brown colored. They nest outdoors but sometimes will enter buildings to feed on sweet materials.

Odorous house ants
Odorous house ants (Tapinoma sessile) are small 2.4-3.3 mm (1/10-1/9 inch) black ants. When disturbed, they will raise their abdomen and emit an odor reminiscent of coconut. They do not nest indoors but may enter to forage on various sweet foods. Heavy mulch adjacent to building foundations has been associated with increased problems with odorous house ants.

Odorous house ants are persistent pests and require appropriate management and follow-up. These ants can be managed by eliminating habitat and contributing conditions and using pesticide baits. Locating foraging trails and nest sites is critical to OHA management. Nest sites and foraging sites are excellent locations to target pesticide applications.

For detailed management strategies of odorous house ants see this factsheet.

Pharaoh ants
Pharoah ants (Monomorium pharoanis) are small, 2 mm (under 1/10 inch) light-brown ants that are adapted to nesting in buildings. Nests can spread extensively through a structure as pharaoh ants may frequently move nest locations, produce multiple queens, and may split to form “satellite” colonies. Pharaoh ants feed on a wide range of foods rich in sugars, fats, and protein. They can be serious pests of hospitals, dormitories and apartments.

Quick Facts

  • Most ants that are found in homes nest outdoors and enter homes only to search for food or water. 
  • Almost all ants are workers, wingless females that search for food and maintain the colony. 
  • A small proportion of an ant colony are winged reproductive forms. These emerge periodically in swarms, mate, and establish new colonies. 
  • Removing sources of food and water will reduce visitation of ants and help to more effectively use baits for ant control. 
  • Slow-acting insecticides in baits allow them to be taken back to colonies and are most effective for eliminating ants. 
Adult male carpenter ant.
Adult male carpenter ant (Camponotus nearcticus) with wings. Image credit: Joseph Berger, Bugwood.org.
Odorous house ant.
Odorous house ant (Tapinoma sessile) tending scale insect. Image credit: Susan Ellis, Bugwood.org
Pavement ant adults and larvae.
Pavement ant (Tetramorium immigrans) adults and larvae. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org.
Field ants with aphids.
Field ants (Formica sp.) with aphids. Image credit: David Cappaert, Bugwood.org.

Life history and habits

New colonies are developed by a queen following a mating flight. After being fertilized, she moves under a rock or some other crevice and sheds her wings and attempts to establish a new colony. Only a tiny fraction of the females is successful in their efforts and most perish without a new colony developing. If they are successful, the colonies will slowly grow, becoming full sized after several years.

Worker ants will forage during the warmer months of the year. The workers lay down chemical trails as they forage that help direct other workers to sources of food or water. Feeding habits of the various species of ants vary, with sugary materials preferred by most species, at least for part of their diet. Others may favor food high in fats such as seeds, or protein-rich foods such as living or dead organisms. Feeding habits may shift during the season with changes in colony needs.

Nuisance

Overall, the activities of ants are beneficial. Many feed on other insects, including pests. Ant scavenging helps to recycle organic matter and their tunneling is useful in aerating and mixing soils. However, occasionally ants form mutualisms with aphids and other honeydew producing insects and protect these common insect pests from their natural enemies while consuming honeydew excreted by them. These mutualisms can be detrimental to the plants.

Ants become problematic mainly when they are found in homes. Most ants found indoors in Colorado are merely foragers, seeking food or water in a home but returning to colonies they have established outdoors. A small number of the ants found indoors may become more permanent residents and establish a nest within a building. This ability to nest indoors is most often associated with pharaoh ants; colonies of pavement ants and carpenter ants may establish within a building as well.

 

Management of household ants

The first and most important step in reducing ant infestation is to eliminate sources of food and water that are attractive. In addition to obvious sources of food left on counters, sinks or floors, spilled food in cupboards and food associated with trash cans may also be visited by foraging ants. Dripping faucets and leaking pipes may also be important water sources that ants will use. If ants have been foraging in a home, wash down counters or floors with a household cleaner to eliminate the odor trails the ants have established to locate sources of food or water.

Most ants found in homes nest outdoors, sometimes adjacent to building foundations. Heavy mulch can provide cover for ant colonies. Keeping the area around the building foundation free of mulch can reduce the likelihood of ants foraging indoors.

Insecticides can be useful to control ants and a wide range of ant control products are available. These are primarily used either to: 1) generally suppress ant numbers in a yard; 2) establish perimeter barriers around a home; 3) spot treat foraging ants found in homes; or 4) incorporate into baits to kill colonies.

Products used for general ant suppression include various sprays, dusts or granules applied to lawns. These surface-applied treatments can temporarily reduce numbers of ants foraging through lawns and on garden beds. However, such treatments do not penetrate colonies, nor do they affect ants that use below-ground tunnels for much of their travel.

Most ants that occur within homes originate from outdoor colonies. An insecticide barrier maintained around the building perimeter can inhibit much of this activity. Such treatments are generally applied as sprays or dusts to the soil immediately adjacent to the building and/or the lower areas of walls. However, such treatments will not be able to well control ants that enter homes through below ground openings in building foundations.

Some household insecticides allow use indoors as a surface spray to kill ants that move across treated surfaces. Such treatments are best applied to cracks and crevices used by foraging ants to enter living areas. However, these have very limited and short-term effectiveness as the insecticides degrade and/or the ants change routes of activity.

Use of ant baits will usually provide the most effective control. This strategy involves use of acceptable food into which a small amount of an insecticide is incorporated. The ants feed on the bait and return it to the colony where it is shared with nest mates.

Additional reading

Green, J. 2023. Ants Around the Home and Landscape. Available https://lancaster.unl.edu/pest/resources/antcontrol.shtml

Karren, J. 2001. Ant Infestation. Utah State University-Extension. Available https://extension.usu.edu/pests/uppdl/faq/ant-infestation

Oregon State University. (n.d.). How to control household ants? Oregon State University-Extension. Available https://extension.oregonstate.edu/ask-extension/featured/how-control-household-ants

CSU Extension Fact Sheet

Download or view the CSU Extension’s PDF fact sheet for your reference.