The odorous house ant, Tapinoma sessile (Say), is considered a pest when it enters structures searching for food, water or nest sites. It cannot sting because it lacks a sting and will likely only bite if you stick a hand into its nest and vigorously disturb the colony. Occasionally winged reproductives found at lights concern residents too. Odorous house ant is common throughout the United States and is the second most common pest ant managed by professionals.
Odorous house ant is a small native ant species found in the United States, southern Canada and Mexico. It survives in a variety of environments found from near sea level to elevations of more than 2 miles and can be found in all the continental states. It’s possible that odorous house ant is really four similar-looking species, but until more data are produced to support this hypothesis, we’ll consider it as one. In 2009, it was discovered on the island of Maui, Hawaii and is now characterized as an invasive species.
Odorous house ant is about 1/8-inch long, dark brown to black and smells like rotten coconut with a hint of other odors when crushed. Its waist is one segmented and lacks an obvious node or bump which easily distinguishes it from other small dark ants, including the Argentine ant. The gaster or abdomen overhangs the waist making the waist difficult to see. Odorous house ant lacks a sting or acidopore (circular ring of hairs) at the end of the gaster, but instead has a slit-like opening on the ventral side of the gaster one segment in from the tip. All odorous house ant workers are the same size or monomorphic.
Behavior and Biology
In addition to their smell, odorous house ants are accurately named as they are often found foraging along the outside base of a home. Increased indoor activity is often associated with rain. Odorous house ant activity can be observed during the day and night and will be found foraging outdoors in greatest numbers when temperatures are between 70 and 86 degrees F. Odorous house ants use edges, ridges or other guidelines to move from one place to another. Natural (vines, trees and shrubs) and man-made (siding, ground/foundation wall interface, wires, pipes, conduits, baseboards, counters and others) objects may serve as guidelines. Outdoors, OHA feed on dead and living insects, dead animals (including those deposited by the family cat), pet food, plant nectar and liquid excrement (honeydew) from aphids, scales and other sucking insects. Indoors, they feed on sweets and other human and pet foods. Odorous house ants are often found foraging to water sources and kitchen and bathroom garbage cans.
Odorous house ants do not build nests within mounds of soil, rather they nest opportunistically. Outdoors, odorous house ant nests in pre-existing spaces that provide some moisture and protection from the sun. They may nest under, near or in logs, landscape timbers, stones, patios, leaves, debris, siding including that laid on the ground, stacked wood or firewood, mulch, pine straw, bee hives, dog houses and near iris rhizomes. Indoors, they may be associated with food or moisture and can be found beneath edges of carpets and toilets, in cabinets or drawers, near or under garbage cans and other similar places. Indoor nests are often associated with outdoor nests. Often many nests are present outdoors and a few may be found inside.
Odorous house ants have many queens per colony (polygyne) but queen number varies. In natural situations, such as forests, colonies tend be small with one queen (monogyne). However, in disturbed, urban environments, many queens and tens of thousands or more workers may be present in a colony that has many nests. Workers are more dominant when present in larger numbers (urban environments) than smaller numbers (forests).
Because indoor odorous house ants are often in contact with outdoor odorous house ants, management efforts during the warmer times of the year can often be directed to the outside to impact the indoor ants.
Steps to manage odorous house ants include:
- Correctly identify the ant. See above for identification details.
- Remove conducive conditions that allow odorous house ant to thrive. Determine the food, water and harborage that the home and near landscape (within 10 ft) provide to the ants and then move/remove as many as possible.
- Monitor and inspect to locate nests and areas of activity. To help locate odorous house ant nests and activity, place index cards with a smear of honey every 10 – 20 ft around the base of the structure. Check the cards in 40 minutes and count the number of odorous house ants. Follow the ants back to their nests and note nest location.
- Bait areas of activity. Bait outdoors where more than 10 odorous house ants are found per index card. Baiting indoors where ants are active as a sole treatment will most likely provide a short-term reduction in indoor ant foraging.
- Treat nests. Because many nests can be found around a structure, it is difficult to locate all of them. Treating nest sites as the sole treatment method would be most effective when just a few small nests are present. Finding nests sites often involves lifting objects to expose the nest. Inspect and treat nests at the same time to avoid disturbing the ants and causing them to move prior to treatment.
- Treat perimeter, entry ways and areas of activity. A typical perimeter treatment involves spraying the ground/foundation wall interface, the siding/foundation wall interface and the area around doors, windows and vents. Recent changes to pesticide labels have restricted the areas where perimeter sprays can be applied. Read labels carefully to avoid misapplying the pesticide and possibly causing unintended run-off. Treat areas of ant activity if allowed by label. Both slow-acting and fast-acting perimeter treatments can dramatically reduce the number of outdoor foraging odorous house ants, but may slowly affect indoor ant activity if baits have not been used.Applying fast-acting crack and crevice sprays or dusts to ants indoors will have little effect on outdoor odorous house ant populations and may prolong indoor activity. Avoid applying fast-acting insecticides to interior cracks and crevices as the sole treatment.
- Combine above. Integrated pest management relies on multiple tactics and managing odorous house ant is no exception. The best management results will be achieved with a combination of the above practices. Correctly identifying the pest ant and correcting conducive conditions should always be used when managing odorous house ant infestations. Monitoring and inspecting to note nest location and activity is especially helpful. Combining different chemical treatments (e.g., slow-acting insecticide applied to the outside perimeter with an exterior bait placed where ants are active) should increase pest management success.
Check with your local Extension service for product recommendations as pesticide registrations differ in each state.
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