Tapinoma sessile (Say), the odorous house ant, is a widely distributed native species found throughout the United States, in Canada, and Mexico. The common name of this insect is derived from a peculiar coconutlike odor produced in the anal glands.

A large population of these ants live in western Washington, between Vancouver, British Columbia and Portland, Oregon. Odorous house ants are less common in the semidesert areas of the Pacific Northwest.

All odorous house ant workers are the same size (monomorphic). They differ from other ant species by the presence of a slitlike cloacal orifice without fringe hairs (Fig. 1). Antennae are 12 segmented and without a club (enlargement) at the tip. The promesonotal and mesoepinotal sutures are both distinct; the latter is even more distinct (Fig. 2). The single-segmented petiole (connection between thorax and abdomen) has no node (Fig. 2), a factor that distinguishes it from the ant in Fig. 3. Workers are approximately ¹/₁₆ inch long and have a uniform brown to black color.

Fig. 1. Transverse ventral orifice (A, lateral view; B, ventral view).

Fig. 2. Tapinoma sessile (odorous house ant).*

Fig. 3. Lasius spp. (Cornfield and other ants).*

Odorous house ants have adapted to a wide range of habitats and thrive nearly everywhere from sea level to about 10,500 feet. They nest in sand, pastures, grass fields, forests, bogs, and houses. They frequently nest under stones and logs. They also build nests under stumps and under the bark of dead trees, in bird and mammal nests, in plant galls, and in debris. Nests in the soil are shapeless, shallow, and temporary, as the ants frequently move.

Colonies can consist of thousands of workers and usually contain many queens. New queens typically mate with their brothers within the colony. Nuptial flights do not occur inside colonies—only outside. Some queens also mate with unrelated males. The nuptial flights occur from June to mid-July. New colonies may form by budding when a new queen(s) leaves the parent colony with workers or as a single foundress queen.

Workers move fast and often travel in columns. When alarmed they run about erratically with abdomens tipped while releasing an alarm pheromone (the peculiar coconutlike odor) which draws more workers to the release site.

Workers collect honeydew (excretions) from mealybugs, aphids, scale insects, and plant hoppers and will protect these insects from predators. Workers also gather nectar from plants and feed on both living, and dead insects.

These ants eat a wide variety of foodstuffs, such as meat, cooked vegetables, dairy products, fruit juices, and pastries. They also feed on both living and dead insects. Odorous house ants have become a major nuisance pest, infesting houses.

If the colony can be traced to an outdoor nest, treat with chlorpyrifos, diazinon, or bendiocarb. Incorporate dusts or granular formulations into the nest area with a shovel to enhance control. Commercial applicators may also use cyfluthrin or cypermethrin. To prevent further infestation around your home treat the area where the foundation and soil meet to form a chemical barrier.

When an outdoor colony cannot be located, apply a registered indoor pesticide such as bendiocarb or chlorpyrifos to cracks, crevices, baseboards, and other floor level entry points. In addition to using chemical control, keep the home free of food debris, exposed stored food, or pet food. Keep counter tops and appliances free of grease or anything these ants will feed upon. These steps will help discourage indoor foraging.

Control can be difficult. A homeowner may choose to hire a licensed pest control operator.

Other ant publications available through WSU Cooperative Extension include:

• EB0818, Carpenter Ants: Biology and Control, $1.50
  
  
• EB1382, Moisture Ants, $1.00
  
  
• EB0929, Thatching Ants, $1.50
  
  
• EB1514, Pharaoh Ant, $1.00
  
  
• EB0671, Identification and Habits of Key Ant Pests of Washington, $2.00