Ants are among the most prevalent pests in households. They are also found in restaurants, hospitals, offices, warehouses, and other buildings where they can find food and water. On outdoor (and sometimes indoor) plants, ants protect and care for honeydew-producing insects such as aphids, soft scales, whiteflies, and mealybugs, increasing damage from these pests. Ants also perform many useful functions in the environment, such as feeding on other pests (e.g., fleas, caterpillars, termites), dead insects, and decomposing tissue from dead animals.
There are over 12,000 species of ants throughout the world. In California, there are about 200 species but fewer than a dozen are important pests (Table 1). The most common ant occurring in and around the house and garden in California is the Argentine ant, Linepithema humile (formerly Iridomyrmex humilis). Other common ant pests include the pharaoh ant (Monomorium pharaonis), the odorous house ant (Tapinoma sessile), the thief ant (Solenopsis molesta), and the southern fire ant (Solenopsis xyloni). The velvety tree ant, Liometopum occidentale, nests in old wood and is a common outdoor species in landscapes.
Less common, but of great importance, is the red imported fire ant, Solenopsis invicta, which has recently gained a foothold in southern California. In some areas, the spread of the fire ant has been slowed by competition from the Argentine ant.
Carpenter ants, Camponotus spp., also invade buildings in California. Although they do not eat wood as termites do, they hollow it out to nest and may cause considerable damage. For more information on carpenter ants, see Pest Notes: Carpenter Ants, listed in “References.” For color photographs and additional information on identifying the different ant species, see A Key to the Most Common and/or Economically Important Ants of California, listed in “References,” or the online Key to Identifying Common Household Ants.
Ants belong to the insect order Hymenoptera and are close relatives of bees and wasps. They are familiar insects that are easily recognized, especially in their common wingless adult forms, known as workers. However, winged forms of ants, which leave the nest in large numbers in warm weather to mate and establish new colonies, are often mistaken for winged termites, which also leave their nests to mate. Ants and termites can be distinguished by three main characteristics:
The ant’s body is constricted, giving it the appearance of having a thin waist; the termite’s body is not constricted. The ant’s hind wings are smaller than its front wings; the termite’s front and hind wings are about the same size. (Shortly after their flights, both ants and termites lose their wings, so wings may not always be present.) Winged female and worker ants have elbowed antennae; the termite’s antennae are not elbowed.
Ants undergo complete metamorphosis, passing through egg, larval, pupal, and adult stages. Larvae are immobile and wormlike and do not resemble adults. Ants, like many other hymenopterans, are social insects with duties divided among different types, or castes, of adult individuals. Queens conduct the reproductive functions of a colony and are larger than other ants; they lay eggs and sometimes participate in the feeding and grooming of larvae. Female workers, who are sterile, gather food, feed and care for the larvae, build tunnels, and defend the colony; these workers make up the bulk of the colony. Males do not participate in colony activities; their sole purpose is to mate with the queens. Few in number, males are fed and cared for by workers.