Argentine ant, Linepithema humile, tending scale insects.
Ants are among the most prevalent pests in households. Ants also invade 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, and termites), dead insects, and decomposing tissue from dead animals.
There are more than 12,400 species of ants throughout the world. In California, there are about 270 species, but fewer than a dozen are important pests (Table 1). The most common ant 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), odorous house ant (Tapinoma sessile), thief ant (Solenopsis molesta), southern fire ant (S. xyloni), and pavement ant (Tetramorium caespitum). The velvety tree ants Liometopum occidentale and L. luctuosum nest in trees and are common outdoor species in landscapes.
Less common, but of great importance, is the red imported fire ant, S. invicta, which gained a foothold in Southern California in 1998. In some areas, competition from the Argentine ant has slowed the spread of the fire ant. Carpenter ants, Camponotus species, and velvety tree ants also invade buildings in California. Although they don't eat wood as termites do, they hollow it out to nest and may cause considerable damage. For more information about red imported fire ants or carpenter ants, see Pest Notes: Carpenter Ants and Pest Notes: Red Imported Fire Ant.
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 or the online Key to Identifying Common Household Ants.
Ants belong to the insect order Hymenoptera and are close relatives of bees and wasps. Ants 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 from one another by three main characteristics:
Inside buildings, household ants feed on sugars, syrups, honey, fruit juice, fats, and meat. Long trails of thousands of ants may lead from nests to food sources, causing considerable concern among building occupants. Outdoors they are attracted to honeydew, produced by soft scales, mealybugs, and aphids. This liquid excrement contains sugars, and other nutrients. Frequently outbreaks of scales and aphids occur when ants tend them for honeydew because the ants protect scales and aphids from their natural enemies.
Adult California harvester ant.
Ants can bite with their pincerlike jaws, although most species rarely do. The velvety tree ant, however, is an aggressive biter. A few ants sting, including native fire ants and harvester ants, which are primarily outdoor species, and are the most common stinging ants in California. An aggressive stinging ant, the red imported fire ant (S. invicta), has recently been found in various southern California counties. If you suspect a fire ant infestation, report it to your county agricultural commissioner. For more information on red imported fire ants, see Pest Notes: Red Imported Fire Ant.
Ants undergo complete metamorphosis, passing through egg, larval, pupal, and adult stages. Larvae are immobile, wormlike, and don't resemble adults. Ants, like many other hymenopterans such as bees and wasps, are social insects with duties divided among different types, or castes, of adults. Queens conduct the reproductive functions of a colony and are larger than other ants; they lay eggs and sometimes participate in feeding and grooming larvae. The sterile female workers gather food, feed and care for larvae, build tunnels, and defend the colony; these workers make up the bulk of the colony. Males don't participate in colony activities; their sole purpose is to mate with the queens. Workers feed and care for males, which are few in number.
Ants usually nest in soil; nest sites vary with species but are often found next to buildings, along sidewalks, or in close proximity to food sources such as trees or plants that harbor honeydew-producing insects. Ants also construct nests under boards, stones, tree stumps, or plants and sometimes under buildings or other protected places. The primary ant that nests indoors in California is the Pharaoh ant. In temperate climates, this species nests in warm, moist locations such as inside wall voids, under flooring, or near hot water pipes or heating systems, but is also found nesting outdoors in warmer parts of California. See Common household ants for nesting sites of other species. Food preferences vary among ant species but may include fruits, seeds, nuts, fatty substances, dead or live insects, dead animals, and sweets.
Ants often enter buildings seeking food and water, warmth and shelter, or refuge from dry, hot weather or flooded conditions. They may appear suddenly in buildings if other food sources become unavailable or weather conditions change.
Although there is some variation among species, a single newly mated queen typically establishes a new colony. After weeks or months of confinement underground, she lays her first eggs. After the eggs hatch, she feeds the white, legless larvae with her own metabolized wing muscles and fat bodies until the larvae pupate. Several weeks later, the pupae transform into sterile female adult workers, and the first workers dig their way out of the nest to collect food for themselves, the queen (who continues to lay eggs), and subsequent broods of larvae. As numbers increase, workers add new chambers and galleries to the nest. After a few years, the colony begins to produce winged male and female ants, which leave the nest to mate and form new colonies.
Argentine ants differ from most other ant species in California in that their nests are often shallow, extending just below the soil surface. However, under dry conditions they will nest deeper in the soil. In addition, Argentine ant colonies aren't separate but linked to form one large "supercolony" with multiple queens. When newly mated queens disperse to found new colonies, they are accompanied by workers rather than going out on their own as most other species do.
Top of page | | Full site