Identification and Habits of Key Ant Pests in the Pacific Northwest

Identification and Habits of Key Ant Pests in the Pacific Northwest

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Laurel Hansen, adjunct entomologist, Washington State University, Department of Entomology, Roger Akre, (deceased), Original Author, entomologist, Washington State University, Art Antonelli
This publication describes the 12 most common ants characterized as structural pests in Washington, Idaho, and Oregon. Drawings and descriptions follow an introduction to ants and their general biology. Illustrated keys are provided for workers, winged males, and females. Detailed photographs of workers are included on the cover. Replaces EM033E and updates EB0671. Cover images are from, as photographed by April Nobile. Top row (left to right): Carpenter ant, Camponotus modoc; Velvety tree ant, Liometopum occidentale; Pharaoh ant, Monomorium pharaonis. Second row (left to right): Aphaenogaster spp.; Thief ant, Solenopsis molesta; Pavement ant, Tetramorium spp. Third row (left to right): Odorous house ant, Tapinoma sessile; Ponerine ant, Hypoponera punctatissima; False honey ant, Prenolepis imparis.
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Ants (Hymenoptera: Formicidae) are an easily recognized group of social insects. The workers are wingless, have elbowed antennae, and have a petiole (narrow constriction) of one or two segments between the mesosoma (middle section) and the gaster (last section) (Fig. 1).

Ants are one of the most common and abundant insects. A 1990 count revealed 8,800 species of ants had been described and that number has increased to more than 14,000. There are still a number of undescribed ant species in the world. The true number is probably more than  30,000.

Ants are also one of the most widely distributed of all insect groups. They occur from the Arctic tree line to the humid tropics, from Alaska to the extreme tip of South America, to the tip of Africa, Australia, and even to all the islands in the oceans. They are the most abundant of all social  insects.

Since there are so many species of ants, and in such diverse habitats, it becomes obvious that ants are one of humans’ principal insect competitors. Ants infest buildings as a nuisance, feed on human foods, and even cause structural damage.

Ants are also annoying because of their biting or stinging habits. Ant stings are responsible for a number of human fatalities in the United States each year. The effects of stings on a human depend on the ant species and the sensitivity of the person. Most deaths are caused by a hypersensitive reaction leading to anaphylactic shock.

General Biology

Many ant colonies are started by a single inseminated female, called a queen. From this single individual, ant colonies can grow to  contain

anywhere from several hundred to millions of individuals. Among the largest ant colonies are the army ants of the American tropics, with up to several million workers, and the driver ants of Africa, with 30 million to 40 million workers. A thatching ant (Formica) colony in Japan covering many acres was estimated to have 348 million workers. However, most ant colonies probably fall within the range of 300 to 50,000   individuals.

Ants normally have three distinct castes: males, queens, and workers. Males are intermediate in size between queens and workers and can be recognized by ocelli (simple eyes) on top of the head, wings, protruding genitalia, and large eyes. The sole function of the male is to mate with a winged female during the nuptial flight.

The winged female loses her wings soon after mating and becomes a queen. However, scars where the wings were attached remain. Queens usually also have ocelli in addition to large compound eyes and  a large gaster for egg production.

The worker, the smallest member of the colony, usually lacks ocelli and is never winged. Workers of a single species may be of one size (monomorphic) or may vary considerably in size (polymorphic). Large workers are usually called soldiers or majors; very small workers are  minors.

Ants pass through several distinct developmental stages in the colony: egg, larva, pupa, and adult. The egg is very small (less than 1 mm) and varies  in shape according to species. The larva also varies in size and shape, but is usually white and is always legless. The pupal stage looks like the adult, but is soft, white, and motionless; many species are enclosed in a cocoon of a brownish or whitish papery material produced by the mature larva.



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