Carpenter ant
© Joseph Berger, Bugwood.org

Ant management is directly related to understanding differences among ants in their colony organization. The top two urban ant problems are carpenter ants and odorous house ants. Consideration of differences between the biology of these two ant groups provides a distinct contrast in organization. An understanding of differences between “colony” and “nests” will lead to successful management.

Is ant control managed by the elimination of the colony or by the elimination of the nest? What is the difference?

In both cases, an ant colony, which is composed of thousands of individuals, is considered a single organism with some individuals devoted to reproduction, some devoted to caring for the young, some devoted to habitat construction, and some devoted to gathering food and defense. A “colony” organization also occurs with other social insects such as honeybees, yellowjackets and termites.

An ant colony has a queen or queens, workers and brood. Each individual ant begins life as an egg that goes through embryonic development, and several larval stages totally dependent on workers for care and nourishment. The larvae grow and develop into pupae. Some species, when mature, spin a cocoon before pupating; some species molt into pupae without cocoons. After undergoing a complete reorganization of their internal and external features, an adult ant is produced. A mature colony may also contain winged males and females usually destined to leave the colony and reproduce to start a new colony.

One of the chief differences in colony organization in these two groups of ants is that carpenter ants (Camponotus spp.) are monogyne (one queen/colony) whereas odorous house ants (Tapinoma sessile) are polygyne (many queens/colony). A “colony” is defined as the complete population including the queen(s), workers, brood and winged forms. The word “nest” refers to the location of the colony and includes the parent and satellite nests in monogyne colonies (as in carpenter ants) or the polygyne colony may refer to any number of subnests (as in odorous house ant colonies).

CARPENTER ANTS. A monogyne colony such as found in species of carpenter ants is initiated by a single queen. Although there are many species of carpenter ants that cause structural damage in North America, a generalized life cycle exists. These are native ants that exist in forested areas or near trees and heavy vegetation. With urban development in areas containing trees, these ants have become structural pests as they excavate cavities in wood for rearing brood.

A queen is produced following a mating flight and mating with several males. Eggs produced by an inseminated queen develop through developmental stages to workers and the colony may grow to thousands. The queen, workers and brood are housed in a “parent” nest. As the colony continues to grow it may divide with some workers and mature brood being transferred to additional areas called “satellite” nests. Winged reproductives are produced in late summer and will overwinter in the nests until the mating flight the following spring. A mature colony of 10 years or more produces winged reproductives and may be contained in either the parent nest or in satellite nests. Most parent nests are located outside structures in wood where there is high humidity and temperature that is conducive to development of eggs and young larvae produced by the queen. Sites for a parent nest include moist wood such as tree stumps, standing dead trees, railroad ties in landscaping, living trees with access to the heartwood, or in buried wood. A parent nest is only moved when disturbed. Workers forage from the parent nest and return with food for the queen, larvae and other workers.

Satellite nest(s) include workers, mature brood and often winged reproductives. Mature larvae and pupae are moved into the satellite nest(s) during the summer months from a parent nest. Some will develop in the fall into workers and winged reproductives, some larvae overwinter to complete development to workers in late winter. Satellite nests are located to areas with less humidity and higher temperatures. These may include structural sites such as wall voids or other hollow spaces, and under insulation in crawlspaces or attics. During the overwintering season, there may be no communication between parent and satellite nests. Satellite nests may be used for several seasons and may vary in number/size. Although a satellite nest cannot produce additional workers without a queen, workers may continue to exist for several years in a satellite nest.

Activities of workers in both the parent nest and the satellite nests include foraging for food during the foraging season, caring for the brood and excavating wood during nest construction. Both activities are more active during the foraging season from March through October. During the winter months, workers from satellite nests in structures may be attracted to moisture and water sources.

ODOROUS HOUSE ANTS. This major nuisance pest is a single species but has many attributes of a tramp or exotic species in its mobility and the formation of a polydomous (many homes) colony. Odorous house ants form a polygyne colony that has many queens and may be founded by a single inseminated queen or by budding where a number of workers and a queen(s) leave the nest to establish subnests.

The difference between a satellite nest (monogyne colony) and subnests (polygyne colony) is that satellite nests do not contain a queen whereas subnests commonly contain a queen(s). The satellite nest cannot reproduce; a subnest has that ability. Subnests are formed when the colony has attained a large population and is foraging at a number of sites. A queen(s), workers and brood are moved to sites near foraging arenas. These sites may be temporary and will move to accommodate changes in the foraging site or for more conducive living arrangements. Formation of subnests occurs during the foraging season and as the season declines, these subnests may unite. The slogan “the brood is moved to the food” is characteristic for polygyne colonies. This is in contrast to monogyne colonies where the nest is more permanent and “food is taken to the brood.”

Subnests in structures are considered nuisance pests because of the sudden appearance of a large population of workers and brood. The ants are attracted to areas that provide humidity and temperatures conducive to rearing brood, often located near water sources such as in bathrooms, laundry rooms and kitchens. They also may be attracted to new foraging sites. Subnests may be located under appliances, in wall voids or temporary sites where there is little construction by the workers. Food often is available from sources inside structures, such as pet foods.

In natural settings, subnests, located near foraging arenas, usually have little or no construction. These nests may be located beneath yard debris, such as boards, shingles or playground equipment. They also may be located under more permanent structures such as landscaping materials. Subnests are often proliferated during the foraging season and will coalesce when the foraging season declines.

DIFFERENCES IN MANAGEMENT. The management of ants is challenging, chiefly because of the diversity of ants infesting structures. Each species has created its own niche for survival and the differences prevent us from having one standard control approach. With the expansion of suburban areas into natural areas, more ants have adapted to new living environments. The introduction of exotic ants also has complicated approaches to control ants.

Homeowners/clients are thrilled to see dead and dying ants following a treatment for either carpenter ants or odorous house ants. The fact that less than 10 percent of the ants in a nest are foraging or, more importantly, more than 90 percent of the ants remain in the nest, is an important factor to consider in the successful elimination of a colony. Treatment of all the subnests or all the satellite nests is an essential factor.

For proper bait placement, location of trails for either carpenter ants or odorous house ants is essential. Both carpenter ants and odorous house ants are omnivores and will forage on a variety of food materials. These ants most commonly feed on aphids and other homopterans. Trails between foraging arenas and structures, plus entry points, should be located for bait treatment.

Perimeter sprays also are more effective when applied to foraging trails and entry points. Trails of both groups of ants will follow structural guidelines such as edges of sidewalks, branches, wires, foundation, hoses, siding, timbers, plumbing, etc.

Development of transferable toxicants either in perimeter treatments or in baits allows ease in its distribution in the colony. The existence of subnests and satellite nests will continue to cause challenges. The correct location, consideration of their size and distribution will facilitate chemical distribution.

The comparison between carpenter ants and odorous house ants illustrates the extremes in differences of colony formation. The polygyne odorous house ant with polydomous nests illustrates the problem of eliminating all the subnests of a colony. When a subnest(s) does not have access to a bait used in treatment or to a perimeter spray that has a high transfer rate, the colony is not only not controlled but with queen(s) in subnests, the colony can continue to reproduce and fill the void produced by the elimination of treated subnests. In a monogyne colony, elimination of the parent nest produces permanent control. Satellite nests located within structures may be difficult to control within a single season but cannot reproduce without a parent nest.

The author is an instructor in the biology department at Spokane Falls Community College, Spokane, Wash.