Home Sweet Home

Being a social insect, ants require a "home base" from which to operate. The site chosen to locate the colony is often mandated by instinct as seen with certain carpenter ant (Camponotus) species that only nest in wood, while other Camponotus species will nest in the soil beneath items. Still other species, such as C. herculeanus, begin their nest in wood and then may extend it down into the soil. Other ants, such as the pavement ant, are strictly soil nesters and will carry soil into voids when they infest structures.

It is very important to understand the specific nesting habits of structural pest ant species. Such knowledge is often the key to quickly locating the offending colony’s nest site so it can be treated. For instance, knowing that acrobat ants prefer to nest in moisture-damaged and often rotting wood eliminates a good portion of the structure that will need to be inspected.

The nests of structural ants can be divided into three basic categories: those that nest in soil, those that nest in wood and those that are opportunistic.

SOIL-NESTING ANTS. The soil is the primary site in which most ant species locate their nests. Soil is certainly plentiful, it is easy to excavate, it contains moisture and food resources and space is not limited. Ants move more earth by their workings than any other animal on earth, including earthworms. Therefore, ants are crucial to the success of the topsoil ecosystem. By tunneling in soil, ants bring oxygen, nutrients and moisture down to plant roots and prevent the soil from becoming too compacted. Without ants, many forms of plant life would likely be impacted severely.

With the large numbers of different soil-nesting ant species, the types of galleries seen in the soil varies greatly. For example, Figure 1 shows the typical gallery of the small honey ant, Prenolepis imparis, which has a single vertical tunnel with branching galleries to each side where the eggs, larvae and food stores are kept. The nest of the Florida harvester ant, Pogonomyrmex badius, differs slightly as shown in Figure 2. The single tunnel may be augmented with other branching vertical tunnels and considerably more branching galleries may be present.

Other species will construct extensive underground galleries that may reach several feet into the soil, including those constructed by fire ants, Solenopsis spp., and the Allegheny mound ant, Formica exsectoides. In these species, a network of interlocking tunnels criss-cross and interconnect with each other. In many cases, the nest will be located beneath some object, such as a stone or log. The mounds can grow quite large in some cases: up to 2 feet (60 cm) or more in diameter with fire ants and more than 3 feet (90 cm) with Allegheny mound ants. One Allegheny mound ant colony in Ohio constructed a mound 13 feet in diameter.

Not all ant mounds will be solely composed of soil. The Allegheny mound ant, for example, may include grasses within the upper layer of the mound to create a thin, thatched layer. This layer serves to insulate the mound as well as to strengthen it. This species also incorporates soil into the mound that it may gather from as far as 100 feet (30 meters) from the nest.

The leaf cutter ants, Atta spp., may dig the most elaborate galleries of all the soil-nesting ants (Figure 4). The galleries of these nests may extend 15 feet down into the soil and contain dozens of chambers connected by tunnels. Both vertical and horizontal tunnels are present, with vertical tunnels running to the soil surface and horizontal tunnels connecting the chambers. Most chambers serve as fungus gardens where the fungi on which the ants feed is grown. Other chambers house the eggs, larvae and pupae. These nests can be quite large, covering an area more than 1,000 square feet (10 square meters).

In the process of constructing nests in the soil, ants must do something with the soil. Obviously, this displaced soil composes the mounds seen around ant colonies. The shape of the mound can be unique to the ants as is seen with leaf cutter ants. These ants leave the soil in crater-shaped mounds with all the soil piled on one side of the nest entrance. This behavior is also seen in the fungus-growing ants of the genus Trachymyrmex, which often can be found in lawns in Florida.

The mounds found around the nests of small honey ants, pyramid ants and other lawn-nesting ants are typically small and form a small crater around the nest entrance. It is not uncommon to see a large number of such small mounds in a single yard.

Displaced soil is a common clue used to determine the location of ant colonies. In many cases, the displaced soil has no distinct shape and only appears as a scattered pile of soil. Although the imported fire ant is known for its dome-shaped mounds, it commonly scatters the displaced soil in irregular piles. This behavior is seen in sandy soils and where nests are located under stones or other objects or within landscape mulch. Pavement ants commonly pile up displaced soil on top of sidewalks, patios and other surfaces next to the cracks through which they enter and exit the nest.

NESTS IN WOOD. Carpenter ants and acrobat ants are the two types of ants most commonly found nesting in wood. Typically, the wood attacked is usually dead and contains considerable moisture and may be weakened by fungi. Carpenter ants are capable of attacking sound wood and even live wood in trees. For example, a nest in the dead wood of a tree hole is expanded into surrounding live wood.

The wood is not eaten by these ants, rather it is deposited in bits outside the nest. The piles of "sawdust" are common at the bottom of tree holes and at the base of trees. The galleries formed by the ants are typically smooth and free from soil or mud. In some cases in floor joists, for example, the galleries will have a uniform appearance with parallel tunnels. In most cases, the galleries vary in size and shape.

Any dead wood is subject to attack, including tree limbs, tree holes, stumps, logs, landscape timbers and the structural wooden members of buildings (Figure 5). In recent years, both carpenter ants and acrobat ants have discovered that the foam elements used in building construction are easier to excavate to create nesting sites.

A number of types of ants will nest in wood previously infested by termites and even carpenter ants. Pavement ants, acrobat ants, Argentine ants, fire ants and the large yellow ant are a few of the pest species with this behavior. The ants usually clear out the soil and other debris within the old galleries and deposit it outside the nest. The sudden appearance of piles of soil in window sills and next to door frames is often an indication of this activity.

NESTS OF OPPORTUNITY. A number of the most important pest ant species take advantage of any suitable location in which to establish all or part of a colony. For example, the Argentine ant, Linepithema humile, is basically a soil-nesting ant that constructs shallow nests within the soil, often under objects. This ant will, however, locate nests in the voids of rocks, in the spaces created by stacked items, in old termite galleries in wood, in wall voids and inside insulation in attics. It has even been found to excavate the foam elements in buildings to create a nest site.

Many other species have similar nesting habits to the Argentine ant, including the crazy ant, Paratrechina longicornis; the white-footed ant, Technomyrmex albipes; the odorous house ant, Tapinoma sessile; and the ghost ant, Tapinoma melano-cephalum.

Although primarily wood-nesting ants, carpenter ants are opportunistic in selecting sites for satellite colonies. Any suitable void within a structure can be utilized by satellite colonies. Colonies have even been found within appliances as small as clocks and radios. Other carpenter ant species primarily nest in the soil beneath objects while others begin nests in wood and may extend the nest into the soil.

The Pharaoh ant, Monomorium pharaonis, may well be the "king" of nest site variability. These ants have been discovered in sites as small as a Band-Aid® box in a medicine cabinet. Other nesting sites include between the folds of sheets in closets, in the hollow tubes of hospital beds, inside electronic appliances and even inside an electric iron.

The ability of these opportunistic species to survive in a variety of locations is a trait that makes them ideally suited to be pests of structures. In most cases, the species that select varied nesting sites are those that exhibit extended colonies with multiple nesting sites. The use of the nest site is often only temporary, lasting from a few days to possibly a few months. If a better site is discovered, the colony will readily vacate one site for another. Established trails connect each nest site to another, thereby linking the entire colony.

SUMMARY. Each species of ant has aspects to its biology that determine how and where they live. The key to managing infestations in and around structures is understanding the target ant’s biology and determining what conditions in the structure may be contributing to the infestation.

The preceding article was excerpted from the Second Edition of the PCT Field Guide for the Management of Structure-Infesting Ants by Stoy Hedges. To order a copy of the book, contact Fran Franzak, manager of books and directories, at 800/456-0707. Hedges is a board certified entomologist, a registered sanitarian and manager of technical services for Terminix International, Memphis, Tenn.