[Annual Ant Control Issue] Paving Their Way

Of the 40+ species of ants in the U.S. that infest buildings, the pavement
ant, Tetramorium caespitum (L.), is one of the more widespread pests. This ant is one of the two or three key pest species found in the northern regions and is considered the primary commercial pest ant in those areas. The pavement ant’s current range extends across much of the northern and midwestern part of the country, being found from Maine to Washington state and south into North Carolina, Tennessee in the east and Southern California and Arizona in the west.

The pavement ant is a significant pest and difficult to control. In most of its range, it is second only to carpenter ants as a structural pest and is the primary ant that invades commercial buildings. In recent years, however, some of its prevalence in commercial situations in the lower parts of its U.S. range is being replaced by that of the odorous house ant.

BIOLOGY AND HABITS. Pavement
ant colonies vary in size and may be considered small to moderate in comparison to some other pest ant species, such as Argentine or odorous house ants. In the book, The Ants by Bert Hölldobler and E.O. Wilson, the number of adults in colonies studied in England ranged in size from 2,603 to 29,571. Others have confirmed these figures listing the colony size ranging up to 30,000 workers and reproductives. This species has multiple queens.

Pavement ants are a soil-nesting species. Structural invasions occur from next to or beneath the foundation, although this ant has been observed to cart soil up into voids and onto floors in which to nest. It also will "clear out" the soil in old termite damage in wood and deposit the soil in piles onto floors and the soil outside infested timbers and walls. It derives its name from its habit of constructing soil nests in association with sidewalks, foundations, and other "pavements" which provide protective structure under which to nest. Any object in contact with the soil can serve as a nest site including logs, stones, landscape timbers, and piles of debris. The presence of ants is usually defined by piles of displaced soil alongside such objects. Where the ant nests beneath pavement, soil piles may appear around cracks. This ant, however, is fully capable of being a "yard ant" where its mounds are in open areas of lawns or fields.

Typically, the entire colony will be located at a single site. Other pest ant species, such as the odorous house ant, establish polydomous colonies composed of many smaller subcolonies often widely dispersed and connected by trails. The author has observed pavement ants extending their colonies in soil beneath numerous consecutive stones in landscaping. In most cases, such colonies occur beneath only two to three stones. In some cases, however, a larger colony may extend 8 to 10 feet under stones and in landscape mulch next to foundations.

Activity begins in spring when ground temperatures rise above 50°F. Under buildings, activity occurs year-round, especially in homes with subslab heating. The author recalls battling a Christmas-time infestation in his home in Chicago when temperatures were well below freezing. The ants foraged beneath the carpeting next to the tack strips and were discovered only when a box of freshly baked strudels had been temporarily set on the floor in the carpeted living room. Needless to say, Mrs. Hedges was none too pleased.

Dispersal flights of winged reproductives occur in the late spring and early summer although fall swarms are not unusual. After mating, dealated queens locate a suitable protected site and deposit their first eggs 48 to 72 hours following swarming. Queens produce up to 20 eggs per day, the larvae undergo three instar stages, and development to adult worker can occur in 36 to 63 days.

Like most pest ants, the diet of the pavement ant is varied. The workers tend homopterous insects, such as aphids and mealy bugs. They are more likely to collect the honeydew from subterranean rather than above ground species. They can be pests of fruit-bearing trees around homes and in orchards where they protect injurious aphids from predators. They may also damage fruits directly. Pavement ants also collect and store seeds in chambers within the nest and will forage on carcasses of animals. They will scavenge on dead insects and any other suitable food. Place a cracker, a drop of jelly, or other food on the floor in an infested home and dozens (or hundreds) of ants may be found covering the food within minutes to hours. Often, such occurrences are the first sign that pavement ants are present.

Pavement ants forage along structural guidelines, often the baseboards and the walls underneath carpeting. Because the ants are more active at night, a professional often may not find visible activity when inspecting a home in response to a customer’s call. Pulling up the carpet along a wall near the site where the ants are seen commonly reveals a trail of ants. These trails can be followed by lifting the edge of the carpet every few feet using needle-nose pliers until reaching the point where the trail disappears through a crack in the floor or beneath the wall.

Infestations typically occur at ground level, but the ants may use plumbing or wiring to move up through walls into upper floors. An infestation in the second story of an office building was traced to ants nesting in the soil along the foundation at the ground level. The swarmers were entering the second story false ceiling and then moving into the light fixtures. Nests also have been discovered among debris and gravel on flat roofs of commercial buildings from which the ants invaded the top floor of the building.

CONTROLLING PAVEMENT ANTS. In most home situations, pavement ants are relatively easy to eliminate because (1) the colonies are fairly easy to find due to piles of displaced soil and (2) the relatively small colony size and the fact the colony is generally in one site. Complications arise, however, when the ants are nesting beneath a slab floor, especially when subslab heat ducts are involved. Ants may also pop up in the middle of large commercial buildings (i.e., hospitals or food-processing facilities) where treatment options can be severely limited.

When conducting a visual survey, any pile of soil that looks out of place should be investigated for pavement ant activity. Even the smallest pile next to a stone could be a sign a pavement ant colony is living underneath. In one case in Chicago, a large commercial building (see photo on page 46) had a vegetation-free border of large stones set in sand around it. This building experienced continual pavement ant activity indoors through the summer despite four separate surface perimeter applications ("power sprays") around the entire exterior. The author’s inspection quickly revealed small piles of soil beside numerous stones in the border. Underneath each such stone, a pavement ant colony remained fully unaffected by the surface treatments. More than two dozen colonies were found in this fashion and each was thoroughly drenched after being exposed by lifting the stones.

In general, perimeter treatments are not as effective for pavement ants as is looking for individual colonies and treating them directly. Those colonies found outside are easily drenched although it is a good idea to use a small hand rake or large screwdriver to rake through the soil and ensure penetration through the entire colony. Also, ants found beneath one stone may extend beneath adjacent stones. Digging in the soil next to the foundation, especially where thick layers of mulch exist, often reveals more colonies.

Colonies beneath sidewalks, patios, etc., where soil is being deposited on the surface next to cracks, may be treated by injecting a suitable volume of water-based residual using a crack and crevice tip or pinstream application. An aerosol equipped with a metal injection tip also may be used.

Ant nests found residing inside walls or other voids may be treated by drilling and treating using a residual aerosol insecticide labeled for wall void treatments.

Indoors, the inspection needs to determine where the ants being seen are originating. Often the offending colony is outdoors in the soil next to the foundation. If the ants appear to be nesting below the slab floor, however, the colony location may be difficult to determine. Just because the ants are disappearing through a stress crack or expansion joint in the floor does not mean the colony is directly underneath. In such cases (and sensitive areas such as hospitals), an ant bait should be attempted.

Fortunately, pavement ants respond well to ant baits. As with other ants, a "buffet" of several different baits should be offered to try and determine which that particular colony prefers. Placement of the bait then becomes problematic in that they should be placed out of sight but still where the ants will forage upon them. If the ants are under the carpet, granular or gel baits can be applied easily next to the tack strip, preferably as close as possible to the site where the ants disappear beneath the slab. If the ants are foraging in a more visible location, ant bait stations should be used or a granular, gel or liquid ant bait applied inside a station, such as Ant Cafe® or D-Sect® stations. A follow-up visit is helpful to see if the ants need more bait or a different bait might be needed.

If baits are not effective, the slab may need to be drilled and the soil underneath treated with an appropriately labeled insecticide, although this should be a last resort measure. Because you may not know exactly where the colony is located under the slab, a number of holes drilled in a checkerboard pattern may be needed. It is also beneficial to apply the insecticide using a foam application to improve spread and coverage. Of course, colonies located under slabs with subslab heat ducts should not be treated in this fashion. Persistent use of a variety of baits can be successful, but weekly visits may be needed and customer patience gained through a thorough explanation of the problems involved.

SUMMARY. Because pavement ant colonies are primarily tied to the soil, inspections need to focus at the foundation level, remembering that roof colonies, although uncommon, also can be involved. Once a colony is located, it is usually easily treated.

Subslab infestations should be controlled with baits and then subslab residual treatments may be attempted unless subslab ducts are involved.

The author is a board certified entomologist, a registered sanitarian and the manager of technical services, Terminix International, Memphis, Tenn. He can be reached via e-mail at shedges@pctonline.com.