Another Exotic Ant On The Prowl

Many of the more serious pest ants in the United States are not native species. Imported via shipping activities, these exotic ants have adapted well to life in this country. Called as a group “tramp ants,” certain pest species have many characteristics in common (see PCT, April). The white-footed ant, Technomyrmex albipes (fr. Smith), is one such species that is becoming a serious pest in South Florida and Hawaii. Pest man-agement professionals working in these areas are all too familiar with this persistent pest ant, and many simply do not care to service buildings plagued by this ant. This article will examine this interesting ant and discuss the reasons why it can be so difficult to control.

Identification. White-footed ants belong to the ant subfamily Dolichoderinae and have many of the same characters as other common dolichoderines, especially the odorous house ant, Tapinoma sessile (Say). Like the odorous house ant, the white-footed ant has a single flattened node between the thorax and abdominal gaster. This node is also hid-den from view by the gaster. Workers are a bit larger than those of the odorous house ant being somewhat more than 1/8-inch (4 mm) in length.

The body shape resembles that of the crazy ants, Paratrechina, spp., especially P. bourbonica (Forel). The legs and antennal scape (first long segment) of the white-footed ant, however, are much shorter. The white-footed ant is black in color and is most easily identified by the pale yellow coloring of the tarsi at the end of the legs which gives this ant its name.

Geographical Distribution. This ant is a particular pest in South Florida in Dade, Broward and Palm Beach Counties and has been found recently in Naples and Sarasota. It is also established in the Hawaiian Islands, particularly on Oahu. Colonies have been reported in San Francisco, California.

Biology. The first detailed, documented ac-count of this species occurred in 1991 where the white-footed ant was found invading a symposium where, ironically, the wholesale invasions of exotic biota was being discussed at the Florida International University in Miami. This is not a new species in South Florida as it has been reported as early as 1922 by W.M. Wheeler, the noted ant specialist. It is currently increasing its status as a structural pest.

This ant is now an established pest of structures in south Florida, and it may very well be the most difficult structure-infesting ant to control where it occurs. Many pest management professionals have literally “thrown up their hands” in surrender when attempting to control this species.

The white-footed ant appears to be more of a pest of homes than commercial buildings, but given time, this species should become more widespread in its range and infest all types of structures.

The colonies have multiple queens and can be huge, containing hundreds of thousands, even one million or more workers. Half of the colony is workers, and the other half consists of “intercastes” that have the characters of both queens and workers and which can produce eggs and assist in colony reproduction. Satellite colonies are formed and connect together by foraging trails, creating a large extended polydomous (multiple nest) colony or “supercolony.” A single extended colony of this ant has been observed to affect several homes, thus confounding control efforts directed at single dwellings.

White-footed ants nest in most of the same types of locations as the crazy ants of the genus Paratrechina. Literally any object on the ground or heavy vegetation lying against foundations or the base of trees may be invaded as a nesting site. A single colony will encompass numerous nesting sites both close to and far away from where a single nest is found. A regular exchange of workers, brood and food occurs between all sites of these extended colonies. This author has found this ant nesting in soil at the base of trees and under loose bark on those trees at Florida International University where Dr. Mark Deyrup reported first seeing this species in 1991.

This ant is fond of nesting in dead wood, and it will invade and short out air conditioners much in the same manner as the red imported fire ant, Solenopsis invicta.

This species establishes set foraging trails that are generally easy to find outside infested buildings. Foragers will follow structural guidelines over much of their trails. Workers maintain contact with edges of sidewalks, edges of bricks on buildings, ledges and corners of soffits. Workers will often move onto buildings from tree and shrub limbs that brush against the walls or roof.

White-footed ants are particularly fond of sweets and, like many ants, will tend and protect homopterous insects (aphids, mealybugs, and scales) that produce “honeydew.” A unique observation is that no trophallaxis (oral exchange of food) seems to occur among adults of this species. Nutrient transfer occurs via specialized trophic eggs that are produced by all females (dealate queens, intercastes and workers).

Huge colonies containing several million adults are formed and winged females emerge annually in large numbers from late May to mid–June. These winged females mate within the colony with wingless males prior to nuptial flights. Mated female alates will found in new colonies but are ultimately replaced by reproductive intercastes.

Managing Infestations. Of course, the key to success with the white-footed ant is to find and treat the colonies directly. Finding all of the subcolonies, however, is not easy and may well be impossible given the fact that many subcolonies may be found on neighboring properties.

Outside, trails of white-footed ants will usually be quite visible on walkways, foundations and the sides of buildings. Once a trail of ants is found, workers whose abdomens appear swollen with food or water will usu-ally be returning to the colony. The trail should be followed in the direction in which these workers are heading.

Any object lying on the ground, such as lumber, stones, or trash, is a potential nesting site.

Piles of any item are especially coveted by this ant (and other ants), particularly items in shaded, damp areas. Landscape mulch should be moved away from the building foundation and the base of trees and shrubs to check for ant colonies. A small hand-held garden rake should be used to avoid contact with stinging pests, such as scorpions or fire ants. These ants may forage long distances from the colony so a thorough inspection of the entire property is necessary.

Branches of trees and shrubs brushing against the building should be checked for trails for foraging ants. Nesting ants are often found behind loose bark on trees. Nests in soffits and attics are not out of the question, especially if the attic is poorly ventilated and humid or excess moisture is evident in the soffits.

The soil of potted plants is a potential nesting site. Where possible, lifting potted plants off the ground or floor will reveal piles of soil displaced by nesting ants.

Inside, these ants may nest in wall voids and in the soil of potted plants and atriums. The workers will forage along baseboards both above and below the edge of carpets.

When this ant is observed foraging indoors, an attempt should be made to follow the trail to the point where the ants enter under a wall or through a crack, e.g., under a doorway. If ants are observed entering under a wall, the foundation and wall outside that area may reveal ants entering from a colony located outdoors.

As with most ants, moisture is a limiting factor in the survival of this ant. It is nearly impossible to significantly impact an infestation of these ants by attempting to limit moisture conditions. If conditions exist which result in excess moisture, e.g., sprinklers that spray onto the foundation, they should be corrected.

A key factor in preventing invasions by white-footed ants is to seal as many cracks as possible in the exterior of the structure. Another important consideration is the role of flowering plants and shrubs next to structures. These plants are subject to aphid infestations, which serve to attract the ants. Limiting such plants can be a helpful control strategy when a building is chronically infested by this species.

Branches of trees and shrubs need to be pruned away from the structure’s roof and walls. As many cracks in the exterior walls as possible need to be sealed. If possible, a vegetation-free border should be installed around the base of the foundation to make the area less attractive to ants and to make inspections of the foundation easier. This vegetation-free border should contain gravel or small stones and NOT bark chips or mulch. Mulch and bark chips hold moisture, creating an ideal nesting site for a variety of ants, including white-footed ants.

Items under which these ants could nest should be removed, including boards, bricks, stones and logs. Firewood should be stored off the ground and as far from the structure as possible.

Treating Infestations. Complete elimination of infestations of this species inside structures on a long-term basis is nearly impossible in areas where this ant is well established. When a structure and its surrounding property is serviced to eliminate the existing colonies, new colonies will quickly move in from neighboring properties. Regular inspections and/or treatments, therefore, are necessary in areas where this ant is common.

Three steps are needed to control this ant: (1) correcting any conditions contributing to the infestation; (2) locating and treating existing colonies; and (3) targeted treatments with a residual, liquid insecticide to help prevent ants from entering the structure.

Correction of contributing conditions was discussed previously.

A thorough inspection of the property must be made inside and out to identify all possible white-footed ant colonies on the property before ANY treatment is performed.

Ant colonies living in the soil or under items, such as stones or logs, should be treated by drenching each individual colony with an appropriately labeled residual insecticide, using a compressed air sprayer or backpack sprayer. Larger colonies will require a greater volume of insecticide. Raking through the soil with a small hand rake before and after treatment will allow more thorough penetration throughout the colony.

Numerous ant colonies living in mulch in landscaping beds can be treated by drenching with an appropriately labeled residual insecticide using a backpack sprayer or a truck-mounted power-treating unit set at a low pressure. The mulch should be raked back to expose the colonies, the colonies drenched thoroughly and the surface treated after the mulch has been raked back into place.

Ant colonies living inside wall voids can be treated by drilling a small hole into the wall and injecting a small amount of an appropriately labeled aerosol or dust insecticide. Insulated walls will require an aerosol application. An aerosol-generating machine, such as the Actisol^® or Micro-Injector,^® can also be used.

White-footed ants may locate nests behind brick and stone veneer. The space be-tween the brick veneer and the outer wall of the building makes an ideal harborage/nesting site for many types of ants (and other pests). Foraging workers enter and exit from weep holes and other exterior cracks and holes and will enter the structure through any available crack behind the veneer. When chronic ant infestations are encountered that have resisted all other efforts of control, this situation may be the source if the building has brick or stone veneer.

Treatment behind brick and stone veneer is best accomplished by drilling ¼-inch holes every 4 to 6 feet through the mortar joints of the veneer. A residual dust insecticide can be applied using a large hand-operated duster or a suitable electric duster. Another option is to apply a residual water-based insecticide using an aerosol-generating machine, such as the Actisol^® or Micro-Injector.^® The holes should be patched following treatment.

Perimeter treatment with a residual liquid insecticide is a method frequently used as a primary control technique for this species. This strategy will usually only provide temporary relief from ant sightings. As many nesting sites as possible on the entire property must be located and eliminated. Nest location takes considerable time and effort but currently is the best approach.

Most perimeter treatments generally are applied to repel foraging ants, especially when pyrethroids are used. This approach, however, may well compound the infestation. Parts of colonies “cut off” inside by re-pellent insecticides may cause more foraging activity indoors than might ordinarily be seen. Increased interior activity prompts more customer complaints and requests for additional service from pest management professionals.

Treatments targeted directly at likely ant nesting sites, such as landscape beds, work better than nonspecific, broadcast applications of insecticides to the entire foundation and 3 to 10 feet out from the building. An appropriately labeled wettable powder or microencapsulated formulation should be used. The treatment can be applied using a backpack sprayer or truck-mounted power-treating unit.

Ant baits are not considered effective for white-footed ant infestations due to their unique food cycle within the colony. Workers feed primarily on liquid foods, especially the honeydew produced by aphids. The workers then produce trophic eggs, which are fed to the larvae.

Any bait that would be effective must be liquid in form. Several manufacturers are currently developing a liquid bait which may prove useful in future situations involving this pest ant.

Stoy A. Hedges, a board certified entomologist, is manager of technical services for Terminix International, Memphis, Tenn.