Pharaoh Ants

Scientific Name:Monomorium pharaonis (Linnaeus) (Insecta: Hymenoptera: Formicidae)

Introduction:

The ant, Monomorium pharaonis (Linnaeus), is commonly known as the Pharaoh ant. The name possibly arises from the mistaken tradition that it was one of the plagues of ancient Egypt (Peacock et al. 1950). This ant is distributed worldwide, is one of the more common household ants, and carries the dubious distinction of being the most difficult household ant to control.

In some of the older literature this species was commonly referred to as the "Pharaoh's ant." This is now incorrect, if it ever was, as the correct common name is "Pharaoh ant," as determined by the Committee on Common Names of Insects of the Entomological Society of America (Bosik 1997). It is only mentioned here as the junior author still sees "Pharaoh's ant" on the Web and in industry publications.

Distribution

Monomorium pharaonis (Linnaeus) has been carried by commerce to all inhabited regions of the earth (Wheeler 1910). This ant, which is probably a native of Africa, does not nest outdoors except in southern latitudes and has been able to adapt to field conditions in southern Florida (Creighton 1950). In colder climates, it has become established in heated buildings.

Description

The workers of Monomorium pharaonis (L.) while monomorphic (same size), do vary slightly in length and are approximately 1.5 to 2 mm long (Haack and Granovsky 1990). The antennae have 12 segments with each segment of the 3-segmented antennal clubs increasing in size toward the apex of the club (Smith and Whitman 1992). The eye is comparatively small, with approximately six to eight ommatidia across the greatest diameter. The prothorax has subangular shoulders, and the thorax has a well- defined mesoepinotal impression. Erect hairs are sparse on the body, and body pubescence is sparse and closely appresssed. The head, thorax, petiole and postpetiole (the petiole, or the petiole and postpetiole, in ants is also called the pedicel) are densely (but weakly) punctulate, dull, or subopaque. The clypeus, gaster, and mandibles are shiny. The body color ranges from yellowish or light brown to red (Smith 1965), with the abdomen often darker to blackish (Smith and Whitman 1992). A stinger is present but is rarely exserted (Haack and Granovsky 1990).

Biology

The Pharaoh ant colony consists of queens, males, workers, and immature stages (eggs, larvae, pre- pupae, and pupae). Nesting occurs in inaccessible warm (80 to 86ºF), humid (80%) areas near sources of food and/or water, such as in wall voids. The size of the colony tends to be large but can vary from a few dozen to several thousand or even several hundred thousand individuals. Approximately 38 days are required for development of workers from egg to adult.

Mating takes place in the nest, and no swarms are known to occur. Males and queens usually take 42 days to develop from egg to adult. The males are the same size as the workers (2 mm), are black in color and have straight, not elbowed, antennae. Males are not often found in the colony. The queens are about 4 mm long and are slightly darker than the workers (Smith and Whitman 1992). Queens can produce 400 or more eggs in batches of 10 to 12 (Peacock et al. 1950). Queens can live four to 12 months, while males die within three to five weeks after mating (Smith and Whitman 1992).

Part of the success and persistence of this ant undoubtably relates to the budding or splitting habits of the colonies. Numerous daughter colonies are produced from the mother colony when a queen and a few workers break off and establish a new colony. Even in the absence of a queen, workers can develop a queen from the brood which is transported from the mother country. In large colonies there may be as many as several hundred reproductive females (Smith and Whitman 1992).

Management

Control of Pharaoh ants is difficult, due to their nesting in inaccessible areas. Treatment must be thorough and complete at all nesting sites, as well as the foraging area. Thus, treatment must include walls, ceilings, floor voids, and electrical wall outlets. Baits are now the preferred method of control for Pharaoh ants and several baits (insecticides) are labeled for indoor ant control. A Pharaoh ant infestation of a multifamily building requires treatment of the entire building to control the infestation. Ants nesting on the outside may be controlled by also using a perimeter barrier treatment (Smith and Whitman 1992).

Baits cannot be placed in just any location and be expected to work. Pharaoh ant trails and their resources (both food and water) must be located for proper placement of baits and effective control (Klotz et al. 2000). Non-repellent baits (such as boric acid, hydramethylon or sulfonamide) should be used, as repellent baits can worsen the situation by causing the colony to fracture and bud. As a result, ant activity will briefly diminish as as the new colonies establish themselves, then again become a problem as the foragers resume activity (Smith and Whitman 1992).

In addition, insect growth regulators (IGR) are marketed for indoor control of Pharaoh ants. The IGR is used as a bait, and ants must be allowed to transport the bait back to their nests. The IGR prevents the production of worker ants and sterilizes the queen. Therefore, it is necessary to allow up to several weeks or months (depending on the size of the colonies or number of colonies) for ants to die naturally with the use of IGR.