Description of
Pharaoh's Ant
(Monomorium pharaonis)

Habitat
Tropical species found only in heated buildings in UK. Small, mobile and active at all times of day. Form large, apparently unstructured, nests in cavities and heating ducts. Many queens per nest. Workers seen in distinct trails between nest and food sources. Forage on wide range of foods and need access to water. Infestations spread by "budding", groups of workers and sometimes queens leaving the main nest with brood to form satellite colonies.

Biology
Egg – larva – pupa – adult.
Eggs – laid by queens within nest, hatch in about 1–2 weeks.
Larvae – legless grubs fed by young workers in nest, full grown in about 3–4 weeks
Pupae – develop in about 2 weeks guarded by workers in nest.
Adults – workers wingless, 2mm long, yellowish, live about 10–12 weeks. Queens winged, 4-5mm, normally stay in nest, live up to 10 months. Minimum development temperature 18 °C, optimum 30°C.


Importance
May carry disease organisms. Contamination of food and sterile materials.
Can penetrate packaging due to small size. Ability to travel through ducting and building structure means infestations can become very widespread. Satellite nests may be transported from one building to another.


Control
Only necessary where ants become serious nuisance within building. Proof entry points with sealants. Residual sprays, dusts or aerosols (some cleared for use by general public) may be used to kill or deter entry of ants. Bendiocarb particularly effective. Slow acting poison bait (sodium tetraborate) also available.

Distribution and Habitat

Successful control only by baiting techniques, not by residual treatments.
Hydramethylnon, a slow acting poison, and S-methoprene, a juvenile hormone that disrupts larval development, are available as ready-to-use baits.


Biology

Unlike the common black ant where one queen lays all the eggs, Pharaoh's ant colonies contain several queens, all of which may lay eggs. Queens have wings when they first emerge but these are soon lost. Winged males will appear from time to time and mate with new queens. And so the colony will continue unabated and probably increase in size, with old queens being replaced by new ones, provided that the necessary physical and food requirements are maintained. The ultimate nest size appears to be dependent more on physical space availability rather than an absolute maximum number, and nests of up to 50,000 workers and 100,000 young stages have been found.

The Pharaoh's ant, in common with all species of the order Hymenoptera (bees, wasps and ants), follows a complete metamorphosis (life cycle). Development times vary mainly according to temperature. The eggs laid by the queen hatch after one to two weeks into tiny larvae, or grubs. These are fed and tended by the sterile female workers and take about two to four weeks and several skin changes (ecdyses) to reach the pupal stage. This stage lasts two to four weeks and is protected by the workers.

Workers will move pupae and young larvae away if danger threatens the nest. This may also happen deliberately to start off new colonies, a process known as budding by which ants gradually spread throughout buildings or complexes.

Sometimes new queens will accompany these migrating groups. This is not essential since the workers can produce new queens, and males, from the developing young.

The tropical origins of the ant are indicated by its requirement of a minimum breeding temperature of around 18 °C, with an optimum of around 30 °C. They are usually associated with central heating systems, ovens and sometimes heat exchange units (calorifiers) in institutions such as hospitals, prisons and increasingly in high-rise apartment dwellings, when these are heated throughout the year. It is the worker Pharaoh's ant that is most commonly encountered. These sterile females are the foraging caste and will search for suitable food sites many metres from their nest location. They communicate information regarding suitable food sites to other workers using chemical (pheromone) trails and it is common to see long narrow trails of worker ants moving to and from the food source. The ants also search out liquid water for drinking and this often brings them into contact with unsavoury areas such as drains and waste-food collection bins.

Importance

It is clear that the foraging activity of the worker ants, combined with their small size and ability to chew through materials, can be responsible for the transmission of disease bringing organisms from dirty and unsavoury situations to clean food, and perhaps more importantly, into otherwise sterile hospital supplies. They have quite a wide taste in food materials but are known to be particularly attracted to suppurating wounds of patients in hospitals causing intense irritation and the ever-present prospect of the transmission of pathogenic bacteria.

The importance of Pharaoh's ant is also related to the difficulty of eradication. The worker ants ability to "bud-off" new colonies where suitable conditions exist, or if the original colony is subjected to unusual stress, frequently results in many smaller colonies existing as satellites. A building may be infested with Pharaoh's ants that will not necessarily be located at one nest site. Additionally, the ants are spread quite easily from building to building by the temporary formation of very small nests in movable items such as clothing, bedding and even in linen baskets. The cost of survey and control for a Pharaoh's ant infestation can be considerably greater than anticipated due to the extent and complexity of the nest-formation characteristics.

Control

General
This pest can be particularly difficult to eradicate and has been present in some premises for many years, despite continued efforts to control it.

Residual sprays and dusts are usually ineffective in eradicating Pharaoh’s ants. Their use may bring temporary relief in small areas but apparent success is usually illusory. The difficulty of accessing nests means that such treatments rely on killing workers in an attempt to starve the nests (containing brood, nurse workers and queens) of their food supply. This rarely works because insufficient numbers of workers are killed and nests also contain reserves of food. Furthermore, the presence of insecticides within range of a nest may induce migration away from the treated area, encouraging the spread of infestation.

The strong trailing habit of the worker ants has encouraged the use of baiting techniques that currently offer the most effective method of control.

Baits may contain either a slow acting stomach poison or an insect growth regulator.

Baiting techniques rely on the continued attractiveness of the bait base over a period of time, long enough to bring about control. It has been noted that Pharaoh’s ants may exhibit a phenomenon known as "bait tiredness" whereby workers will cease to be attracted to a particular bait formulation if this is used for a long period.

Whichever product is used a thorough treatment must be carried out to ensure that all nests are treated. It is important that a detailed survey of the premises is undertaken to determine the extent and degree of infestation. It is also important to ensure that no residual insecticide treatments have been carried out prior to baiting, which might reduce the foraging activity of worker ants.

Survey
Except in very heavy infestations of Pharaoh's ants, visual survey does not easily detail the extent and degree of an infestation. It is usual to carry out a pre-treatment survey by placing small amounts of attractive bait throughout the entire site, including adjacent properties. The baits are then examined after 24 hours for ant presence or signs of being eaten by ants.

Any attractive bait base may be used including plain raw liver.
Mixtures containing combinations of liver (fresh or powdered), peanut butter, sponge cake and honey presented in short lengths of large diameter drinking straws or tubing are more convenient and less messy. Ready-to-use survey/monitoring bait stations are also available.

Baits may be applied at a density of one bait per 10–50m2 in areas likely to attract workers. It is important to ensure an even distribution of baits so that no infested areas are missed.

Slow acting poisons
Slow acting poisons formulated in an attractive bait base are collected by worker ants and transported back to the nest where they are fed to larvae and adults. The delayed toxicity allows time for the workers to transport a sufficient quantity of bait to the nest before they themselves succumb. With this method all stages, including workers, should be killed.

Ready-to-use bait stations containing hydramethylnon formulated specifically for Pharaoh’s ant control are applied at a recommended average rate of one bait per 10m2.

Juvenile hormone baits containing S-methoprene
S-methoprene is a juvenile hormone analogue, a substance that mimics a naturally occurring insect hormone. Mixed with an attractant bait base it is collected by workers and fed to the larvae and queens, This has the effect of (i) sterilising queens to prevent further production of viable eggs, and (ii) preventing developing larvae from reaching the adult stage. The workers are not directly affected but as they die at the end of their natural life span (10–12 weeks) they are not replaced by emerging adults and the colony gradually declines.

Currently S-methoprene is available, formulated with an attractant bait base, as ready-to-use bait stations. These are applied at a recommended average rate of one or two baits per 5m2, -depending on the level of infestation. Control should be achieved in 12–14 weeks if used correctly.

S-methoprene treatments do not have an immediate dramatic effect but in most cases it is better to rely on bait alone, used as directed. In some sensitive situations (e.g. hospital wards or operating theatres) where ant worker numbers must be reduced quickly, a localised treatment could be carried out using a residual insecticide at 10–14 days after the completion of the baiting treatment. Most pyrethroid or carbamate/OP active ingredients will kill Pharaoh's ants. Bendiocarb is particularly effective in either wettable powder or dust formulation. Residual insecticides should not be used immediately before or during baiting treatments because if workers are killed (too quickly) the bait may not be transported back to the nest. Disruption of the colony structure may also result in the nest being "moved" to a new location.