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The Pharaoh Ant (Monomorium pharaonis) is a small (2 mm) yellow or light brown, almost transparent ant notorious for being a major indoor nuisance pest, especially in hospitals. The origin of this "tramp" ant[clarification needed] is uncertain, although favoured proposals include West Africa and Indonesia. The pharaoh ant has been introduced to virtually every area of the world including Europe, the Americas, Australasia and Southeast Asia. Pharaoh ants are a tropical species but they thrive in buildings anywhere, even in temperate regions provided central heating is present.
Pharaoh workers are about 1/16-inch, or 2.0 millimeters, in length. They are light yellow to reddish brown in color with a darker abdomen. Pharaoh ant workers have a non-functional stinger used to generate pheremones. The petiole (narrow waist between the thorax and abdomen) has two nodes and the thorax has no spines. Pharaoah ant eyesight is poor and they possess on average 32 ommatidia. The antennal segments end in a distinct club with three progressively longer segments.
The pharaoh ant queen can lay hundreds of eggs in her lifetime. Most lay 10 to 12 eggs per batch in the early days of egg production and only four to seven eggs per batch later. At 27 °C (80 °F) and 80 percent relative humidity, eggs hatch in five to seven days. The larval period is 18 to 19 days, prepupal period three days and pupal period nine days. About four more days are required to produce sexual female and male forms. From egg to maturity takes about 38 to 45 days depending on temperature and relative humidity. They breed continuously throughout the year in heated buildings and mating occurs in the nest. Mature colonies contain several queens, winged males, workers, eggs, larvae, prepupae and pupae.
Each colony produces sexually reproductive individuals roughly twice a year. However, colonies raised in a laboratory can be manipulated to produce sexuals at any time of year. Colonies proliferate by "budding" (also called "satelliting" or "fractionating"), where a subset of the colony including queens, workers and brood (eggs, larvae and pupae) leave the main colony for an alternative nest site.
Pharaoh ant colonies appear to prefer familiar nests to novel nests while budding. This suggests the ability for colonies to remember certain qualities of their living space. However, if the novel (unfamiliar) nest is of superior quality, the colony may initially move toward the familiar, but will eventually select the unfamiliar. The colony assumes the familiar nest is preferable, unless they sense better qualities in the novel nest. This decision-making process seeks to minimize the time the colony is without a nest while optimizing the nest the colony finally chooses.
Pharaoh ants utilize 3 types of pheromones. One is a long lasting attractive chemical that is used to build a trail network. It remains detectable even if the ants don’t use the trail for several days. Pharaoh ants cease activity at night and begin each day of work at around 8 am. Yet, parts of the trail network are identical each day. The second pheromone is also attractive, but will decay to imperceptible amounts in a matter of minutes without reapplication. This pheromone is useful in marking food sources, as these are unpredictable and the colony must be able to respond to environmental changes quickly. Individuals will not waste their time on an unprofitable trail route. The third pheromone is repellant.  Pharaoh ants were the first species found to use a negative trail pheromone. If an individual finds an unprofitable area with little food or danger, it will release this repellant pheromone, which will warn others and cause them to look elsewhere. While positive pheromones indicating lucrative foraging sites are very common in social insects, the pharaoh ant's negative pheromone is highly unusual.  Like the food source marker, the negative pheromone is volatile. It may even be insecticidal in some cases. The chemical will decay roughly two hours after being emitted. In addition, it is so powerful that an individual can detect it 30 millimeters prior to an encounter. Pharaoh ants utilize this pheromone near bifurcations in the trail network, and an ant that detects it will begin to walk in a zigzag manner. These different pheromones allow the ants to make accurate predictions about food availability and danger, and therefore increases their decision-making ability.
Pharaoh ants use a positive feedback system of foraging. Each morning, scouts will search for food. When one finds it, it will instantly return to the nest. This causes several ants to follow the successful scout’s trail back to the food source. Soon, a large group will be upon the food. Scouts are thought to use both chemical and visual cues to remain aware of the nest location and find their way. If the colony is exploring a new region, they employ a land rush tactic, in which a large number of foragers randomly search, constantly releasing pheromones.
Even though scouts search independently, they use a distinct trail system that remains somewhat constant from day to day. The system consists of one to four trunk routes. Every scout uses one of these trunks in the beginning and end of its food search. In this way, the trunks get continuous chemical reinforcement and do not change much. Each trunk divides into many branch routes. These will change based on food availability.
Upon scouts’ return with food, the queens will attempt to beg for their share. Depending on food availability and each individual’s condition, a scout may refuse the queen’s entreaties and even run away from her. The decision of an individual to give up food to the queen may be beneficial in situations of plentiful food, as a healthy queen can reproduce and propagate the colony’s genes. However, when food is highly scarce, an individual’s own survival can outweigh this potential benefit. He will therefore refuse to give up food. A queen may also feed on secretions from larvae. This creates a positive feedback loop in which more larvae will provide more food to queens who can in turn produce more larvae. If a large amount of larvae results in a surplus of secretions, pharaoh ants will store the excess in the gasters of a unique caste, the replete workers. Members of this group have enormous gasters and can regurgitate their stored food when needed. In this way, the colony has a cushion against food shortages.
Monomorium pharaonism, similar to other invasive ants, is polygynous, meaning its colonies contain many queens (up to 200). It is hypothesized that polygyny leads to lower levels of nestmate recognition in comparison to monogynous species due to the expected higher levels of genetic diversity. Because these colonies lack nestmate recognition, there is no hostility between neighbouring colonies, which is known as unicoloniality.
Pharaoh ant colonies contain many queens. The ratio of queens to workers is variable and dependent on the size of the colony. An individual colony normally contains 1,000–2,500 workers, but often a high density of nests gives the impression of massive colonies. In a small colony, there will be more queens relative to workers. In addition, individuals will be larger than those in a more populous colony. This ratio is controlled by the workers in the colony. Larvae that will produce workers have characteristic hairs all over them, while larvae that will produce sexual males or females are bare. It is thought that workers can use these distinguishing features to identify larvae. Workers may cannibalize larvae in order to ensure a favorable caste ratio. This decision to cannibalize is largely determined by the present caste ratio. If plenty of fertile queens are present, for example, the workers may eat sexual larvae. The caste ratios are controlled in an attempt to maximize the growth of the colony.  For example, in a small colony, the ratio of queens to workers is increased. This in turn increases the potential for reproduction, allowing colony growth. Conversely, in a large colony, the high worker to queen ratio maximizes the foraging capacity of the nest, helping sustain the population size. The decision of a worker to cannibalize a certain larva is made in order to increase the probability its relatives in the nest will be successful, thereby propagating its genes.
The pharaoh ant is a polygynous species that has a relatively low worker to queen ratio of around 12.86. This allows the pharaoh ants to be able to exert social control over the size of the colony and the size of each caste. In the average nest, there are around 170 ± 8 queens every nest, which compromises around 5.2% of the total population in the nest whereas there are around 2185 ± 49 workers, which make up around 66.6% of the population. This low worker to queen ratio is usually associated with swift changes in the nest and may be why pharaoh ants form many new nest buds quickly. To branch out and form a new bud nest, pharaoh ants only need a minimum of 469 ± 28 individuals to establish a new colony, which explains how they proliferate so quickly.
Mating for pharaoh ants occurs within the nests with males that are usually not from the colony in order to ensure genetic variety. The queen can typically produce eggs in batches of 10 to 12 at once, but can lay up to at least 400 eggs every time she mates. The eggs that are produced usually take up to 42 days to mature from an egg to an adult. Each queen within the nest can survive between four to 12 months before passing away.
When the queen ant first moves to a new nest, she will rear the first group of workers. Once a worker threshold has been reached, resources will then be invested into new males and queens. When a new nest is formed, queens are not a necessity; workers can raise new queens after finding a suitable nest site.
In pharaoh ant colonies new males and queen ants can be produced when the existing fertile queen is removed. When queens are absent, the workers in the nest can do two things: either rear existing sexual larvae or transport sexual larvae from other bud nests or from the main nest to its own nest. However, when there are fertile queens still within the nest, the worker ants will cannibalize the sexual larvae and will either reject or consume sexual larvae from other nests. On the other hand, the worker ants will always accept and nurture worker larvae from other nests.  Furthermore, according to Schmidt et al, polygamous species such as pharaoh ants will have higher resource allocations towards the female caste instead of the worker caste to ensure rapid growth of new budding colonies.
When social ants encounter ants from another colony, behavior can ebe either aggressive or non-aggressive. Aggressive behavior is very commonly seen; the attacking worker usually bites the opponent at the petiole. In non – aggressive behavior, antennation occurs when the two ants meet. In the case for Monomorium pharaonis, behavior is almost always non – aggressive even when the ants are from different colonies and of different castes. 
After foraging, Pharaoh ants will wash themselves when a worker enters the cell. Pharaoh ants will also wash after a long feed. It has been proposed that washing has a hygienic value, keeping the nest area clean, staving off disease and disorder. Right before workers leave to forage, they also may wash themselves. However, in this instance the behavior is extremely violent, often causing the ants to fall over. It is thought that here, the washing behavior has no hygienic value and instead may be a displacement activity, a sign that the ants are deliberating whether or not to exit the nest. The decision to wash, which takes energy and time, must have sufficient hygienic benefits to have evolved in this species.
Budding is a major factor underlying the invasiveness of pharaoh ants. A single seed colony can populate a large office block, almost to the exclusion of all other insect pests, in less than six months. Elimination and control are difficult because multiple colonies can consolidate into smaller colonies during extermination programs only to repopulate later. Pharaoh ants are a major hazard in hospitals, where their small size means they can access wounds and medical instruments, causing the spread of infection and electrical interference.
Pharaoh ants have become a serious pest in almost every type of building. They can feed on a wide variety of foods including grease, sugary foods, and dead insects. They can also gnaw holes in silk, rayon and rubber goods. Nests can be very small, making detection even more difficult.  They are usually found in wall voids, under floors, or in various types of furniture. In homes, they are often found foraging in bathrooms or near food.
Pharaoh ants have been exterminated by placing baits, consisting of ground liver mixed with boric acid, in places where the ants forage. Renewing the baits once or twice may be necessary. It is recommended not to exterminate using sprays and dusts because they will cause the pharaoh ants to scatter.
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