Pharoah Ant

Brief:

The Pharaoh ant, Monomorium pharaonis, so named out of a belief that it may have been one of the insect plagues of Ancient Egypt, 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 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 2 mm in length. They are light yellow to reddish brown in colour with a darker abdomen. There is a non-functional stinger used to generate pheromones. The petiole has two nodes and the thorax has no spines. Eyes are poor and possess on average 32 ommatidia. The antennal segments end in a distinct club with three progressively longer segments.

Habits

The Pharaoh ant is polygynous, meaning its colonies contain many queens (up to 200). An individual colony normally contains 1,000–2,500 workers but a high density of nests gives the impression of massive colonies. Colonies also lack nestmate recognition so there is no hostility between neighbouring colonies, which is known as unicoloniality. They produce sexually reproductive individuals roughly twice a year in established colonies but in the laboratory colonies 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. 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 made difficult because multiple colonies can also consolidate into smaller colonies and be unaffected by a baiting program only to repopulate when baiting is withdrawn.

Pharaoh ants have become a serious pest in hospitals, rest homes, apartment dwellings, hotels, grocery stores, food establishments and other buildings. They feed on a wide variety of foods including jellies, honey, shortening, peanut butter, corn syrup, fruit juices, baked goods, soft drinks, grease, dead insects, toothpaste and even shoe polish. They can also gnaw holes in silk, rayon and rubber goods.

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. 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.

Habitat

Pharaoh ants use pheromones as the primary means of communication. Parts of certain colonies often migrate to new locations. Nests can be very small, located between sheets of paper, in clothing or laundry, furniture, foods, etc. Nests usually occur in wall voids, under floors, behind baseboards, in trash containers, under stones, in cement or stonewall voids, in linens, light fixtures, etc. They prefer dark, warm areas near hot water pipes and heating tapes, in bathrooms, kitchens, intensive care units, operating rooms, etc. They are ‘trail-making’ ants and often are found foraging in drains, toilets, washbasins, bedpans and other unsanitary sites as well as in sealed packs of sterile dressing, intravenous drip systems, on surgical wounds, food and medical equipment.

Threats

Pharaoh ants are a major hazard in hospitals, where their small size means they can access wounds, driplines, and instrumentation, causing the spread of infection and electrical interference.

Prevention

Pharaoh ants are often 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.

Others

Some photos and information are provided by WR Gay Pest Control Pty Ltd and Bayer Environmental Science.

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