Pharaoh Ants: Why Spraying Splits the Colony
Pharaoh ants are tiny yellow to light brown indoor ants, about 1.5 to 2 mm long, and the University of Florida calls them the most difficult household ant to control. Colonies hold many queens and split into new nests when disturbed, so repellent sprays usually scatter them rather than kill them. Slow-acting baits, including insect growth regulator baits, carried back to every nest are the method extension entomologists recommend.
The species, Monomorium pharaonis, has been carried by commerce to every inhabited region of the world. In cold climates it lives only inside heated buildings, nesting in wall voids, behind baseboards and in the folds of stored linen. It is the reason apartment buildings and hospitals sometimes battle the same ant for years, and the reason a can of ant spray can turn one trail into five.
Pharaoh ant identification
Three features together point to a pharaoh ant: its very small size, its pale yellowish to reddish colour with a darker abdomen, and a two-segmented waist. Under a hand lens, the antennae have 12 segments ending in a three-segmented club, which is the reliable way to separate it from the thief ant.
| Pharaoh ant | Thief ant | Ghost ant | Odorous house ant | |
|---|---|---|---|---|
| Worker length | 1.5 to 2 mm (about 1/16 in) | About 0.8 mm (1/32 in) | 1.3 to 1.5 mm | About 1/8 in (3 mm) |
| Colour | Yellowish or light brown to red, abdomen often darker | Yellow to light brown | Dark head and thorax, milky white abdomen and legs | Dark brown to shiny black |
| Waist segments | Two | Two | One, hidden | One, hidden |
| Antenna | 12 segments, 3-segmented club | 10 segments, 2-segmented club | 12 segments, thickening towards the tip | Not a field mark |
| Preferred food | Fats, proteins, sweets | Greasy and fatty, sometimes sweets | Sweets | Sweets, sometimes proteins |
| Usual nest | Indoors in warm voids, all year in cold regions | Outdoors in soil; indoors behind baseboards | Outdoors in Florida, Hawaii, parts of Texas | Shallow mounds outdoors, or indoor wall voids |
Compiled by The Pest Control Brief from UF/IFAS EENY290 (pharaoh ant, thief ant antennae) and EENY310 (ghost ant), UC IPM Pest Notes: Ants and its household ant key (thief ant and odorous house ant size, colour, foods and nests). Wikipedia’s pharaoh ant article gives worker lengths of 2.9 to 4.5 mm, which does not match UF’s measured 1.5 to 2 mm.
Pharaoh ants vs ghost ants
The two get confused because both are tiny, pale and trail to kitchen and bathroom sinks. A ghost ant is two-tone, with a dark front half and a see-through, milky abdomen, and has one waist node. A pharaoh ant is one fairly even honey colour with a slightly darker abdomen and has two nodes. Our ghost ant guide covers that species in detail.
Are pharaoh ants ever black?
The males are. UF describes pharaoh males as black, the same 2 mm as the workers, with straight rather than elbowed antennae, but notes they are not often found in the colony. The workers you see on a trail are yellow to light brown; a trail of tiny black ants is usually a different species, which our ant identification guide helps narrow down.
Why spraying splits the colony
Most ants start new colonies with a winged queen on a mating flight. Pharaoh ants do not. UF reports that mating takes place inside the nest and no swarms are known; new colonies form by budding, when a queen and a few workers break off from the mother colony. Workers can even raise a new queen from brood they carry with them. Large colonies may have several hundred queens, so there is no single nest to destroy.
That is why repellent treatments backfire. UF advises non-repellent baits because repellent baits can worsen the situation by causing the colony to fracture and bud, with activity briefly dropping and then returning as the new colonies settle in. Texas A&M’s urban entomology programme says liquid or dust repellent insecticides are not recommended indoors because they typically cause colony budding.
What the laboratory test showed
For decades this was received wisdom with little direct evidence: Purdue entomologists wrote in 2005 that no detailed experimental data existed to support or refute it. Their study, published in the Journal of Economic Entomology, gave pharaoh ant colonies of about 4,000 workers and 20 queens a choice between staying beside treated surfaces and moving to empty nests 100 feet away.
- Only the repellent pyrethroid cypermethrin made reproductive colony fragments, with queens or brood, move out: in three of three replicates on vinyl tile and one of three on ceramic tile.
- The move came late, around day 35, long after the treatment, which may be why people who spray see a lull and then a return.
- On vinyl tile, colonies exposed to cypermethrin grew by 27 per cent over six weeks.
- The non-repellent fipronil killed 99 to 100 per cent of workers on concrete and mulch, and 100 per cent of queens.
These were professional-use products tested on laboratory colonies, and the authors themselves suggest combining baits with non-repellent treatments in large buildings. The homeowner lesson is narrower: do not spray a pharaoh ant trail with a household ant spray, and do not use a spray while baits are out. UC IPM gives the same advice for all ants: no insecticide sprays while you are using baits.
Are pharaoh ants dangerous to humans?
Not directly in a home. The risk is contamination, and it matters most in hospitals. UF notes that pharaoh ants can carry over a dozen pathogens, including Salmonella, Staphylococcus and Streptococcus, and have been seen seeking moisture from the mouths of sleeping infants and from in-use IV bottles. In Texas, an infestation ran through a seven-floor medical centre.
| Study | Setting | What it found |
|---|---|---|
| Beatson, Lancet, 1972 | Hospitals | Described pharaoh ants as pathogen vectors in hospitals |
| Edwards and Baker, Journal of Hospital Infection, 1981 | NHS hospitals in England, about half surveyed | 11.6 per cent of surveyed hospitals infested, as cited to Parliament in 1986 |
| Polish hospital survey, 1997 | 748 hospitals, 1990 to 1995 | Pharaoh ants in 17 per cent of hospitals, behind German and Oriental cockroaches |
| Lise and colleagues, 2006 | Two hospitals, Santa Catarina, Brazil | Pharaoh ant was the most frequent of seven ant species (71.5 per cent) |
| do Nascimento and colleagues, 2020 | Emergency hospital, Macapá, Brazil | 39.2 per cent of 9,687 ants collected were pharaoh ants; 38.3 per cent of them carried bacteria |
Compiled by The Pest Control Brief from the published abstracts and UF/IFAS EENY290. These studies show the ants carry bacteria on their bodies in hospitals; they do not show that pharaoh ants in a house make people ill. Throw away food the ants have reached, and see a doctor about any health concern.
Do pharaoh ants bite?
Rarely, and not as a recognised household problem. UF notes that workers have a stinger but rarely extend it, and Texas A&M states they do not sting humans. A 1978 German hospital report described pharaoh ants injuring the skin of premature infants, which is one reason hospitals treat them as serious. If you are getting bites indoors from something you cannot see, our bite identification guide is a better starting point.
Where do pharaoh ants come from?
UF considers the species probably native to Africa, while Texas A&M calls its origin uncertain. The name comes from the mistaken idea that it was one of the plagues of ancient Egypt. It now occurs throughout the United States, where Texas A&M says it is generally limited to heated structures, nesting outdoors only in subtropical areas such as Hawaii, Florida and southern Texas. UF gives nest conditions as warm, 80°F to 86°F, and humid, about 80 per cent, near food or water.
Our analysis of iNaturalist research-grade records shows why outdoor ant seasons do not explain a pharaoh ant problem. The Monomorium genus, which mixes pharaoh ants with native outdoor species, runs at 124 to 132 on our index from March to August and falls to 40 in January; odorous house ant sightings peak at 153 in spring. Pharaoh ants themselves are rarely recorded: iNaturalist held only about 1,300 observations of the species worldwide when we checked in October 2026. An indoor colony in a heated building keeps breeding all winter.
Why do I suddenly have pharaoh ants?
- A neighbouring unit. UF states that an infestation in a multifamily building needs treatment of the entire building, because colonies spread through walls along wiring and hot water pipes.
- A recent spray. If you or a neighbour sprayed a trail a few weeks ago, budding colonies may now be foraging from new nests.
- Something carried in. UF records nests between sheets of stationery, in layers of bed linen and clothes, in appliances and in rubbish, so colonies travel with belongings.
- A new food or water source. Sweet, fatty or oily food left uncovered even briefly, UF notes, will draw a trail. Bathrooms are common because of the moisture; see our guide to ants in the bathroom.
How to get rid of pharaoh ants
- Confirm the species with the size, colour and two-node waist above. Treatment for sweet-feeding ants such as those covered in our sugar ants guide is not the same.
- Stop spraying. Put away contact sprays and foggers; they kill the foragers you see and can trigger budding.
- Test which bait they take. UC IPM says combination fat-based baits with sweets work longer than sweet baits alone, and suggests offering small portions of each type first. Place enclosed bait stations on every active trail, away from children and pets, and follow the label.
- Choose slow-acting baits. UC IPM lists hydramethylnon, sulfluramid, low-concentration boric acid (under 1 per cent) and fipronil as effective for pharaoh ants. Fast killers defeat the purpose: as few as 1 per cent of a colony’s foragers can feed the queens and larvae.
- Consider an insect growth regulator bait. UF explains that IGR baits stop worker production and sterilise queens, and need several weeks to months to work. Methoprene and pyriproxyfen are the two studied most.
- Keep baits topped up and wait. Texas A&M says a baiting programme may take several months to a year. Clean up spills and store food sealed, so the bait is the best meal available.
The bait trial behind that chart, by Oi, Vail and Williams in 2000, explains why patience pays. A fast-acting hydramethylnon bait cut workers and brood near the bait by at least 80 per cent within three weeks, but ants from other colonies moved into the emptied nests. The slow pyriproxyfen IGR bait spread to all four colonies, cutting brood by 95 per cent at every nest by week three and workers by 73 per cent by week eight. Earlier work by Vail and Williams found pyriproxyfen baits at 0.25 to 1 per cent outperformed the older methoprene bait, while a 1983 trial by Rupes and colleagues eradicated pharaoh ants from health buildings and an apartment house with methoprene bait, and expected eradication within 100 days where bait reached every colony.
When to call a professional
Call a licensed pest control company if you live in an apartment or condo building, if the trail keeps returning despite steady baiting, or if ants are in a care home, clinic or anywhere food is prepared commercially. Building-wide baiting and treatment of wall voids and electrical boxes is professional work. Our guides to apartment and condo pest control and ant control costs explain what to expect.
Frequently asked questions
Are pharaoh ants hard to get rid of?
Yes. The University of Florida calls the pharaoh ant the most difficult household ant to control, because colonies have many queens, nest in inaccessible wall voids and split into new nests when disturbed. Slow-acting baits placed on every trail, including insect growth regulator baits, work best, and Texas A&M notes a baiting programme can take several months to a year.
Are pharaoh ants harmful?
They are a contamination risk rather than a bite or sting risk. In hospitals they have been found carrying bacteria such as Salmonella, Staphylococcus and Streptococcus, which is why infestations there are treated as serious. In homes the main harm is spoiled food that has to be thrown away. For any health concern, speak to a doctor.
Why are they called pharaoh ants?
The University of Florida says the name possibly comes from the mistaken idea that the ant was one of the plagues of ancient Egypt. Older literature calls it Pharaoh’s ant, but the Entomological Society of America’s approved common name is pharaoh ant. The species is probably native to Africa and has been spread worldwide by commerce.
Why do I suddenly have pharaoh ants?
Usually because a colony has budded into your space from a neighbouring unit or wall void, often after someone sprayed it, or because ants arrived in belongings, appliances or stored linen. They follow wiring and hot water pipes between floors. A new food or water source, such as uncovered sweet or greasy food, then draws a visible trail.
Sources
- Nickerson, Harris and Fasulo, Pharaoh Ant, Monomorium pharaonis, UF/IFAS Featured Creatures EENY290 (revised October 2024).
- Nickerson, Bloomcamp and Pereira, Ghost Ant, Tapinoma melanocephalum, UF/IFAS EENY310.
- Texas A&M AgriLife Extension, Urban and Structural Entomology Program: Pharaoh ant.
- University of California Statewide IPM Program, Pest Notes: Ants; Key to Identifying Common Household Ants: Pharaoh ant and Managing pharaoh ants with baits.
- Buczkowski, Scharf, Ratliff and Bennett, Efficacy of simulated barrier treatments against laboratory colonies of pharaoh ant, Journal of Economic Entomology 98: 485 to 492, 2005.
- Oi, Vail and Williams, Bait distribution among multiple colonies of pharaoh ants, Journal of Economic Entomology 93: 1247 to 1255, 2000.
- Vail and Williams, Pharaoh ant colony development after consumption of pyriproxyfen baits, Journal of Economic Entomology 88: 1695 to 1702, 1995.
- Rupes, Chmela, Hrdý and Krecek, Effectiveness of methoprene-impregnated baits in the control of Monomorium pharaonis, Journal of Hygiene, Epidemiology, Microbiology and Immunology 27: 295 to 303, 1983.
- Beatson, Pharaoh ants as pathogen vectors in hospitals, Lancet, 1972; Edwards and Baker, Journal of Hospital Infection 2: 249 to 254, 1981, as cited in Hansard, House of Commons written answers, 28 April 1986.
- Rocznik Państwowego Zakładu Higieny 48: 295 to 303, 1997 (Polish hospital survey); Lise, Garcia and Lutinski, Revista da Sociedade Brasileira de Medicina Tropical 39: 523 to 526, 2006; do Nascimento and colleagues, Journal of Medical Entomology 57: 1619 to 1626, 2020.
- Angewandte Parasitologie 19: 28 to 31, 1978 (pharaoh ants in a ward for premature infants).
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
