Pharaoh Ants: Why Spraying Them Can Make an Infestation Worse

Pharaoh ants (Monomorium pharaonis) are tiny — workers run only about 1.5 to 2 millimeters long — and a pale yellowish-brown color that makes them easy to mistake for several other small household ant species. The University of Florida's EDIS fact sheet (EENY-290) and Ohio State University Extension both single this species out as among the most difficult indoor ants to control in North America, and the reason has little to do with the ant's size and everything to do with how its colonies respond to being disturbed.

A colony structure built around splitting apart

Unlike many ant species, pharaoh ants don't start new colonies through a mating flight — in fact, according to UC IPM, neither males nor queens in this species can fly at all. Instead, new colonies form through a process called budding: a group of workers carries brood, and often one or more queens, to an entirely new nesting site, and colonies routinely operate with multiple queens at once rather than a single one. UC IPM notes that budding tends to happen specifically when a colony grows too large, or comes under stress from temperature swings, water shortages, or food scarcity — and, critically, when it's disturbed by a repellent insecticide application.

Why a spray treatment backfires

That last trigger is the core problem with treating a pharaoh ant sighting the way most people treat an ordinary ant trail: with a surface spray. A repellent insecticide doesn't need to kill the colony to make things worse — it only needs to disturb it enough to trigger budding. Workers and queens scatter outward into new voids and wall cavities in response, and a single nest can fracture into multiple new satellite colonies spread through a structure, each starting fresh and out of reach of whatever treatment triggered the split in the first place. Reports on pharaoh ant infestations in large buildings describe exactly this pattern: an infestation that was concentrated in one area spreading into many more locations shortly after a spray treatment, rather than resolving.

Why hidden pathways make the problem worse

Pharaoh ants nest in wall voids, behind baseboards, inside insulation, and — in larger buildings — inside wiring chases, plumbing voids, and elevator shafts, all of which function as internal highways that let a colony move and forage across a structure largely out of sight. That combination of hidden nesting and a tendency to split under pressure is exactly why university extension guidance consistently steers away from contact or repellent sprays for this species and toward slow-acting bait products instead. Ohio State University Extension notes pharaoh ants can also be transported between locations in laundry, groceries, and paper products, which is part of why isolated sightings in a building can be harder to trace to a single source than expected.

Why bait, and slow bait specifically, actually works

Because workers forage and share food with the rest of the colony (including queens and brood) through trophallaxis, a bait that kills slowly enough to be carried back and distributed throughout the nest before it takes effect reaches far more of the colony than a fast-acting product ever could — and crucially, it doesn't trigger the same disturbance response that a spray does. Guidance on pharaoh ant control describes exactly that logic: a slow-acting bait lets workers keep feeding and sharing the treated food over an extended period, so that nearly all of a colony's workers and brood die off gradually rather than all at once, avoiding the sudden disruption that provokes a colony to bud and scatter.

Why this matters beyond household annoyance

Pharaoh ants are specifically documented as a recurring pest concern inside hospitals, and the reason isn't purely about nuisance. Concern about ants as a hospital pest dates back decades — a 1972 study published in The Lancet first raised pharaoh ants specifically as a potential vector for spreading bacteria between patients in clinical settings. More recent entomological research has continued examining this: a study published in Neotropical Entomology on ants collected inside Brazilian hospitals found Monomorium pharaonis among several ant species carrying a range of bacterial species, some showing resistance to common antibiotics, while a separate hospital-ant survey in the Brazilian state of Santa Catarina found pharaoh ants were the single most frequently collected species. Researchers in that line of study describe ants as a potential, if still not fully quantified, route for moving bacteria between surfaces, equipment, and patients in a hospital setting — which is part of why pest management guidance for healthcare facilities treats a pharaoh ant sighting as a genuine priority rather than a routine nuisance call.

• Pharaoh ant workers are tiny (1.5-2mm) and pale yellowish-brown, often confused with other small household ants.
• Colonies reproduce by budding rather than mating flights — workers carry brood and queens to new sites, especially when the colony is stressed or disturbed.
• Repellent insecticide sprays are a known trigger for budding, which is why spraying a visible trail can cause an infestation to spread into new areas rather than end it.
• Hidden pathways — wall voids, wiring chases, plumbing voids — let colonies forage and relocate largely out of sight.
• Slow-acting bait is the standard professional approach: it's shared colony-wide through trophallaxis before it takes effect, killing gradually without triggering a disturbance response.
• Published entomological research has found pharaoh ants among ant species carrying bacteria, including antibiotic-resistant strains, collected inside hospitals — a concern first raised in a 1972 Lancet study and still studied today.

Because the wrong first response can genuinely make a pharaoh ant problem larger and more spread out, identifying the species correctly before treating it matters more here than with most household ants. A free Sevacorp inspection can confirm what's actually present before any treatment decision is made.

• Sources:
• University of Florida, EDIS (IFAS Extension) — "Pharaoh Ant" (EENY-290) — https://edis.ifas.ufl.edu/publication/IN533
• Ohio State University Extension — "Pharaoh Ants" (HYG-2136) — https://ohioline.osu.edu/factsheet/HYG-2136-10
• University of California Integrated Pest Management (UC IPM) — "Pharaoh Ant Biology" — https://ipm.ucanr.edu/TOOLS/ANTKEY/pharbio.html
• SciELO Brasil, Neotropical Entomology — "Ants as carriers of antibiotic-resistant bacteria in hospitals" — https://www.scielo.br/j/ne/a/QpfhjjLCrnQWB5GC8bd5cPc/