Ant Infestation in the House: Identification and Treatment

Seasonal Foraging Versus True Infestation

Most ant activity in UK houses in summer is not a true infestation — it is seasonal foraging by black garden ants (Lasius niger) whose nest is outside in the garden or beneath the building path. Scout workers enter the house through gaps around doors, pipes or skirting boards, find a food source, and recruit trail followers from the colony. This foraging activity is seasonal (June–September), ceases in winter when the colony becomes dormant, and is typically resolved by removing the food source and treating the entry point. The colony is outside the house — you have a foraging problem, not an infestation.

A true ant infestation involves one or more of: ants nesting inside the building fabric (not outside foraging in); multiple nests or colonies present in or immediately beneath the structure; year-round ant activity without seasonal cessation; or pharaoh ants (Monomorium pharaonis) — a species that cannot survive outdoors in UK conditions and forms permanent colonies inside heated structures. These situations are significantly more difficult to resolve than seasonal foraging and generally require professional intervention.

Garden Ants Nesting Inside the Building

Black garden ants can establish nests inside buildings where the conditions are suitable — typically in sub-floor voids beneath concrete slabs, within cavity walls (particularly at low level where soil and insulation are in contact), under kitchen floor tiles, within the rubble fill of older building foundations, and beneath conservatory bases and extension slabs. These are indoor nests in the structural sense — the colony is physically within or immediately beneath the building — but the species is still Lasius niger and can be treated with residual insecticide and nest drench approaches, provided access is achievable.

Signs that an ant nest is inside rather than outside the building: ants are entering the house from below (emerging through cracks in the floor, around the toilet base, under kitchen unit kickboards) rather than from external doors or windows; activity is heavy early in the season before the nest has started foraging widely outdoors; the nest location cannot be found despite searching the garden and exterior path edges; and — in severe cases — the floor in the affected area feels hollow or sounds different under foot as nest excavation has removed substrate material.

Treating Under-Floor Ant Nests

Professional treatment of an ant nest beneath a concrete floor requires either treatment through drill holes (fine holes drilled through the floor covering into the sub-floor void, through which insecticide is injected); treatment through gaps around pipes, under kickboards or at floor/wall junctions; or — where access allows — lifting a section of flooring. A professional pest controller assesses the most practical access route and applies a nest drench or residual insecticide in sufficient volume and concentration to penetrate to the queen and brood chambers. This is not achievable with consumer products poured along a crack in the floor.

A black garden ant colony can contain 5,000–15,000 workers and is supported by a queen that can live for 15 years or more. A nest established beneath a kitchen floor will continue producing foraging workers season after season unless the queen is killed or the nest is physically disturbed and eliminated.

— Entomological research; BPCA species guidance

Pharaoh Ant Infestation — The Most Serious Indoor Ant Problem

Pharaoh ants (Monomorium pharaonis) represent the most significant ant infestation scenario in UK buildings. They are a tropical species that lives exclusively in continuously heated indoor environments in the UK. Workers are 1.5–2 mm — very small — and pale yellow to orange in colour. They are present year-round and their colonies contain multiple queens (polygynous), can occupy multiple interconnected nesting sites throughout a building, and reproduce by budding — a colony fragmentation process triggered by stress (including insecticide exposure) that multiplies the infestation.

Pharaoh ant infestations are most common in hospitals, hotels, large residential blocks of flats with communal heating, care homes, food businesses and any large building with centralised heating that creates warm, stable conditions throughout. The nesting sites are within building cavities — wall voids, under flooring, in roof insulation, in electrical conduit runs, within hollow skirting boards. The scale of a pharaoh ant infestation is often not apparent from surface observations — only professional assessment and a systematic bait monitoring programme reveals the full extent of colony distribution through the building.

Why Residual Spray Makes Pharaoh Ant Infestations Worse

If residual insecticide is applied anywhere in a building with a pharaoh ant infestation, the chemical stress triggers budding — sub-groups of workers and queens fragment from the main colony and disperse through the building, establishing new nesting sites in locations not yet reached by the spray. Each spraying event potentially multiplies the number of active colony units and extends the geographic range of the infestation through the structure. Buildings where pharaoh ant infestations have been repeatedly sprayed by inexperienced operators are often the most complex and expensive to remediate — the infestation has been distributed throughout the entire structure by multiple budding events. The only appropriate treatment is a professional slow-acting bait programme using commercial protein and sugar bait formulations, applied by a pest controller experienced specifically in pharaoh ant management.

Ant Infestations in Commercial Buildings and Food Businesses

Ant infestations in food businesses — restaurants, food production, catering, retail food units — represent a food hygiene risk and a potential EHO finding under Regulation (EC) 852/2004 (UK retained). Any ant activity on food contact surfaces, in food storage areas or in food preparation environments requires immediate professional assessment and a documented treatment and monitoring programme. Pharaoh ants in food businesses are a particular concern — their small size allows access to food packaging, they forage across food contact surfaces, and they are very difficult to eliminate without a comprehensive bait programme. A professional treatment record demonstrating appropriate pest management response is essential for EHO inspection due diligence.

When to Call a Professional Pest Controller for Ants

Professional ant pest control for an infestation (as opposed to casual seasonal foraging) is warranted in all of the following situations: any suspected pharaoh ant infestation (mandatory — no DIY treatment should be attempted); ants present year-round or in winter; ant activity emerging from below floor level with no accessible nest outside the building; ant activity across multiple rooms or floors rather than a single trail from an external entry point; ant activity in a food business where EHO compliance is required; and any situation where repeated DIY treatment has failed to produce lasting results. In Leeds, Sheffield, Barnsley and Rotherham, Apex Pest Control Ltd provides ant species identification and a treatment programme appropriate to the specific infestation type — contact us for a professional assessment.

If ants are causing problems, these related guides may help: how to get rid of ants in the garden, how to get rid of ants in the house, and how do you get rid of ants.

Frequently Asked Questions

How do I know if I have an ant infestation or just seasonal ants?

The key distinctions: seasonal garden ant foraging occurs in summer (June–September) and ceases completely in winter as the colony becomes dormant. If you have ants indoors in October through March, it is not seasonal garden ant foraging — it is either an indoor nest or pharaoh ants. Seasonal foraging typically presents as a trail from a single entry point (a gap under the back door, around a pipe under the sink) — it is directional and traceable.

A true infestation presents as ants across multiple areas, emerging from below floor level or from the building fabric rather than from external entry points, or as small pale ants (1.5–2 mm, yellow-orange) present year-round regardless of season.

Can ants cause structural damage to a house?

Black garden ants nesting beneath a concrete floor or under building foundations excavate the substrate — removing sand, soil and fine aggregate from beneath the slab. In most residential properties, this does not cause structural damage in the engineering sense, but it can cause floor void collapse, uneven surfaces, slab cracking and — beneath thin tiles — hollow-sounding areas. Damage to electrical cables in a sub-floor void from ant activity is rare but possible.

The more common consequence of an under-floor nest is the cosmetic and nuisance impact rather than structural harm. Pharaoh ants nesting in wall insulation or electrical conduit are more likely to create issues with insulation performance and, in extreme cases, issues around cable integrity.

How do professionals treat an ant infestation in walls?

Professional treatment of an ant nest in wall cavities or under floors uses one of three approaches: treatment through drill holes into the void, through which insecticide (a residual formulation or dust) is injected to reach the nest galleries; application through existing gaps (behind skirting boards, under kickboards, around pipework entry points) of residual insecticide in sufficient volume to penetrate the void; or — for pharaoh ants, which cannot be sprayed — placement of commercial-grade protein and sugar bait stations at every foraging location throughout the building, without any spray use. The choice depends on the species, the nest location, and the accessibility of the void. A professional pest controller assesses all three options and selects the most practical approach for the specific building.

Are ant infestations worse in summer or winter?

For black garden ants, infestation impact is highest in summer — warm weather triggers foraging activity, colony size is at its seasonal peak, and nuptial flights occur. Ants are dormant inside the nest in winter. For pharaoh ants, there is no seasonal variation — the infestation is present and active year-round.

Pharaoh ant activity may intensify in late winter and spring as temperature fluctuations in the building trigger increased foraging. A property that has an ant problem only in summer (June–September) has a garden ant foraging issue. A property with ant activity in winter, or with ants present continuously without seasonal cessation, has an indoor infestation that requires professional investigation and treatment.

Can ants come through the floorboards?

Yes. Ants can enter the living area from below through gaps between floorboards, through cracks in concrete floors, around the bases of floor penetrations (pipes, cables), and through the gap between the floor covering and the skirting board. In older properties with suspended timber floors, the sub-floor void provides a substantial habitat for ant colonies that enter the living area through the gaps inherent in the floor construction.

In properties with solid concrete floors, ants nesting in the sub-base material enter through any crack or gap in the concrete or through pipe entry points. Ant activity emerging from below floor level — as opposed to entering from external doors and windows — is a strong indicator of an under-floor nest.

How long does it take to get rid of an ant infestation professionally?

For garden ant nests treated with professional nest drench or residual insecticide, colony activity reduces significantly within 48–72 hours of treatment. A follow-up visit at 4–6 weeks confirms the nest has been eliminated and applies further treatment if any activity remains. For pharaoh ant infestations, the treatment period is substantially longer — a professional bait programme typically requires 8–12 weeks minimum with monitoring visits throughout, as the slow-acting bait must reach all queens in all colony units distributed through the building.

Complex pharaoh ant infestations in large buildings can require 3–6 months of managed bait programme before the infestation is resolved. Ongoing monitoring after active treatment confirms resolution.

Is an ant infestation in a rented property the landlord's responsibility?

Under the Landlord and Tenant Act 1985, landlords are responsible for maintaining properties in a habitable condition, including structural defects that allow pest entry. An ant infestation arising from gaps in building fabric, poorly fitted external doors, cracked foundations or structural voids that allow indoor nesting is the landlord's responsibility to address. An ant problem arising from tenant food hygiene practices — food left accessible, spills not cleaned — may be considered a tenant responsibility.

In practice, most ant infestations have both a structural component (accessible entry points) and an attractant component (available food). Tenants should report the infestation to the landlord in writing; if the landlord does not respond, the local authority environmental health department can provide advice.

Ant Infestation Treatment Across West and South Yorkshire

Apex Pest Control Ltd treats ant infestations across Leeds, Sheffield, Barnsley and Rotherham — including under-floor and structural nest treatments for garden ants and professional bait programmes for pharaoh ant infestations in commercial and residential buildings. Species identification included with every assessment.

Contact Apex Pest Control Ltd for an ant infestation assessment.