Deconstructing the brown ant

The brown ant (Lasius brunneus) is common in Europe from the Mediterranean area to the south of Great Britain, south Sweden and south Norway. Like all members of the Formicidae brown ants are social insects. The nests contain female workers, a queen, and the brood (eggs, larvae and pupae). The female workers are 2.5-4mm long, with yellow-brown legs and antennae. Their petiole (the part connecting the abdomen to the thorax – see diagram) consists of a broad scale-like segment. The opening of the cloaca at the tip of the gaster is surrounded by a ring of hairs. Female workers do not possess a stinger – to defend themselves they spray a liquid which contains formic acid. The dark-brown 6.5-8mm long queens possess wings which are brown coloured at the base. These wings are thrown off after copulation.

The brown ant is a monogyne ant species (each colony has only one queen). However, observations of some authors suggest multiple young queens may be accepted after the nuptial flight by an existing colony (polygyne). The black, ~4mm long males also develop once per year and die after the nuptial flight.


The females throw their wings off after the nuptial flight and copulation and create a nest chamber under the bark of standing trees, usually 3-12m above ground level. The first eggs are laid 1-4 weeks later and the queen takes care of the first offspring. During this time the female lives off her body fat and her flight musculature which is no longer needed. With the increasing size of the nest the colony moves down from the tree towards the soil. In the second year the colony can already contain several thousand individuals and after 10 years up to 100,000. Depending on their age, the workers have different tasks. The development from the egg to the adults takes five weeks under favourable conditions. During the swarming period which is mainly between June and July individual-rich swarms of queens and males leave the nest for the nuptial flight. The winged reproductive ants may already be present in May depending on the climatic conditions.


The brown ant occurs frequently in hardwood forests in rotten trunks, in dead roots and in decayed portions of trunks up to 4m high. Members of this species avoid shady coniferous forests, but it has no further special requirements in habitat. If the young queen finds decayed timbers with sufficient humidity at the front or inside a building, it may also settle there. Foundation of the nests occurs behind door frames, in partition walls made from gypsum board, insulation below the floor as well as in insulating material in close proximity to sources of humidity (e.g. cold and warm water pipes, underfloor heating and sewage drains).
Nest foundation in a building occurs usually after damage to the construction followed by the penetration of water into wood or stonework. Further causes of the establishment of ant colonies can be penetrating humidity through cracks in the external wall, defective water pipes within the building, leaky roofs etc. Brown ants act therefore as indicator species for any hidden moisture damage in the construction. The number of workers rise slowly in in the first years and workers generally avoid travelling over open surfaces. The infestation is therefore often noticed by the home owner 5 to 10 years after settlement of a brown ant queen, often when the first swarms of the winged reproductives occur. Swarms may occur several metres from the nest, and in colonies within heated buildings, may occur at different of the year from those based outside.
At first only decayed wood or insulation material is used by the brown ant to create the nest. However the ants will begin to gnaw on sound wood, creating tunnels in softer new growth, with the surface usually remaining intact. The wood serves only as a harbourage and not as food. The ants obtain moisture via condensation from heaters, water pipes or other sources around the nest.
The workers take honey dew from aphids outdoors, and indoors they are attracted to high-sugar foods such as jam, fruit juices or honey. Temporarily protein is needed for the developing larvae, which mainly comes from captured arthropods. At this stage the workers from indoor colonies may consume cooked or raw meat. Bins and dishwashers are ideal food sources for foraging worker ants, who take food back to other workers in the nest which then feed the queen and the brood.
Ants are of particular concern in food processing and catering establishments due to contamination of food with pathogens. Also ant secretions (i.e. formic acid) can sometimes cause skin irritation.

In the past ten years ants have been seen more and more frequently within the insulation substrate of buildings. Besides the brown ant, various other native ant species are now frequently found in the insulation material of outer walls, particularly the black garden ant (Lasius niger) and the shining black wood ant (L. fuliginosus). Carpenter ant species are also found in insulation materials, especially C. ligniperdus and C. herculeanus. Another species which immigrates temporarily into buildings is the invasive ant species Lasius neglectus, which has spread over the past decades from southern Europe to southern England. This species is characterised by very individual-rich colonies with many queens.

Facades insulated with polystyrene sheets and polyurethane foam also offers ants ideal conditions for nest foundation. Glass wool, rock wool and cellulose wool have no fixed structure and are rarely used by the ants as nesting sites. However, ants contaminate these materials with moisture and through gnawing and waste deposition from the nest.

Ant activity within buildings

The average householder will be unaware of brown ant activity because the ants move mostly hidden in cracks and crevices or behind baseboards, the infestation only becoming noticeable with the first annual flights. However, activity of the workers is signposted by small heaps of gnawed particles (wood, pupal cases, dead ants and the carcases of captured insects) on the ground, under insulating material and within timber constructions, the composition and colouring of the particles being dependent on the materials in which the nest was built. Because workers stay away from open areas, baiting with different food sources (proteins and sweets) or with a liquid ant gel is helpful to locate the nest. The measurement of moisture in the structure can also give an indication of the nest location, so a moisture meter is a helpful addition to any technician’s toolkit. The homeowner should be interviewed to ask about possible moisture damage during the past 10 years, as this may be the origin of ant infestation.

External insulation

Ant infestation within external insulation can be detected from the ant activity on the foundation surfaces, as well as by tracking worker ants, which enter the insulation layer through gaps and cracks. It is less likely to see the typical gnawed particles at the exits (tunnel ends) since they are blown away by wind or washed away by rain. Winged ants rarely stay on the outer facade.

To determine the extent of an ant infestation, the insulation must be removed in the infested area. Insulation boards damaged by ant activity may cause heat loss in the building. Cold spots on the wall inside the house combined with increased humidity and mould are an indicator for ant activity in external insulation.

Infestation prevention

Modern foam-based insulation is an excellent habitat for many organisms, therefore it is important to permanently shield such insulating layers from the outside. Reinforce the insulation surface using a gauze layer with one millimetre mesh size covering 100% of the insulation even around windows, doors and other potential openings to prevent ant penetration. The terminal edge of the insulation board to the foundation must be protected by a metal profile so that ants cannot penetrate the insulation from the soil. Ensure the façade does not allow growth of ivy or Virginia creeper which encourage ant activity. Bitumen sheets with glass fleece provide protection against the penetration of ants around the foundation of the building. Covering the insulated façade with thin clinker slices offers a good long-term protection against ants, but most ant invasion is via the foundation.


The first step to successful ant control in buildings is species identification and adjustment of the control strategy accordingly. Since the brown ant builds a colony indoors usually only in decayed wood or insulation with moisture damage, it is necessary to locate and remove the nest and the decayed wood or other moisture sources which facilitate the settlement of the ants (e.g. incorrectly repaired building damage, rotten wood within the foundation range or other sources of humidity). After the elimination of the nest, care must be taken to make good all damaged areas to avoid further settlement of brown ants. The remaining workers can be killed with sweet baits after the removal of the nest. Swarming ants which are found seasonally in large quantities at windows and on walls, should be eliminated by physical means using a vacuum cleaner.