Ants indoors: the field card
What you see is only the vanguard: the colony is somewhere else.
Card 05
Lasius niger, Monomorium pharaonis
In short
The black garden ant (Lasius niger) comes indoors in spring and summer looking for sugar; the pharaoh ant (Monomorium pharaonis), tiny and yellowish, nests inside heated buildings. The workers you see are only a small part of the colony. A slow-acting bait carried back to the nest is the only way to reach the queens.
Ants leave a chemical trail that other workers follow, which is why they march in single file. Squashing the line or spraying insecticide scatters the workers without touching the nest.
The species matters. Black garden ants usually nest outside, under paving or at the foot of a wall, while pharaoh ants live indoors, inside wall linings and ducts, with many queens. Spraying a pharaoh ant colony can even make it split into several new nests.
Tell-tale signs of ants
- 1Lines of workers between a crack and a source of sugar or fat.
- 2Little heaps of soil or fine sand at the foot of walls and between paving slabs.
- 3Winged ants emerging in large numbers on hot, stormy summer days.
- 4Ants in honey jars, the sugar bowl or pet bowls.
Risk index
DeratiseurPRO rating, from 1 (low) to 5 (high).
- Health
- 1/5
- Damage
- 1/5
- Spread
- 3/5
The risks
- Contamination of food and worktops.
- Pharaoh ants can turn up in sensitive areas such as communal kitchens and care facilities.
- A nuisance that keeps coming back while the nest survives.
- Swarms of winged ants indoors, very unpleasant for a few days.
How we treat it
- 01
Detect
Species identification, tracking trails back to entry points and pinpointing the likely nest.
- 02
Neutralise
Slow-acting gel bait placed on their routes, carried back and shared by the workers all the way to the queens.
- 03
Lock
Sealing of cracks in sills and skirting, plugging of duct openings and food storage advice.
- 04
Monitor
Activity check, bait top-up if needed, photo report and written guarantee.
Prevention checklist
Simple habits that lower the risk of the intruder coming back.
- Wipe up sugar, honey and juice spills straight away.
- Keep food in closed containers and don't leave pet bowls full.
- Don't spray insecticide near bait: ants would avoid it.
- Trim branches touching the façade that act as bridges.
Frequently asked questions
Why do ants come back every spring?
Because the colony, often outdoors, survived the winter. As long as the queens are alive, new workers pick up the same trail.
Will vinegar or lemon do the trick?
They wipe out the chemical trail for a while but never reach the nest. The ants find another way in within a few days.
How long does bait take to work?
Usually a few days to two weeks: the bait needs time to be carried and shared throughout the colony. A temporary increase in ants around the bait is normal.
Got ants at home?
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