Ants come into a house for three things: food, water and a dry place for their nest. Heavy rain pushes them inside for the dry spot. A soaked nest in a lawn or under paving is too wet, so the colony looks for better ground. A hollow space inside a wall, called a cavity, or the dry area under the floor often does the job. Dry weather does the opposite and sends workers looking for water, which is why they turn up in bathrooms with no food. A line of ants across the bench after a storm is usually a colony on the move or scouts looking for food.
Why one ant becomes a trail
Ants follow chemical signals. The Victorian Department of Health's ant guidance explains that workers use pheromones to mark the path to food, to call for help and to tell nest groups apart. A pheromone is a scent the ant leaves on the ground so other ants can follow it. One scout finds a crumb of sugar or a smear of fat. It walks back laying that scent. The workers behind it make the line stronger. Wipe away the food and the trail fades, but the nest that sent them is still there. The same guidance notes that ants usually keep permanent nests. Nest changes in bad weather are still common.
Why ants come in after rain
Most pest ants around homes nest in soil, under pavers, along path edges and in rockeries. When that soil gets soaked, workers carry their young to drier spots. A house wall is often the closest one. Those young ants are called brood. Brood looks like small white bundles in the jaws of the workers. The Victorian guidance says Argentine ants usually nest at tree bases, along building and path edges and in lawns, and may move indoors in wet weather. It also describes white-footed house ants budding. That is when a queen leaves with workers carrying the white young. A column carrying small white bundles is a colony on the move, not a food raid. Timing is the useful clue. Ants that appear just after a storm and fade within a few days as the ground dries were pushed out of a wet nest. Ants still marching a week later across a dry lawn have probably nested inside the building. A Picnic Point kitchen that still has a wall trail a week after the Georges River flats dry out has an indoor nest, so call ant control in Picnic Point before treating the bench again.
Why ants come in during dry weather
Dry spells pull ants inside for water. A dripping tap, damp air under the sink, a wet shower recess or a pet's water bowl can be enough. The Victorian guidance describes white-footed house ants entering through small cracks. Once they find food or water, they form trails. When ants climb the grout of a Revesby bathroom with no food in the room, look for a nest in the wall or under the slab, then book ant control in Revesby for that spot.
What the ants eat tells you which ant it is
The right control method depends on the species. The food on the trail narrows it down.
- Black house ant (Ochetellus glaber): 2.5 to 3 mm, jet black, with a strong smell when crushed. It prefers sweet food and commonly nests in ceilings, wall cavities and under floors. It tends aphids on pot plants, so ants on an indoor plant may be farming them.
- Coastal brown ant (Pheidole megacephala): shiny brown workers of 1.5 to 3 mm and big-headed soldiers of 3.5 to 4.5 mm. It prefers meat, grease and dead insects. The Western Australian Department of Primary Industries and Regional Development coastal brown ant fact sheet says it digs large amounts of sand from between brick pavers. In high numbers it enters homes for fat and protein.
- Argentine ant (Linepithema humile): slender and brown. It moves in slow trails up to three or more ants wide. It often climbs tree trunks for honeydew from aphids and scale insects.
- Pharaoh's ant (Monomorium pharaonis): yellow-brown. It nests in the warmer parts of buildings and prefers protein and fat. Sand between Punchbowl pavers plus brown ants on the dog bowl means a coastal brown nest under the paving, so ant control in Punchbowl should cover the yard nest as well as the kitchen.
Why spraying the trail can make it worse
Surface spray kills the foragers you can see. It does not reach a nest in a wall or under a slab. The Victorian guidance warns that ants are good at finding new routes to food. For Pharaoh's ants it goes further. Spraying splits the colony. Groups bud off into new colonies and the problem often gets worse. Yellow-brown ants that reappear in a new Chester Hill room after every spray are often Pharaoh's ants splitting, so skip another can and get urgent ant control in Chester Hill for the nest.
How to stop ants coming inside
Take away what they came for. Seal sweet and fatty food. Wipe benches before bed. Clear pet food, since the Victorian guidance notes that food left outside draws ants close to the house. Fix dripping taps and wet spots under sinks. After a storm, give displaced ants a few dry days before deciding the house has a nest. Write down what the trail leads to, where it enters, and whether workers carry white brood before you book pest control in Bankstown, so a sweet black house ant is not treated like a grease-eating coastal brown ant.