A rat colony will often start with a single pregnant female and unless there are obvious signs of damage, such as torn packaging or droppings, the infestation may not be noticed in the early stages. The exception is when there are a lot of rats or food is scarce. They usually only come out at night and daylight sightings are unlikely.
Pregnancy lasts about 21 days and a single female can have a litter of about 6 young. These can feed themselves after 3 weeks and reproduce when they are about 8-12 weeks old. Mortality of the young is high but even so, a rat infestation can grow quickly.
As the number of rats increases, signs of damage become obvious. There is damage to food containers and cartons as rats gnaw at them, droppings (black, about the size of a peanut) become noticeable and a rat running across the attic in the quiet of night can be easily heard. Eliminating rats is a job for the expert. Rat traps and cats will only remove some rats without eradicating the infestation and rats are resistant to some of the older poisons on sale to the public. A pest control technician can choose the more effective poisons which are only on sale to trained personnel and can vary the way the bait is laid to take advantage of the rats' natural habits. Rats are highly intelligent and suspicious of new objects or food and some rats will dominate others and grab the best food.
Eradication of your infestation starts with a thorough survey to check the size of the problem. Poison will then be laid in the best locations and may be supplemented with traps.
Rats should start to die within a week and if the infestation is a small one, control will be fairly quick. In a larger infestation, it may take some time until the more timid rats are able to feed freely and eat enough bait to be killed. Most of the poisons used require the rat to feed for several days. This makes them safer to use around houses, as one dose does not kill, but it is important that the rats should have little other food than the poison bait. Your pest control technician will explain how you can help by removing other sources of food.