The answer to 'when to treat for rat' is to treat for rats as soon as you suspect an infestation. Rats can reproduce quickly, with females giving birth to litters of 6-12 pups every 3-4 months. This means a small rat problem can turn into a major infestation in a short period of time. The sooner you can identify and address a rat problem, the better.
Signs of a rat infestation include droppings, gnaw marks, nests, and sightings of the rats themselves. If you notice any of these signs, it's best to take action right away by setting traps, sealing entry points, and removing food sources. Consulting a professional pest control expert can also help ensure the problem is properly identified and treated.