It depends how many rats there are in the area. The quantity laid by a professional pest controller is expected to be sufficient to remove all the rodents and also leave some bait to deal with any new arrivals for the next 6-12 months.
How long do rat baits last?
Most pest controllers will replace or supplement baits after 12 months, to be sure the homeowner gets an effective result. However, it is certain that under many circumstances, solid blocks and pellet baits will last longer. The manufacturers give an expiry date, but that is the date to which they have tested the bait and can guarantee a certain level of chemical activity. As with most “use by dates” it is a conservative estimate of the expiration date.
The commonly used poisons are organic in nature with the active ingredient absorbed onto a base of grain and usually waxes. In the presence of that bait matrix, poisons slowly degrade over time through chemical interactions between the grains, dyes and flours which contain natural enzymes. The rate of degradation completely depends on the environment and the chemistry of the compound. Deterioration is affected by factors like moisture, attacks by insects that eat the grain and especially heat. High temperatures speed up enzyme action and drive off volatile compounds which include additives that give the bait an odour and taste that is attractive to rodents. So baits in a dry bait station under a house may well be effective for longer than those placed in a hot roof space.
So after the expiration the baits may still work, they are just less effective.
Some pest controllers have claimed effective rodent control from baits over 2 years old but it’s certain those baits were less effective than fresh ones. But if a rat is hungry it will probably eat anything that seems digestible, even stale baits.
For the best rodent control, baits should be refreshed after 12 months or just before the colder times of the year when new rodents can be expected to investigate your property. That annual pattern of new arrivals suggests that fresh baits each autumn is the best control option, so we usually recommend an annual treatment. If a property is close to creeks, large drains or long grass, or you have chickens in the garden, then 6 monthly treatments may be required.
Keep in mind even old baits will probably still be toxic to both rodents and humans so they should always be treated carefully and disposed of safely.
