Rodenticides Resistance By Rats & Mice:
Although resistance in my opinion is a problem that is over exaggerated it is not surprising. Since Warfarin was first used in the 1950’s the modern poisons have all been the same, anti coagulants. These all kill by thinning the blood. An acute poison based on Calciferoul was tried but although extremely successful for mice did not have the same result on rats. These bait required ‘pre baiting’ prior to putting the poison down. The reasons for this is that Acute poisons are quite fast acting and although it didn’t seem to bother mice rats would become suspicious if their friends and relatives stated dying quickly after feeding on a particular food source. The pre bait was a bait of the same type of bait but without the active ingredient in.
The idea is that the whole of the rat population happily feeds on the non toxic pre bait and then the bait with the poison is put down. The advantage of the Chronic (of which anti coagulants are) poisons is that as they are slow acting the rats do not become suspicious of the bait until it is too late. Some of the later baits will kill in a single feed if enough is consumed. Some of these 2nd generation baits are restricted to use inside which reduces their usefulness. The reason for this is secondary poisoning. This is where other non target animals may eat the rat and get poisoned. Birds of prey and foxes are a good example. This is a problem with the slow acting poisons. As it is slow acting a rat may consume a dose several times higher than is needed to kill it, creating even more danger to the non target species that may eat the rat.
Rats are not stupid so any new type of poison will have to consider this but using different types of poisons that kill the rat in the same way (blood thinning) will eventually lead to more resistance problems. What is needed is a slow acting poison that targets the animal in a different way and is not too toxic to other animals and birds.