Effective rodent control

First and foremost it is essential to conduct rodent pest control operations following widely available codes of best practice (see also Integrated pest management for rodent control). Only by following best practice guidance and use recommendations given on product labels will applications of rodenticides be fully effective. Such applications are those least likely to promote the development of resistance.

Especially when anticoagulants are used, it is important to remove all rodents from an infested site. This is because it is likely that those rodents that survive into the latter parts of treatments are those that are intrinsically less susceptible to anticoagulants or may even be those physiologically resistant to the active substance in use. Therefore, these are the most important individuals to remove in order to prevent resistance development, although they are often the most difficult.

All traces of rodenticide bait should be removed at the end of baiting operations. Leaving small remnants of bait in position will mean that susceptible individuals will succumb if they find and consume them but others that are more tolerant or resistant will not do so. Anticoagulants should not be used routinely as permanent baits. These applications are generally serviced, and baits replenished, at intervals of four, six or eight weeks. It is therefore inevitable that, occasionally, rodents encounter bait stations containing only limited quantities of bait. The most susceptible are likely to succumb in such circumstances, while the less susceptible will survive and breed. Permanent baiting should take place only where there is a direct and immediate risk of immigration of rodents and permanent bait stations should be visited frequently to ensure that they do not run out of bait.

Resistance to the second-generation anticoagulants includes resistance to the first-generation anticoagulants in rats and in mice. The use of the first-generation compounds to control populations already containing a proportion of resistant individuals, e.g. in resistance areas of the Norway rat and the house mouse, would promote the survival of individuals that are resistant to anticoagulants, and thereby increasing the frequency of the resistance gene in the population. However, first-generation anticoagulants have well-known environmental benefits that they are less acutely toxic to non-target animals and are less persistent in the bodies of non-targets, and the environment in general, and so less likely to cause secondary poisoning. In particular for the control of Norway rats outside known foci of resistance, the use of these compounds therefore is recommended. Even when applied indoors, the probability is high that poisoned rats stay outdoors, posing some risk of secondary poisoning to predators and raptors. Where these important environmental advantages of this group of anticoagulants are not required, in particular for the control of house mouse infestations which are confined to indoor locations, having been under control-pressure for prolonged periods, careful consideration should be given to the use of one of the most potent second-generation anticoagulants or a non-anticoagulant to avoid the selection of anticoagulant resistant mice.