Control Of Rats Resistant To Second-Generation Anticoagulant Rodenticides

Second-generation anticoagulant rodenticides were introduced to control Norway rats that had become resistant to first-generation compounds. Unfortunately, some rats have become resistant to these as well. The lack of alternative rodenticides with the same attributes of ease of use and relative safety is potentially a serious problem should resistance become so widespread that anticoagulants are no longer effective. However, the second-generation anticoagulants difenacoum and bromadiolone can still be effective provided most rats in a population possess only a low degree of resistance to them. Measures that maximize the uptake of bait, such as using the most palatable formulation, baiting burrows and saturation baiting have to be implemented. The low levels of resistance discovered so far mean that the most potent anticoagulants, such as brodifacoum and flocoumafen, should also control most populations if baits containing either of them are properly applied. These two rodenticides are restricted to indoor use in the United Kingdom and are thus not available to control those rats living outdoors that are highly resistant to all other anticoagulants. Those rats can, however, be controlled with either zinc phosphide or calciferol, preferably after prebaiting. Strategies to manage resistance in the long-term should be implemented before high-degree resistance spreads. One potential tactic is to stop using anticoagulants altogether and allow deleterious pleiotropic effects to reduce the prevalence of resistance in a population. Any attempts to manager resistance are only relevant if the intention is to retain anticoagulant rodenticides, with their undoubted advantages, as the main method of controlling rodent pests.