Control of a population of norway rats resistant to anticoagulant rodenticides

For the first time, it has been unequivocally shown that multiple‐feed second‐generation anticoagulant rodenticides were ineffective against a population of rats in N.W. Berkshire, UK because of an unusually high prevalence and high degree of resistance. Use of the non‐anticoagulant rodenticide calciferol led to a substantial reduction in the population, although primary poisoning of small birds appeared to be greater than with anticoagulant baits. There was strong evidence that many of the surviving rats had developed an aversion towards calciferol‐treated bait. A reduction in the degree of anticoagulant resistance in the population was evident after a period of 17 months without anticoagulant use. The long‐term strategy to manage the resistant population should integrate non‐anticoagulant and anticoagulant rodenticide use to take advantage of possible pleiotropic costs of resistance.