| Summary: | Roughly 50 years ago, the Wisconsin Alumni Research Foundation developed warfarin, the first anticoagulant rodenticide. This product was something close to that desired elusive "magic bullet" of pest management. Warfarin effectively killed rats and mice, required multiple feedings, and had a good margin of safety for non-target species. The widespread adoption of anticoagulants somewhat changed the conduct of rodent control with a shift in interventions toward toxicants and away from education and physical measures. The discovery of warfarin resistance in the United States in Rattus norvegicus in 1971, and later in Mus musculus and Rattus rattus, heralded in another shift in rodent pest mitigation. This shift was the development of more toxic anticoagulant products capable of killing with one or a few feedings and with concomitantly greater risks to non-target species. Development of the more toxic products both anticoagulant and non-anticoagulant continues today, although there is an increasing trend favoring comprehensive approaches (i.e., integrated pest management [IPM]) which: emphasize educating clients and reducing causative conditions; diminishing the role of toxicants; and, when necessary, using products of the least practical toxicity. In this paper, the concept of counteracting anticoagulant resistance is blended with the sometimes necessary use of anticoagulant rodenticides as part of IPM. Nationwide data from the former New York State Department of Health Rodent Control Evaluation Laboratory (in cooperation with the Centers for Disease Control’s former Urban Rat Control Program) are examined regarding warfarin resistance in Rattus norvegicus. In samples from two dozen project cities, population resistance levels ranged from 1.6% to 76.2% using the standard World Health Organization (WHO) testing criteria. However, most survivors (i.e., resistant rats) of the initial test succumbed upon one or more re-exposure(s) to warfarin using the same WHO testing protocol. The results are surprising and have implications on interpreting the phenomenon of anticoagulant rodenticide resistance and on the pragmatic designing of rodent management programs.
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