Proceedings of the Vertebrate Pest Conference

While the problem of anticoagulant resistance in commensal rodents has been well-documented from certain areas of northern Europe in recent years, this paper describes its first known occurrence in North America in 1971. Over a rural area of about 5 square miles in Johnson County, North Carolina, it was noted that attempts to control Norway rats using typical warfarin rodenticide bait were increasingly ineffective. Diphacinone was alternated with warfarin with no success. On one farm, 200 lbs of bait had been used in bait boxes in a single month. Laboratory trials on rats captured at this location demonstrated resistance. It was concluded that at this location, intensive use of anticoagulants (mostly warfarin) over a decade, in the absence of adequate sanitation and building maintenance, provided the selective agent to develop resistant populations. This likely will be repeated elsewhere in the U.S.