Laboratory and field trials of the rodenticide brodifacoum against warfarin‐resistant Norway rats

Abstract Norway rats (Rattus norvegicus) trapped in a low‐income housing area in Raleigh, North Carolina, and on a poultry farm in Garner, North Carolina, manifested high levels of resistance to warfarin in laboratory tests. Of the rats tested, 59.1 and 91.7% from the Raleigh and Garner trap sites, respectively, survived a 6‐day no‐choice test of a diet containing 0.005 % warfarin. The warfarin‐resistant rats were subsequently given a choice of a 0.005% brodifacoum bait and a placebo diet. Of 27 rats, 19 died after a 3‐day test. Survivors had consumed significantly less brodifacoum (0.1 < P > 0.05) than those that died, but the survivors also died when subjected to an additional 1‐day no‐choice test of the brodifacoum bait. Field trials, in which warfarin‐resistant rats in four poultry houses were fed with poison bait for 14–16 days with 0.005% brodifacoum in a pelletised bait (‘Talon’), resulted in 87–99 % control being achieved.