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SUMMARYTrials of rodenticidal baits containing 50 p.p.m. difenacoum, 50 p.p.m. bromadiolone or 20 p.p.m. brodifacoum were carried out on farmsteads against populations of Rattus norvegicus containing difenacoum-resistant individuals. Six difenacoum treatments failed in 14-42 days of baiting. Two treatments with bromadiolone succeeded in 23 and 33 days, but four further treatments lasting 35-56 days failed to eradicate the populations. Brodifacoum gave virtually complete control of six populations in 21-73 days and of the ten residual populations left behind by the other two compounds, after baiting for a further 11-85 days. The performance of both bromadiolone and brodifacoum was well below that reported by previous investigators, indicating the possibility of low-grade resistance to these compounds in the difenacoum-resistant strain.

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S U M M A R YObservations are presented to show that resistance to difenacoum, a rodenticide commonly used for the control of warfarin-resistant rodents, is widespread in populations of the Norway rat, Rattus norvegicus, in an area centred in north-east Hampshire. Of 202 rats trapped on 4 2 farms in this district and tested in the laboratory, 85% were resistant to warfarin and 14% were resistant to difenacoum. Only rats that were resistant to warfarin showed difenacoum resistance, indicating it to be a form of cross resistance. The difenacoum-resistant rats survived a dose of difenacoum that was lethal to more than 99% of susceptibles, but succumbed when this dose was increased fivefold. The results of four treatments carried out with difenacoum on farms illustrate that this level of resistance seriously reduces the effectiveness of difenacoum as a rodenticide in the field.