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SUMMARYFeeding tests were carried out in the laboratory to evaluate VVBA 8119 as a potential new rodenticide against wild common rats (Rattws norvegicus), ship rats (R. rattus) and house mice (Mus musculus). The results obtained are compared with data previously obtained for difenacoum, another member of the same series of 4-hydroxycoumarin anticoagulants.With warfarin-resistant and non-resistant common rats, complete kills were obtained using a concentration of 0.0005% for 2 days, or 0 001 % for 1 day: a 1-day test at 0.0005% killed 6 out of 10 and 17 out of 20 of the two types respectively. At 0.005% complete kills of resistant ship rats were obtained after 2 days exposure and of resistant house mice after 1 day, but at 0.002% for 2 days there was some survival. Non-resistant ship rats and house mice were all killed after 2 days feeding on 0.002% bait. In 2-day palatability tests, R. norvegicus showed no significant aversion to the poison at 0.002% and 100% mortality was obtained. The poison was significantly unpalatable to R. rattus at 0.005% and to M. musculus at 0.005% and 0*002 %, although with the last species these concentrations gave complete kills.It is concluded that WBA 8119 has greater activity than other know-n anticoagulants against the three commensal species examined. The laboratory results suggest that concentrations between 0.0005% and 0 002% would be suitable for field use against common rats, and between 0.002% and 0.005% for ship rats and house mice.

SummaryThe 2-stage determination is based on changes in blood coagulation activity brought about both by the administration of warfarin in conjunction with vitamin KJ epoxide and by feeding a vitamin K-free diet for 4 days. When it was applied to laboratory-bred rats of known warfarin-resistance genotype, 35/35 homozygous susceptible, 44/44 homozygous resistant and 131/133 heterozygous rats were correctly classified. This method was equally effective in identifying the genotype of wild rats carrying the warfarin-resistance gene, Rw 2 • The procedure is rapid and accurate.

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.