Development Of An Innovative Cholecalciferol Rodenticide Bait

BASF have developed a non-anticoagulant rodenticide soft bock bait, Selontra® (0.075 % cholecalciferol). In
mammals, cholecalciferol toxicity causes death by hypercalcemia - the calcification of soft tissues such as heart, kidney, liver, stomach (inducing a stop-feeding effect). The resulting “stop-feeding” effect means that cholecalciferol rodenticide baits must be both potent and palatable to ensure a lethal dose of bait is eaten before this effect occurs. The mode of action of cholecalciferol means that once a lethal amount has been eaten, time to death is between 2-5 days, compared to 4-10 days for anticoagulants. All reported choice feeding (palatability) studies and field trial studies undertaken either complied with or were a modification of that prescribed in the Biocidal Products Directive Technical Notes for Guidance on Product Evaluation, Product Type 14. Choice feeding studies (4 days) were undertaken using male anticoagulant susceptible R. norvegicus against either Selontra® (n=50) or Storm® (0.005 % flocoumafen) wax block bait (n=50). The mean times to death were 2.8 days for Selontra® and 5.7 days for Storm®. This quicker time to death means that, providing the bait formulation is palatable, control of an infestation can be quicker with a cholecalciferol bait than an anticoagulant bait. Field trial studies undertaken have shown that against R. norvegicus, R. rattus and M. musculus control can be achieved within as few as 7 days of baiting. Choice feeding studies with Selontra® undertaken against anticoagulant resistant strains of R. norvegicus, Y139S and L120Q resulted in 100 % mortality within 4 days. This effectiveness, against resistant rats was confirmed with two field trials. The first trial was in France, where the Brown rats were identified, by DNA sequencing, as Y139F - resistant to first generation anticoagulants (FGARs) rodenticides and bromadiolone. Total control, 100 %, was affected after 12 days of baiting with Selontra®. The second site was in undertaken in South England where the rats were identified, by DNA sequencing, as L120Q - resistant to FGARs plus bromadiolone and difenacoum. The population of approximately 2,000 rats was also considered to be extremely neophobic. After 18 days of baiting with Selontra® at this highly challenging site, 83 % control was achieved. Unfortunately, after 18 days the baiting was terminated due to harvesting and subsequent significant re-invasion affecting population assessment. The laboratory and field data presented, demonstrates the efficacy against Rattus norvegicus, Rattus rattus, Rattus exulans and Mus musculus including anticoagulant resistant strains. Selontra® is palatable and efficacious against both anticoagulant susceptible and resistant strains of rats and mice. Field trials have shown that pre-baiting is not required. This bait will be an invaluable resistant management tool for the effective control of rodent infestations.