Rodenticides impact on wildlife

The control of rodent pests is essential in many environments, indeed it is a legal requirement for land owners and local authorities. The main weapons in the battle against rats and mice are anticoagulant rodenticides, which inhibit the vitamin-K cycle, prevent blood clotting and cause death from internal bleeding. Warfarin, developed in the 1950s, is probably the best known anticoagulant, but second-generation compounds, such as difenacoum and bromadiolone, are now the most widely used. Anticoagulants are acutely toxic to most mammals and are highly effective control agents because of their cumulative effect and low lethal dosage.
    Usage of anticoagulants is most common in and around buildings, and certain active ingredients, such as flocoumafen and brodifacoum, are only licensed for indoor use. However, farmers and gamekeepers also commonly use anticoagulants away from farm buildings, to protect feed hoppers and root crop clamps. When rodenticides are used away from buildings it becomes much harder to control their impact on non-target species. Other mammals, such as rabbits, mice and voles are just as likely to ingest the cereal based bait and these species form a major part of the diet of many predators. Anticoagulants have long biological half lives, so predators may be especially vulnerable as residues from each contaminated prey item accumulate in the liver. 
    The impact of anticoagulants on non-target wildlife is largely unknown and studies are few and far between, mainly because of high analytical costs. The latest study, to be published in Environmental Pollution, comes from Bristol University and the Central Science Laboratory and is based on the analysis of livers from 40 stoats Mustela erminea and ten weasels M. nivalis killed by gamekeepers on English shooting estates. Nine stoats (23%) and three weasels (30%) contained sub-lethal residues of three anticoagulants: brodifacoum, bromadiolone and coumatetralyl. Perhaps because of dietary differences between the sexes, the prevalence of residues in female stoats was significantly higher than in males. As a result of this sex bias, there is concern that the overall breeding success of stoat and weasel populations may be reduced. Residue concentrations were generally quite low, but several livers contained residue concentrations nearing levels (0.1-0.4 mg.kg-1) at which mortalities have been recorded in other trials. 
    It should be remembered that these animals had been killed by gamekeepers and had survived exposure until they were trapped. It is hard to say just how many stoats and weasels receive higher doses and may die as a result of exposure to anticoagulants. A statutory monitoring programme, the Wildlife Incident Investigation Scheme, is in place for monitoring anticoagulant and other sources of pesticide poisoning. The scheme has recorded small, but increasing, numbers of foxes and badgers dying from exposure to anticoagulants. However, this programme depends on the public sending in animals found dead in suspected poisoning incidents. Unfortunately, stoats and weasels are inconspicuous species, they die out of sight and their bodies are hardly ever found or sent in. Consequently, there are only one or two records of stoat and weasel deaths caused by any form of pesticide poisoning. 
    It is unclear whether exposure has any long term effect on stoat and weasel populations. Trapping records suggest that both species may be declining in Britain, and increasing usage of rodenticides is one of several factors, together with habitat loss and competition with other predators, that have been implicated in a possible decline. Other predators, such as polecats and barn owls are also commonly exposed to rodenticides. Polecats are especially vulnerable because of their tendency to overwinter in farm buildings, where rats form a substantial part of their diet. In contrast, while barn owls are commonly exposed to rodenticides, owl populations are probably more affected by habitat loss and lack of prey than any other factor. The same may apply to stoat and weasel populations, which are also very sensitive to prey availability. Projected work at Bristol will further investigate the impact of anticoagulant exposure on non-target wildlife, with the ultimate aim of minimising the risks associated with essential rodent control.

Robbie McDonald works at the School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK, Tel. 44 (0)117 928 9000, Fax 44 (0)117 925 7374, Email: r.a.mcdonald@bristol.ac.uk

 [This article first appeared in Pesticides News No. 41, September 1998, page 17]