Berlin Highlights—The Use of Rodenticides, a Nagging Issue on Effectiveness and Risks
Nico van den Brink, Alterra Wageningen UR and John Elliott, Environment Canada
Rodenticides are used globally and most usage involves anticoagulants, both first and second generation compounds. Use of first-generation anticoagulant rodenticides (FGARs) resulted in resistance in target populations, and second-generation anticoagulant rodenticides (SGARs) were subsequently developed. Some of the SGARs, are known PBT-compounds (persistent, bioaccumulative, toxic) which would normally restrict their use. However, rodenticides are considered "essential use" products due to a lack of acceptable alternatives. This implies that the advantages of their use in rodent control outweigh the risks to non-target organisms.
Several studies show exposure to SGARs not only in non-target rodents but also in a wide array of wildlife predators. In some cases, effects on such non-target species were evident. Exposure may be related to baiting schemes and protocols, types of compounds used, food web relationships and species-specific sensitivity. Studies have shown that the toxicodynamics and the toxicokinetics of anticoagulant rodenticides are extremely complex, which hampers a proper risk assessment based on field cases and derived dose-response relationships.
At our session in Berlin, we learned the following from an experienced group of speakers:
- The extent of food chain contamination by SGARs is even broader than envisioned, even little penguins (Eudyptula minor) from New Zealand contain detectable SGAR residues
- Up to 80% of animals analysed for rodenticides in the UK contained one or multiple SGARs and that incidence rate is still increasing
- Resistance problems may also arise from the use of SGARs and not just result from exposure to FGARs
- Pathways of rodenticide movement to non-target wildlife are complex and may be affected by land use, however direct consumption of rats appeared to be the primary vector for urban owl species
- Proper application of rodenticides according to the guidance decreases the environmental risks, as was shown by the massive use of rodenticides during the Foot and Mouth Disease outbreaks in the UK that was not accompanied by an increase in non-target species exposure
- Non-target species caught up to 800 m from an application with rodenticides may contain detectable levels of SGARs
- Multiple exposure to FGARs may result in higher risks than previously expected
- Environmentally relevant species may be much more sensitive to rodenticides than standard test species and thus higher- tier tests should be performed with more sensitive species
- Calculated PEC/PNEC ratios for specific SGARs may be well above 1, clearly indicating risks
Overall, the session illustrated the unmistakable societal need for methods to control outbreaks of rodents. However, the presentations also showed that the use of anticoagulant rodenticides, first and second generation, may result in undesired toxic effects on non-target organisms while SGARs may result in food web transfer to predators. This demands further research on exposure, dose-response relationships and monitoring of non-target species. This should be performed in relation to the class of rodenticides, the potential for resistance development and the methods of application. Finally, alternatives to the use of anticoagulant rodenticides should be developed since risks appear evident as well as the issue of development of resistance. Thus, we have another interesting example of the difficulties encountered in trying to balance societal needs with environmental risks.
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