The animals examined were either found dead or they had been shot or trapped for some other purpose. The incidence and concentrations of active substances (bromadiolone, difenacoum, brodifacoum, flocoumafen, chlorophacinone, difethialone, coumatetralyl) were determined from liver samples taken from the animals. Active substances are transferred from the rodents that have ingested the poison to predators who eat these animals.
In Finland, the percentage of active substances found in animals was higher than in Central Europe where similar studies have also been carried out. In Spain, only half of the birds of prey and mammal predators studied had been exposed to the active substances of rodenticides. On the other hand, three out of five foxes in Germany had been exposed to rodenticides, while in Finland all of the twelve foxes examined were found to have the active substances sought.
– We do not have an answer to the question why more residues are found in Finland than in Germany or Spain. Efforts have been made to reduce the exposure of other animals to rodenticides with various restrictions to the use of rodenticides. The common discovery of rodenticides in predators and scavengers raises concerns whether the restrictions set for rodenticides are complied with or if the current restrictions are sufficient. However, it is clear that rodenticides will inevitably end up in other animals for as long as poisons are used, says Senior Officer Sanna Koivisto of Tukes.
Rodenticides have a delayed effect, and rodents will not die until about a week after they have ingested a lethal dose. At first, rodents can move about in the normal way and be hunted by predators.
In Finland, the detected rodenticide concentrations found in the majority of mammals and birds studied were small. However, in 12.5 per cent of the animals studied, the concentrations were so high that it is possible that rodenticides have had a harmful impact on the animals.
Rodenticides were found commonly in eagle-owls, tawny owls, foxes, raccoon dogs, and weasels. The most frequently detected active substance was bromadiolone, and its concentrations were also the highest. Bromadiolone has been the most commonly used active substance in rodenticides in Finland since the early 2000s. The next most common active substances detected were coumatetralyl, difenacoum, brodifacoum, flocoumafen. The observations corresponded quite well to the amounts of active substances sold. There was a lot of variation in the concentrations between animal species and between individual animals of the same species.
In addition, active substances with the most restricted use, i.e. the use of which is only permitted for professional pest control operators indoors, were found in the animal species studied, although to a lesser extent than the more commonly used substances.
The current restrictions on using rodenticides are not sufficient
The Chemicals Act obliges to comply with the instructions for use when using rodenticides. Individuals may exterminate mice in indoor areas and rats in buildings and in their immediate vicinity. Baits must always be kept in locked-up bait boxes to prevent, for example, dogs from ingesting them. Surplus poison baits must be destroyed as hazardous waste. However, all users do not necessarily comply with the instructions for use. In Finland, rodenticides can be sold in shops, but not bait boxes. You need to go out of your way to search for bait boxes elsewhere.
In professional pest control, the prevention of rodents with poison baits seems to be common practice, although that kind of use is not in accordance with the instructions for use. Long-term use of rodenticides increases the exposure of other animals and promotes the building of resistance in rodents.
The Tukes website contains information about the prevention of disadvantages and the correct use of rodenticides for consumers and professional pest control operators.
The risks of the active substances used in rodenticides are currently being reassessed in the EU. Methods of reducing the accumulation of rodenticides in domestic animals and wildlife will be agreed on in connection with the risk assessment.
Survey: Prevalence of anticoagulant rodenticides in non-target predators and scavengers in Finland (pdf)
‘Rodenticides in Finnish animals’ graph (in Finnish)
Sanna Koivisto, Senior Officer, Tukes, tel. 029 5052 030
sanna . Koivisto (at) tukes.fi