Second Generation Anticoagulant Rodenticides (SGARs) are currently used for controlling small mammalian pests. Evidences of their negative impact on wildlife have been reported worldwide. In some countries, particularly in Europe, the SGAR bromadiolone is intensively used in the field. It is the only rodenticide authorised in France for controlling the population outbreaks of the Water vole Arvicola terrestris Sherman. Vole control operations using bromadiolone are undertaken over large areas (e.g., hundreds of km²), and dozens to hundreds of secondary poisonings of Red foxes (Vulpes vulpes) have been reported each year.
The first aim was to measure bromadiolone persistence in wheat baits and its variability in field conditions. The persistence of bromadiolone in artificial galleries is short (half-life 3-6 days) and weakly influenced by environmental conditions (soil type and climatic conditions). However, this persistence is dramatically increased in baits when they were stored (27< half-life < 45 days). This may cause rodent exposure on a longer duration.
The second objective was to assess the kinetic of bromadiolone residues in rodent populations in a treated area. Here, both the Water vole (the target species) and the Common vole Microtus arvalis (a non target species), two rodent species having high density in grassland and both eaten by predators, were studied. Bromadiolone residues in rodent population reached the maximum concentration from the third to the fifth day depending on the species or the tissue and the high concentrations are maintained during 15-20 days. However, the re-colonization in the treated parcel by rodents from the neighbourhood may induce consequences with exposure of rodent until three months after bait distribution. This may explain in part why rodents with bromadiolone residues may be potentially available to predators more than four to six months after a treatment.
Finally, we assessed the feasibility of monitoring the exposure of foxes to bromadiolone by analysing the residues in faeces following field treatments. A new LC - MS method was developed. Bromadiolone residues in faeces and blood exhibited similar patterns over time: a dramatic increase during the exposure period (2 or 5 days according to foxes) and then a gradual decrease after the last exposure. Bromadiolone was detected in faeces 15 hours after the first exposure and for at least 24 days after the last exposure (end of the experiment). Two of the foxes presented severe external haemorrhages six days after the first exposure. Their prothrombin time (PT) was multiplied by 6 compared to the day before exposure and they would probably have died without vitamin-K administration. Then, two other studies were achieved in situ. The first demonstrated that 48 % of faeces sampled in an area treated between 15 to 45 days before, contained bromadiolone residues. The second demonstrated an exponential relationship between the rectum content of foxes trapped in an area treated 1 to 6 months before the field surveys, and the residues measured in the liver.
These findings provide a basis for programs aiming to monitor the exposure in situ of wild fox populations to bromadiolone using non-invasive methods based on standard sampling and analysis of residues in faeces. After optimisation, it will be possible to determine if different environmental factors may affect fox population exposure without trapping and/or animal death.