Introduction

While the main goal of rodenticides is to eliminate rodent populations, they can also bring about unintended effects on non-target species. Rodenticides are targetted at 3 main rodent species: the black rat (Rattus rattus), the brown rat (Rattus norvegicus), and the house mouse (Mus domesticus) (Shore & Coeurdassier, 2018) (Figure 1). However, non-target species could also be exposed to rodenticides. This is especially so in agricultural settings where there are severe issues of rat infestations and rodenticides are heavily deployed on a large spatial scale. Specifically, there are 2 main exposure pathways: primary exposure and secondary exposure

Figure 1: Target species for rodenticides (Source: Gallura Disinfestazioni, 2016)

 

Primary exposure

Primary exposure among non-target species occurs when they directly ingest contaminated baits. Primary exposure is likely to cause acute mortalities due to the ingestion of lethal doses. Vertebrates with similar diets as the target rodents are the most likely to feed on contaminated baits (Shore & Coeurdassier, 2018). Since baits are usually in the form of cereal grains, they are likely to attract non-target granivores, herbivores and omnivores that compete with rodents for food. Examples of such vertebrates include birds and small mammals such as non-target rodents (bank voles and field voles) (Brakes & Smith, 2005). Invertebrates such as insects and worms could also feed on baits and be unintentionally poisoned.

 

Baiting practices can also affect the levels of primary exposure in non-target species (Shore & Coeurdassier, 2018). The strategic placement of baits near rat burrows is likely to lower the risk of unintentional poisoning of non-target species (Brakes & Smith, 2005). However, it might not always be possible for farmers to identify the most strategic locations to deploy rodenticides and places to avoid. Using bait protection measures such as bait boxes could also prevent primary exposure among larger mammals and birds. But these protection measures are not 100% foolproof since non-target species that are of similar or smaller sizes as target rodents are still likely to gain access to the baits.

 

Secondary exposure

Secondary exposure among non-target species occurs when they consume preys that have been contaminated with rodenticides. While secondary exposure is less likely to cause acute mortalities, chronic bioaccumulation in non-target species can lead to a decreasing population in the long run. Predators such as owls that feed on rodents are the most likely to be affected in areas where rodenticides are widely deployed (López-Perea & Mateo, 2018). Scavengers that feed on the carcasses of poisoned rodents are also at high risk of secondary exposure.

 

Non-target species that have been primarily exposed to rodenticides also become sources of secondary exposure for their predators. For instance, insectivorous birds might be exposed to rodenticides when they feed on contaminated invertebrates (López-Perea & Mateo, 2018).

 

Conclusion

Due to the wide variety of pollution pathways that can affect non-target species, rodenticides pose high risks to local and potentially regional biodiversity. To minimise unintentional impacts and maximise efficacies, there should be more studies on methods to deploy rodenticides more strategically.

 

References

Brakes, C. R., & Smith, R. H. (2005). Exposure of non-target small mammals to rodenticides: Short-term effects, recovery and implications for secondary poisoning. The Journal of Applied Ecology, 42(1), 118-128. https://doi.org/10.1111/j.1365-2664.2005.00997.x

Shore, R.F., Coeurdassier, M. (2018). Primary Exposure and Effects in Non-target Animals. In: van den Brink, N., Elliott, J., Shore, R., Rattner, B. (eds) Anticoagulant Rodenticides and Wildlife. Emerging Topics in Ecotoxicology, vol 5. Springer, Cham. https://doi-org.libproxy1.nus.edu.sg/10.1007/978-3-319-64377-9_6

López-Perea, J.J., Mateo, R. (2018). Secondary Exposure to Anticoagulant Rodenticides and Effects on Predators. In: van den Brink, N., Elliott, J., Shore, R., Rattner, B. (eds) Anticoagulant Rodenticides and Wildlife. Emerging Topics in Ecotoxicology, vol 5. Springer, Cham. https://doi-org.libproxy1.nus.edu.sg/10.1007/978-3-319-64377-9_7