Rats are persistent pests with a long history of causing problems in human society. In agriculture, these pests cause problems by feeding on crops and acting as a vector for diseases, which can lead to socio-economic issues for many actors in the supply chain. Therefore, these problems make it necessary to keep rat populations on farms at bay.
Before the proliferation of chemical rodenticides, there were several methods to control rat populations. Historically, in ancient Egypt, Rome, and India, house pets such as cats and ferrets were considered to be effective at preventing rat infestations (van den Brink et al., 2018). In addition to relying on house pets, people also used mousetraps and hired rat catchers for rat control (van den Brink et al., 2018). However, these measures are often labour-intensive and do not eradicate entire rat populations.
Today, the most common type of rodenticide is anticoagulants. Anticoagulants are chemical substances that prevent blood clotting, causing rats to die from excessive bleeding (US EPA, n.d.). However, the type of chemical compound people use today is very different from the chemicals that were first introduced in the 1950s.
First-generation anticoagulant rodenticides (FGARs) first came into use in the 1950s and effectively “revolutionised rodent control” because of how effective they were (Rodent Resistance Action Committee, n.d.). The FGARs tend to be most effective when rats feed on them for several days. It takes about 5 to 8 days for the FGARs to kill the rat. At the time, many were under the impression that the FGARs were environmentally-friendly alternatives because existing chemicals were less effective and potentially threatened biodiversity (van den Brink et al., 2018). However, by the 1960s, rats grew increasingly resistant to the FGARs, causing a decline in its efficacy against rodent control.
Consequently, pest resistance to FGARs led to the development and wide use of second-generation anticoagulant rodenticides (SGARs) in the 1970s (van den Brink et al., 2018). These SGARs consist of completely different chemical compounds that are more acutely toxic. Instead of taking several days to kill rats, SGARs can kill rats after a single feeding. As a result, they were effective against rats that had developed resistance to FGARs, making it a more attractive option to control rat populations.
However, the story of how anticoagulants evolved over the years is a worrying sign for the future of rodent control because pest resistance is a recurring problem. As rats develop growing resistance against current pesticides in the market, there is a growing need to introduce stronger chemicals that are potentially more harmful to the environment. SGARs are PBT compounds (Persistent, Bioaccumulative, and Toxic) (van den Brink et al., 2018) that are more harmful than FGARs because of their longer half-lives (Horak et al., 2018). They persist in the tissues of rats, posing threats to non-target animals that feed on the poisoned rats. In recent years, there have already been reports of growing resistance among rat populations against SGARs (Buckle, 2013), indicating a potential need for more potent chemicals for rodent control.
References:
Buckle A. (2013). Anticoagulant resistance in the United Kingdom and a new guideline for the management of resistant infestations of Norway rats (Rattus norvegicus Berk.). Pest management science, 69(3), 334–341. https://doi.org/10.1002/ps.3309
Horak, K.E., Fisher, P.M., Hopkins, B. (2018). Pharmacokinetics of Anticoagulant Rodenticides in Target and Non-target Organisms. 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_4
Rodent Resistance Action Committee. (n.d.). 1st generation anticoagulants. RRAC Resistance Guide. https://guide.rrac.info/rodenticide-molecules/1st-generation-anticoagulants.html
US EPA. (n.d.). Restrictions on Rodenticide Products. https://www.epa.gov/rodenticides/restrictions-rodenticide-products
van den Brink, N.W., Elliott, J.E., Shore, R.F., Rattner, B.A. (2018). Anticoagulant Rodenticides and Wildlife: Introduction. 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_1