Apparent resistance to brodifacoum in

Abstract

Context. Anticoagulants have been used in New Zealand for decades, but few data are available on the sustainability of these toxins for rodent control. It is important to regularly monitor for resistance in long-term brodifacoum-use areas and establish a database for future references.Aims. This study aimed to estimate the effective dose (ED 50 ) of brodifacoum for ship rats from an area of New Zealand with no history of brodifacoum use, in order to establish a bloodclotting response test for assessing resistance in rodent populations from other areas. Methods. A ranging study was conducted whereby successive groups of ship rats were administered brodifacoum doses that were increased or decreased progressively, until an International Normalised Ratio (INR) of 3.6 was reached. Linear regression was used to model the relationship between dose and INR, and ED50 dose was estimated using the resulting model. Results. None of the rats appeared susceptible to brodifacoum at previously reported LD 50 exposures for this species. The ED 50 of brodifacoum was estimated to be 2.88 mg/kg for males and 3.81 mg/kg for females. These values are 6-8 times greater than the previously published lethal dose values for ship rats in New Zealand. Conclusions. Blood-clotting inhibition was detected in the rats only following high doses of brodifacoum, which may indicate resistance within the sampled population. Implications. Relatively low susceptibility, or resistance, to brodifacoum in New Zealand ship rats may be mediated by spatial connections between areas with different histories and patterns of anticoagulant rodenticide use.

Rodent management in Aotearoa New Zealand: approaches and challenges to landscape‐scale control

“…2022; Sran et al. 2022). Using anticoagulant poisons to target invasive carnivores by secondary poisoning is illegal in United Kingdom and European Union.…”

“…Most rodent management in NZ depends on toxins, especially 1080 for large-scale mainland control, brodifacoum for island eradications, and first-generation anticoagulants such as pindone and diphacinone for ground control in ecosanctuaries and in diverse urban and rural projects. A better understanding of toxin humaneness, non-target and secondary effects, and resistance to toxins is needed to address public concerns about these matters so that New Zealanders can keep using these and other tools (Hansford 2016;Eason et al 2017;Baker et al 2022;Sran et al 2022). Using anticoagulant poisons to target invasive carnivores by secondary poisoning is illegal in United Kingdom and European Union.…”