Review of the toxicity and impacts of brodifacoum on non‐target wildlife in New Zealand

Abstract Brodifacoum, an anticoagulant used in cereal‐based baits for the control of vertebrate pests, especially rodents, may be accidentally ingested by non‐target species. In birds, its acute toxicity varies from an LD 50 of < 1 mg/kg in pukeko to >20 mg/kg in paradise shelduck. Fourteen indigenous and eight introduced bird species have been reported killed by field use of brodifacoum in New Zealand. Populations of three species (western weka, Stewart Island weka, and pukeko) have been severely reduced in poisoned areas. There are no published data on the acute toxicity of brodifacoum in bats, reptiles, or amphibians. Invertebrates are unlikely to be killed by anticoagulants. Because of the high toxicity of brodifacoum, all vertebrates that eat baits or poisoned prey are at risk. Brodifacoum is only slowly eliminated from the liver, and therefore accumulates in vertebrates if there are repeated exposures to the toxin, which increases the risk of death. In New Zealand, indigenous non‐target species most at risk from eating brodifacoum in cereal‐based baits are herbivorous and omnivorous birds (e.g., weka, pukeko, and saddleback). The species most at risk from secondary poisoning are predatory and scavenging birds such as weka, Australasian harrier, southern black‐backed gull, and morepork. Insectivorous birds, bats, lizards, and frogs are probably least at risk. However, laboratory and field trials are essential to determine the actual risks, and reduce scientific uncertainties. The risks of pest control must be carefully balanced against the benefits. These benefits can be substantial where introduced mammals threaten native species with extinction.