Toxicology and ecotoxicology of zinc phosphide as used for pest control in New Zealand

Zinc phosphide (Zn₃P₂) has been used overseas as a vertebrate pest control tool for several decades. It has been favoured in the USA and Australia for the field control of rodents and other animal pest species because of its comparatively low risk of secondary poisoning and lack of environmental persistence. Zn₃P₂ paste was approved for use as a possum control agent in New Zealand by the Environmental Protection Authority in August 2011. A micro-encapsulated form of Zn₃P₂ has been developed for use in paste and in the future will be developed in solid cereal bait, initially for controlling possums and as a rodenticide. New Zealand research over the last 10–15 years has focused on several factors, including determining Zn₃P₂ effectiveness for controlling possums, animal welfare, understanding and reducing non-target risk, and environmental fate. Zn₃P₂ is fast acting when delivered at toxic doses in baits to possums, with clinical signs first appearing from 15 min, and death after a lethal dose generally occurring in 3–5 h. Its toxicity is largely mediated by phosphine, which is formed as a breakdown product when paste is digested; Zn₃P₂ interacts with stomach acid. A toxic dose for possums will be delivered in 5 g of paste containing 1.5% Zn₃P₂ w/w. When this paste is applied in bait stations in field settings following prefeeding, possum numbers will be rapidly reduced. There should be no long-term residue risks. However, considerable care must be taken when using Zn₃P₂ for the control of animal pests because, despite low secondary poisoning risks, it has the potential (like other toxins) to cause primary poisoning of non-target species, and treatment of accidental poisoning is difficult. Exposure to sublethal doses has the potential to cause adverse effects, and strict safety precautions must be enforced to protect contractors and workers in the pest control industry. Despite extensive use of Zn₃P₂ overseas there has been only limited research and practical experience with Zn₃P₂ paste in New Zealand, especially when compared with alternative tools such as baits containing sodium monofluoroacetate (1080). Additional research efforts and practical experience should enable the effective use of Zn₃P₂ in New Zealand as a tool to achieve conservation outcomes or to control vectors of bovine tuberculosis.