The potential of sodium nitrite as a new tool for vertebrate pest control from formulation, efficacy and safety perspectives

Introduction. The commonly used food preservative sodium nitrite (NaNO₂) has been explored as a potential vertebrate toxic agent (VTA) for the control of feral pigs (Sus scrofa) and brushtail possums (Trichosurus vulpecula). In sufficiently high doses NaNO₂ is toxic because it can induce profound methaemoglobinaemia. NaNO₂ is highly reactive and has a bitter and salty taste, encapsulation has been explored as a method of improving its stability and palatability. New candidate VTAs require proven efficacy on target species and to be evaluated for the risk to non-target species from primary and secondary poisoning, and for persistence in the environment. Aims. The aims of this study were (i) to determine whether NaNO₂ can be encapsulated in a form that keeps it stable, masks its taste and enables it to be used as a VTA in an effective and commercially viable manner to control possums and feral pigs, and (ii) to determine the risks to non-target species through primary and secondary poisoning from this formulation. Materials and methods. The corn protein zein was identified as a suitable encapsulant material for NaNO₂. Polyethylene glycol (PEG), glycerol and polyvinylpyrrolidone (PVP) were each added to zein to test their suitability as potential plasticizers for the encapsulation of NaNO₂. The most effective zein-plasticizer formulation was used to coat NaNO₂ for efficacy trials. This formulation was mixed through palatable paste and presented to possums and feral pigs in cage and field trials to determine its lethal efficacy. The acute toxicity of encapsulated NaNO₂ was assessed for non-target species by oral gavage and free-feeding trials with toxic paste and pellet baits. These trials were undertaken with four non-native bird species including chickens (Gallus gallus domesticus), Pekin ducks (Anas platyrhynchos domestica), pigeons (Columba livia f. domestica) and budgerigars (Melopsittacus undulates). Weta (Family: Rhaphidophoridae), an invertebrate native to New Zealand, were also presented baits. Secondary poisoning risk for non-target species was assessed through observing the health of dogs, feral cats and chickens fed carcasses of possums poisoned with encapsulated NaNO₂. Blood and tissue samples were taken from dogs and cats to identify any potential damage to internal organs or tissues. The environmental degradation of NaNO₂ was determined in three parts (i) by comparing the solubility of encapsulated and unencapsulated NaNO₂, (ii) by analysing the breakdown rate of NaNO₂ in bait spilled onto the ground, and (iii) a desktop analysis of the likely pathways of NaNO₂ from bait to soil and water. Results. PVP at 1 g kg-1 was the most effective plasticizer added to zein for encapsulation of NaNO₂. PVP improved the ability of zein films to resist water transmission and absorption, whilst the mechanical properties of zein films were not significantly altered. This formulation mixed through paste bait was palatable to possums and pigs. In no-choice cage trials, all 12 possums consumed a lethal dose of toxic paste bait and died on average after 95.6 minutes (±4.9 SE). In two-choice cage trials seven out of eight possums consumed a lethal dose of toxic paste bait and died on average after 96.7 minutes (±11.4 SE). Two field trials targeting possums using this toxic paste bait reduced their abundance by 81.2% (± 2.5% SE) and 72.7% (± 1.6% SE) respectively. In pen trials, eight out of nine pigs consumed a lethal dose of toxic paste bait and the average time to death was 59.5 minutes (± 8.47 SE). In a field trial, 11 out of 12 radio-collared feral pigs consumed a lethal dose of toxic paste bait. The oral LD₅₀ value for NaNO₂ in solution was approximately 68.50 mg/kg (95% CI 55.00 – 80.00 mg/kg) for both chickens and Peking ducks. The LD50 for chickens that consumed a lethal dose of toxic paste bait was approximately 254.6 mg/kg (95% CI 249.1 - 260.2 mg/kg). There was no evidence of weta feeding on toxic baits. After consuming whole possum carcasses, minced possum meat, vital organs or stomach, no dogs, cats or chickens displayed any obvious physiological signs of methaemoglobinaemia. Blood samples from dogs and cats confirmed that liver and renal function remained normal and there was no damage to muscle tissue after consuming poisoned carcasses. Encapsulated NaNO₂ took approximately 17.5 minutes to dissolve in distilled water compared to two minutes for unencapsulated NaNO₂. The concentration of encapsulated NaNO₂ in paste bait on the ground declined on average by 50% after two weeks and on average by 98% after four weeks. Its concentration in pellet baits on the ground declined by 50% after one week and by 85% to 93% after three weeks. Discussion and conclusions. Improved palatability and stability of NaNO₂ as a result of successful encapsulation enabled the corresponding efficacy for possums and feral pigs to be achieved. Acute toxicity trials showed that NaNO₂ is toxic to birds and that baits containing encapsulated NaNO₂ are potentially hazardous. When manufacturing and using NaNO₂ paste and pellet baits it is therefore appropriate to apply similar precautions to those applied to other VTAs that are ground laid, this includes colouring the baits green and using them in appropriate bait stations. The low risk of secondary poisoning makes encapsulated NaNO2 an ideal toxin for use in areas where non-target species like dogs are likely to scavenge carcasses of poisoned possums. In November 2013, NaNO₂ paste, known as Bait-Rite paste (ACVM V009563), was registered in New Zealand as a VTA for the control of possums and feral pigs. This represents the first registration of NaNO₂ worldwide for use as a VTA and the only toxin currently registered for feral pig management in New Zealand. The same formulation of encapsulated NaNO₂ is currently being trialled for feral pig management in the USA and Australia.