Development and use

Zinc phosphide is an inorganic chemical compound first used as a rodenticide in Italy 1911. It became popular in the 1930s in Europe and was introduced in the USA in 1943. Zinc phosphide was registered in Australia in 1997 (Caughley et al., 1998). Today it is the one of the few acute poisons still used for the control of pest rodents because alpha-chloralose, phosphorus and strychnine have been phased out (McLeod and Saunders, 2013). It is mainly used to control mice populations in broadacre crops, but it can also be used to control rat populations in specific circumstances (typically for plague incursions).

Mode of action

When ingested, the bait reacts with stomach acids to produce poisonous phosphine gas, causing central nervous system depression, irritation of the lungs, and damage to the liver, kidney and heart. Death occurs suddenly with minimal outward signs. Dead rodents are frequently found on their belly with legs and tail spread out (Freeman et al., 1954).

Birds (including chickens) are highly sensitive to zinc phosphide, so give extra consideration to separating birds from bait sources (Christopher et al., 1982). Zinc phosphide does not accumulate in the tissues of rodents (Robertson et al., 1945), therefore the risk of secondary poisoning is greatly reduced compared to anticoagulant compounds.

Time to death

Time to death is dependent on the size of dose. Large doses have caused death in rats within 20 minutes, while low doses may take up to several days (Freeman et al., 1954; Schoof, 1970).

Evidence of resistance

There is no evidence of resistance to zinc phosphide.

APVMA-registered products containing zinc phosphide

ZP Mouse (20g/kg), ZP Rat (20g/kg), Mouseoff Zinc Phosphide (25g/kg), Rattoff Zinc Phosphide (25g/kg), Farmalinx Zincphos (25g/kg), Imtrade Deadmouse Zinc Phosphide (25g/kg), Surefire Zinc Phosphide (25g/kg), Pestmaster ZnP (25g/kg), Last Supper (25g/kg), 4 Farmers Zinc Phosphide (25g/kg)

Available formulation

• Grain bait
• Pellet bait
• Sachet bait

Acute toxicity

Species	LD50	Average bodyweight	Amount of bait consumed for a LD50	Reference (for LD50)
Mouse	25.8–53.3mg/kg	20g	0.02–0.04g*	Bell, 1972
Norway rat	27–48mg/kg	320g	0.3–0.6g*	Dieke and Richter, 1946
*Calculated using a bait concentration of 25g/kg

The table above shows the oral median lethal dose (LD₅₀) of zinc phosphide for the house mouse and Norway rat, the typical bodyweight for an adult animal from each species, and the total amount of commercial bait needed to be eaten to cause death. An adult rat (bodyweight of 320 grams) will eat about 20–30 grams of food daily; an adult mouse (bodyweight of 20 grams) will eat 2–5 grams of food daily (Hadler and Buckle, 1991). Zinc phosphide rodenticides have a standard active concentration of 25g/kg. Therefore, 0.3–0.6 grams of bait would be considered a lethal dose for rats and 0.02–0.04 grams of bait is lethal for mice. For both species, this is a fraction of daily food requirement. It is possible for a lethal dose to be consumed in a single feed.

Poison scheduling and regulatory requirements

Zinc phosphide is classified as a Schedule 7 Dangerous Poison. It is available only to specialised or authorised users. Regulations restricting availability, possession, storage or use may apply. Please check with your state health authority before purchasing.

Handling, storage and user safety

Store in the closed original container in a dry, cool, well-ventilated area out of direct sunlight. Do not handle near food, animal foodstuffs or drinking water. Keep out of reach of children. Do not use near heat sources, open flame or hot surfaces. As soon as possible, wash hands thoroughly after applying bait.

Read the label before use. For detailed instructions on handling and user safety, please refer to the relevant Safety Data Sheet.

References

Bell, H. B. (1972). The hazards of secondary poisoning from zinc phosphide to selected vertebrate species. Masters Thesis, University of Tennessee.

Caughley, J., Strong, K. and Hinchliffe, P. (1998). Report on the zinc phosphide baiting program to control mice in central Queensland in 1997. Queensland Department of Natural Resources.

Christopher, M. J., Philip G. H., Purushotham, K. R. and Ramamurthi, R. (1982). Incidence of a secondary poisoning with zinc phosphide in a poultry farm. Rodent Newsletter (India), 6:4.

Dieke, S. H. and Richter C.F. (1946). Comparative assays of rodenticides on wild Norway rats. Public Health Reports, 61:672-679.

Freeman, R. B., Elton, C., Leslie, P. H., Ranson R. M., Rzoska, J. and Thompson, H. V. (1954). Properties of the poisons used in rodent control. Pages 25-146 in D. Chitty and H. N. Southern, editors. Control of Rats and Mice. Vol 1 Rats. Oxford University Press, London.

McLeod, L., & Saunders, G. (2013). Pesticides used in the management of vertebrate pests in Australia: A review. NSW Department of Primary industries.

Robertson, A., Campbell, J. G., & Graves, D. N. (1945). Experimental zinc phosphide poisoning in fowls. Journal of Comparative Pathology, 55, 290-300.

Schoof, H. F. (1970). Zinc phosphide as a rodenticide. Pest Control, 38:44.