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Types of pesticides

Pesticides are substances that are designed to control pests. They include insecticides (substances used to kill insects), rodenticides (substances used to kill rodents, such as rat poison), molluscicides (substances used to kill molluscs, such as snail baits), and herbicides (substances used to kill weeds).

How do poisons and baits harm wildlife?

Although pesticides are designed to kill specific animals, insects or plants, these products commonly poison non-target species, including wildlife, through either primary or secondary poisoning. 

Primary poisoning occurs when an animal ingests a pesticide directly – for example, a brushtail possum or antechinus (a small mouse-like native marsupial) eating rat bait. Secondary poisoning occurs when an animal eats another animal that has itself ingested a pesticide – for example, a greater sooty owl (such as the one pictured above) eating an antechinus that had eaten rat bait. 

Pesticides can kill wildlife, as well as cause illness which increases an animal’s risk of being preyed upon. The exact effect of both primary and secondary pesticide poisoning on wildlife depends on the species, and on the type and amount of poison consumed.

Common poisons and how they work

Rodenticides are the most common and harmful pesticides to Australian wildlife. Though no comprehensive monitoring of non-target exposure of rodenticides has been conducted, numerous studies have documented the harm rodenticides do to native animals. In 2018, an Australian study found that anticoagulant rodenticides in particular are implicated in non-target wildlife poisoning in Australia, and warned Australia’s usage patterns and lax regulations “may increase the risk of non-target poisoning”.

Most rodenticides work by disrupting the normal coagulation (blood clotting) process, and are classified as either “first generation” / “multiple dose” or “second generation” / “single dose”, depending on how many doses are required for the poison to be lethal. 

These anticoagulant rodenticides cause victims of anticoagulant rodenticides to suffer greatly before dying, as they work by inhibiting Vitamin K in the body, therefore disrupting the normal coagulation process. This results in poisoned animals suffering from uncontrolled bleeding or hemorrhaging, either spontaneously or from cuts or scratches. In the case of internally hemorrhaging, which is difficult to spot, the only sign of poisoning is that the animal is weak, or (occasionally) bleeding from the nose or mouth. Affected wildlife are also more likely to crash into structures and vehicles, and be killed by predators. 
 
An animal has to eat a first generation rodenticide (e.g. warfarin, pindone, chlorophaninone, diphacinone) more than once in order to obtain a lethal dose. For this reason, second generation rodenticides (e.g. difenacoum, brodifacoum, bromadiolone and difethialone) are the most commonly used rodenticides, due to their high lethality. Second generation rodenticides only require a single dose to be consumed in order to be lethal, yet kill the animal slowly, meaning the animal keeps coming back. This results in the animal consuming many times more poison than a single lethal dose over the multiple days it takes them to die, during which time they are easy but lethal prey to predators. This is why second generation poisons tend to be much more acutely toxic to non-target wildlife, as they are much more likely to bioaccumulate and biomagnify, and clear very slowly from the body.

Which species are most at risk from which poisons? 

Small Mammals

Small mammals including possums and bandicoots often consume poisons such as snail bait, or rat bait that has been laid out to attract and kill rats, mice, and rabbits. Poisons such as pindone are often added to oats or carrots, and lead to a slow, painful death of internal bleeding. Australian possums often consume rat bait such as warfarin, which causes extensive internal bleeding, usually resulting in death. 

There is a very poor chance of survival. Possums are also known to consume slug bait, which results in a prolonged painful death mainly from neurological effects. There is no treatment.

Small mammals can also be poisoned by insecticides. Possums, for example, can ingest these poisons when consuming fruit from a tree that has been sprayed with insecticide. Rescued by a WIRES carer, the brushtail possum joey pictured below was suffering from suspected insecticide poisoning. Though coughing up blood, luckily the joey did not ingest a lethal dose as he survived in care and was later released.

Large Mammals

Despite their size, large mammals including wallabies, kangaroos and wombats can also fall victim to pesticide poisoning. Wallabies and kangaroos have been known to suffer from rodenticide poisoning, while poisons often ingested by wombats include rat bait from farm sheds, and sodium fluroacetate (1080) laid out to kill pests such as cats and foxes.

Australian mammals are also impacted by the use of insecticides. DDT, although a banned substance, has been reported as killing marsupials.

Birds

Birds have a high metabolic rate and therefore succumb quickly to poisons.

Australian birds of prey – owls (such as the southern boobook) and diurnal raptors (such as kestrels) – can be killed by internal bleeding when they eat rodents that have ingested rat bait. A 2018 Western Australian study determined that 73% of southern boobook owls found dead or were found to have anticoagulant rodenticides in their systems, and that raptors with larger home ranges and more mammal-based diets may be at a greater risk of anticoagulant rodenticide exposure.

Insectivorous birds will often eat insects sprayed with insecticides, and a few different species of birds may be affected at the same time. Unfortunately little can be done and death most often results. 

Organophosphates are the most widely used insecticide in Australia. Birds are very susceptible to organophosphates, which are nerve toxins that damage the nervous system, with poisoning occurring through the skin, inhalation, and ingestion. Organophosphates can cause secondary poisoning in wild birds which ingest sprayed insects. Often various species of insectivorous birds are affected at the same time as they come down to eat the dying insects. After a bird is poisoned, death usually occurs rapidly. Raptors have also been deliberately or inadvertently poisoned when organophosphates have been applied to a carcass to poison crows.

Organochlorine pesticides (OCPs) are persistent, bio-accumulative pesticides that include DDT, dieldrin, heptachlor and chlordane. OPC’s have been used extensively in the agriculture industry since the 1940s. Some of the more common product names include Hortico Dieldrin Dust, Shell Dieldrex and Yates Garden Dust. Although no OCP’s are currently registered for use in the home environment in Australia, many of these products still remain in use on farms, in business premises and households. OCP poisons remain highly toxic in the environment for many years impacting on humans, animals, birds and especially aquatic life. They can have serious short-term and long-term impacts at low concentrations. In addition, non-lethal effects such as immune system and reproductive damage of some of these pesticides may also be significant. Birds are particularly sensitive to these pesticides, and there have even been occasions where the deliberate poisoning of birds has occurred. Tawny frogmouths are most often poisoned with OCP’s. The poisons are stored in fat deposits and gradually increase over time. At times of food scarcity, or during any stressful period, such as breeding season or any changes to their environment, the fat stores are metabolised, and with it, the poison load in their blood streams reaches acute levels, causing death.

Although herbicides, or weed killers, are designed to kill plants, some are toxic to birds. Common herbicide glyphosate (Roundup) will cause severe eye irritation in birds if they come into contact with the spray. Herbicides also have the impact of removing food plants that birds, or their insect food supply, rely on. Birds can also readily fall victim to snail baits, either via primary or secondary poisoning.

Reptiles and Amphibians

As vertebrate species, reptiles and amphibians are also at risk of pesticides. Though less is known about the effects of pesticides on reptiles and amphibians, these animals have been known to fall victim to pesticide poisoning. Blue-tongue lizards, for example, often consume rat bait and die of internal bleeding. A 2018 Australian study also found that reptiles may be important vectors (transporters) of rodenticides in Australia.


How can I keep pests away and keep wildlife safe?

Remember, pesticides are formulated to be tasty and alluring to the target species, but other species find them enticing, too. It is safest for wildlife, pets and people for us to not use any pesticides, and prevent or deter the presence of pests practically, rather than attempt to eliminate them chemically. 

Tips to prevent and deter wildlife deaths from poisoning:

If turning to pesticides as a last resort:

      • Use only animal-safe slug baits.
      • Place tamper-proof bait stations out of reach of wildlife.
      • Avoid using loose whether pellets or poison grain, present the highest risk, the latter being particularly attractive to seed-eating birds and to many small mammal species.
      • Read the label and use as instructed.
      • Avoid products containing second generation products difenacoum, brodifacoum, bromadiolone and difethialone, which are long-lasting and much more likely to unintentionally poison wildlife via secondary poisoning.
      • Cover individual fruits when spraying fruit trees with insecticides.

What about pets and children?

Because of their poisonous nature, pesticides pose a risk to animals and people alike, including pets and children. Roaming pets like cats and dogs are most at risk of being poisoned, with one 2016 study at the Norwegian University of Life Sciences finding that one in five dogs had rat poison in its body, and a 2011 study by the Humane Society in the United States finding that 74% of their pet poisoning cases are due to second-generation anticoagulants such as rat baits. 

It is best to avoid the use of all pesticides, or otherwise use them sparingly, carefully and only after researching each poison and its correct usage. Always supervise pets and children, keep poisons locked out of their reach, and be vigilant in public spaces where pesticides may have accumulated, e.g. poisons can accumulate in streams or puddles where herbicides have recently been sprayed. 

If you suspect your pet has been poisoned, seek veterinary help immediately.

If you suspect your child or another adult has been poisoned, do not induce vomiting and call the NSW Poisons Information Centre on 13 11 26 for 24/7 medical advice, Australia-wide.

References

Lohr, M. T. & Davis, R. A. 2018, Anticoagulant rodenticide use, non-target impacts and regulation: A case study from Australia, Science of The Total Environment, vol. 634, pp. 1372-1384.
 
Lohr, M. T. 2018, Anticoagulant rodenticide exposure in an Australian predatory bird increases with proximity to developed habitat, Science of The Total Environment, Volume 643, pp.134-144.

Lohr, M. T. 2018, Anticoagulant Rodenticides: Implications for Wildlife Rehabilitation, conference paper, Australian Wildlife Rehabiliation Conference, awrc.org.au

Olerud, S., Pedersen, J. & Kull, E. P. 2009, Prevalence of superwarfarins in dogs – a survey of background levels in liver samples of autopsied dogs. Norwegian University of Life Sciences, Faculty of Veterinary Medicine and Life Sciences, Department of Sports and Family Animal Medicine, Section for Small Animal Diseases.

Healthy Wildlife, Healthy Lives, 2017, Rodenticides and Wildlife, healthywildlife.com.au

Society for the Preservation of Raptors Inc. 2019, Raptor Fact Sjeet: Eliminate Rats and Mice, Not Wildlife!, raptor.org.au/factsheetpests.pdf

poisonsinfo.nsw.gov.au

All WIRES species manuals