Brodifacoum - risky for rats, safe for the environment by Jo Ritchie

Environmental News Issue 15 Winter 2008

The concept and practice of broadscale application of toxins to effect eradication of introduced pest mamals has been developed and fine tuned in NZ such that we are regarded as world leaders in the science. Yet locally the practice is regarded with suspicion. Jo Ritchie lucidly illustrates the science and methodology behind the sucessful use of brodifacoum in eradicating rats from islands and fence-enclosed areas.

The aerial eradication tour and the eradication planning work I have been doing in the last year or so for Kaikoura Island, Rotokare Sanctuary (Taranaki) and Glenfern Sanctuary have reinforced how poor the availability and accessibility of information to the public is as it relates to brodifacoum and vertebrate toxins in general. Much of the information is science-based and not easy to read.
Brodifacoum is one of the main vertebrate pest toxins currently used for animal pest control (i.e. reducing numbers of pests) and eradication (removing every last animal permanently), yet many people know little about it. Myths rather than facts abound. I’d like to try and change this. This is the first of two articles on brodifacoum. The second in the next newsletter will look more closely at its effects on native and introduced animals and examine the practical aspects of its use in domestic rat control.
How does it work?
Brodifacoum is the active ingredient in both Pestoff and Talon. A second generation anticoagulant, it stops the blood clotting in animals with haemoglobin (the iron-containing protein attached to red blood cells that transports oxygen from the lungs to the rest of the body). This includes mammals and birds but not invertebrates (because they have a different blood clotting system to vertebrates). Brodifacoum is slow acting compared to other toxins such as 1080 or cyanide, which are acute or fast acting. This may raise some humane issues because people may think that if a toxin is slow acting it must be cruel. However, fast acting toxins, while extremely effective, can cause some animals to get sub-lethal doses and become neophobic or bait shy. This happens because they taste a bit of something that makes them sick very quickly so they relate the illness to the food. In contrast, brodifacoum effects happen progressively over time, so the animals do not associate their illness with the bait, and eat more of it. However, it is important to point out that most animals that eat brodifacoum die within 3-4 days.
What forms does brodifacoum come in?
It depends on what species you are trying to target. Brodifacoum is highly effective against rodents, rabbits and possums. These are the only species that it is licensed for use on. It comes in the form of a paste and a small pellet for possum and rabbit control and as a block or pellet for rodent control. For household use it is usually in a waxy block/pellet containing grain or another lure as an attractant. For aerial baiting work (larger scale use) it is put in a cylindrical cereal pellet.
This pellet bait (Pestoff Rodent Bait 20R for rodents) is about 2 grams and is cereal based. It’s dyed green to deter birds (most birds cue into the colours of ripe fruits and seeds so all baits have to be dyed green or blue). Its use is restricted to rodent eradication operations (as opposed to control) on offshore islands or behind pest proof fences. It is normally applied by helicopter, requires resource consent and is only available to D.o.C and authorised persons operating under a strict Code of Practice. The restrictions are because this is currently the most effective means of eradicating rodents and needs to be used by experienced people to ensure it is not misused.
How much toxin is in the bait?
The actual amount of brodifacoum in Pestoff bait is very small at 0.02g/kg (0.002%) — the equivalent of 20 grams of toxin in 1 tonne of bait or the equivalent of a pound of butter in 25 tonnes of bait. This is for two reasons:
• Brodifacoum is very effective against rodents (so only a small amount is required to kill one). Half of one cereal pellet kills a mouse and generally around 4-6 pellets will kill a rat depending on its size. Block baits are larger so one bait can kill multiple animals.
• The cereal based bait in which the toxin is bound is extremely tasty to rodents — in fact studies of various types of bait formulations, toxin concentrations and different toxins have consistently shown that Pestoff is the preferred bait for rodents and in fact is generally preferred over their natural foods.
Talon WB baits (the standard wax eggs that you can buy over the counter) only come as a block bait but have a higher toxin concentration of 0.05g/kg or 50 grams per tonne Because the bait recipe is not as tasty as the Pestoff one, more toxin is required.
What happens to brodifacoum in the environment?A valid concern that many people have is how these toxins break down in the environment and how quickly they do so. I believe that many concerns arise either because it’s hard to find information or because those doing the work do not provide sufficient information in a readily digestible form for concerned people to have access.
There is also the issue of misinformation, exaggeration and confusion of one toxin with another. The most common of these is confusion between 1080 and brodifacoum which are two completely different products. I have also found confusion between aerial baiting and aerial spraying. The latter is further confused when consent authorities issue consents for aerial baiting under that section of the Air, Land and Water Plan that deals with aerial spraying. The two activities are very different; spraying has the potential for drift onto neighbouring properties and baiting does not, especially on unoccupied and remote offshore islands. This is because spray is made up of many fine light particles that float and can therefore drift. Baits are cylindrical hard pellets that drop straight down. The method of application is also different; spraying uses a bar made up of a series of nozzles, baiting uses a bucket with a single exit point.
There are a number of factors that influence bait breakdown. These include: whether the bait has been consumed (e.g. by a rodent) or directly by invertebrates, the bait formulation, (a large block or a small cereal-based bait) and local conditions. A considerable amount of information is available, both from actual operations as well as laboratory and field studies.

CASE STUDIES of Brodifacoum

Breakdown in soil and water and uptake by plantsThe key here is how brodifacoum breaks down in soil and water. Brodifacoum is insoluble in water and is broken down following rainfall by soil micro-organisms (e.g. fungi bacteria,). Breakdown is also influenced by moisture, temperature and soil type. For example, it will break down much faster in topsoil than clay. The cereal baits commonly used in aerial baiting operations are designed to breakdown following absorption of moisture from the soil, or after rain. Where toxic baits disintegrate on the ground, the brodifacoum attaches to organic matter in the soil where it is broken down by soil micro-organisms into non-toxic products. Breakdown of the poison starts as soon as rain begins to fall following bait application (Haydock & Eason 1997).
 Baits will break down by swelling, cracking, then crumbling, depending on the temperature and humidity. Mould and fungi can appear rapidly as breakdown proceeds and help to speed up the process. Once this has happened baits are less likely to be eaten by non-target species. Although the cereal component of the bait disappears quickly, the toxin takes longer to breakdown (Fisher & Fairweather 2006). A common concern is whether plants can take up brodifacoum. The short answer is NO. Because of its low solubility and rapid breakdown in soil Brodifacoum is not accumulated in plants.