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
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
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.
The house rat bait widely used on GBI - chocolate flavoured, blue in
colour and yes, it contains the toxin brodifacoum.
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 plants
The 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.