- First, the facts:
Metaldehyde is a poison that is highly toxic if inhaled and
moderately toxic if ingested. It is not a carcinogen. It is
metabolized and does not build up in the body. Poisoning symptoms
will appear in 1 - 3 hours, and include severe abdominal pain,
nausea, vomiting, diarrhea, fever, convulsions, and coma. It is
toxic to all mammals and birds, but not to aquatic life. Breakdown
in the environment is rapid. Metaldehyde is in regular use in
farming, with about 48,000 pounds of active ingredient used per
year in the US. It is mainly used on seed crops, lemons,
artichokes, oranges, strawberries and grapes.
Easily the most common poison baits used to
control slugs are the ones containing metaldehyde. These are
typically bran pellets, often dyed blue, which contain 4% or less
of the chemical. Metaldehyde is poisonous to mammals, so does pose
a threat to pets if used in the garden. If pets ingest enough of
it, it can be fatal.
The average amount necessary to be fatal for a
dog is somewhat variable, with the lowest being 100mg/kg, or 100
milligrams per kilogram of dog weight. This would roughly be a
tablespoon of bait for the average dog. For cats, twice the amount
per kilogram of body weight. As cats are smaller, this would be
less than a teaspoon of bait. These amounts are not so large to be
impossible for dogs and cats to ingest, so the possibility of
poisoning is real in the home garden if they are used. Children
could be poisoned as well, but a lethal dose has not been
established, as no child deaths were ever reported from
metaldehyde slug baits..
To get a look at how often child poisoning
happens I checked with the EPA and found that there were 1,892
incidents reported of children being taken to poison centers
because of concern they had ingested metaldehyde slug bait during
the years 1993 - 2004 in the US. Of these, 3 exhibited moderate
symptoms that had no long term effects, 29 had minor symptoms that
cleared quickly, and the remainder did not show any symptoms. None
of the incidents were life-threatening.
Pet poisoning statistics are not kept in
the US, but they are in Great Britain, where perhaps even more
metaldehyde baits are used per person. In a 2000 study, 10 dogs
were found to have been fatally poisoned by metaldehyde. These
were classified as follows:
4 were due to abuse, such as leaving piles of it to kill pest
animals or intentional poisonings.
5 were due to misuse, such as getting into stored baits or
spillages in agricultural use.
1 was due to proper use, although it was not included whether it
was agricultural or garden use.
No cases of cat poisoning were recorded, but it is expected they
would be significantly less than dog poisonings.
- There are often
anecdotal reports of pet poisonings which tend to be confusing.
Usually in these cases, veterinarians treat dogs for symptoms, but
do not know for certain what caused them. These situations are
often referred to as "probable" or "possible
poisonings". Because these are so difficult to pin down, no
numbers on these are included here. Pets can be poisoned by many
things in the home and garden, including bad pet food, and often
neither the owner or the attending veterinarian knows for certain
what the problem was. I was unable to find any reports of bird
poisonings in garden use, but it remains a possibility that birds
could be poisoned. Secondary poisoning by animals eating poisoned
slugs, snails, or even mice has been disproven in studies and is
apparently just a myth.
Because of concern over poisoning from what
could be an attractive bait, makers of slug baits containing
metaldehyde were required to include denatonium benzoate, commonly
known as Bitrex, the most bitter-tasting substance in the world.
The amount of Bitrex was upped from 30ppm to 300ppm in late 2003,
a tenfold increase, to help further reduce the number of
poisonings.
From the above information it can be seen
that poisoning of children does occur, but no serious
life-threatening poisonings were reported. Increasing the amount
of Bitrex seems likely to prevent many of the mild poisonings that
have occurred in the recent past. Cats in general do not have
excessive interest in the bait products, so are not considered to
be very much at risk. Dogs are likely to remain at risk because of
their interest in eating almost anything edible, but the most
danger to them lies in improper use and storage rather than from
areas in the garden where it has been properly applied.
- Whenever the
choice is made to use toxic chemicals in the garden, we need to
understand the dangers they present and be careful to act in the
best interests of those who might be harmed by our actions.
Whether a toxic chemical is "organic" or synthetic is
largely irrelevant when it comes to assessing how dangerous it is.
Safety information is often very easy to find on synthetic
chemicals, but actual information can be much more difficult for
"organic" products, which often are not tested to any
real extent. To find safety information on the internet, just type
(chemical name) + safety into a search engine.
- Genuine U.S.
safety information is available in the form of EPA documents and
MSDS papers from OSHA. Other good sources are university sites,
and other government safety organizations. Be wary of misleading
information from various manufacturers and "activist"
groups.
Recent Discovery about how Metaldehyde Works:
Recent studies
have shed a clearer light on how metaldehyde kills slugs, according to
an Ohio State University fact sheet (link below). There seems to be a
largely held misconception that metaldehyde baits work by dehydrating
slugs, and that in rainy weather they may survive.
"The
activity comes from its ability to cause the mucus-producing cells found
in slugs to burst, producing death of the slug. Contrary to popular
belief, death does not occur because of excess "sliming";
slime production is only a symptom. Because of that belief, many people
assume that slugs can overcome the "sliming" if they can
uptake water in a wet environment. Studies have disproved
this, and have shown that the inability of metaldehyde to sometimes kill
slugs is because of the slug not having consumed a toxic amount."
- Proper Handling of Metaldehyde Bait
Products
Proper storage - Bait products should be stored where they cannot
be reached by children, dogs, or other pets. Even with Bitrex
included, there is still the possibility of ingestion of enough to
do at least some harm. Because the amounts necessary to cause
severe symptoms are not especially large amounts, the consequences
of a torn bag or box could still be serious. Metaldehyde is a
moderately dangerous poison, and in bait form it is attractive to
animals. Its size and coloring could make it appear to be candy to
small children as well.
Proper use in the garden - Metaldehyde baits should be used
sparingly and always spread evenly. Never apply in piles or lines.
It is very toxic to slugs, and one small bait pellet half the size
of a rice grain can kill several slugs. For larger areas, use
Deadline MP and a handheld "windmill" spreader. No more
than 3 or 4 pellets per square foot should be applied, as heavier
rates would not likely accomplish anything and increase the risk
of a pet being able to get a lethal dose by eating them from the
ground. Spills, indoors or out, should be thoroughly cleaned up.
If allowing pets or small children into treated areas while the
product is still dangerous, they should be supervised to prevent
them from licking pellets off the ground.
Application time - The first application should be made very early
in spring to eliminate as many overwintering adults as possible.
Eggs also overwinter and begin hatching as the weather warms. A
second application should be made as soon as any small young slugs
are seen or 4-6 weeks into the season. After this, monitor for
slugs and apply if needed. Many slugs hide in areas around beds,
against foundations or in lawns. Applications should include these
areas as well to be effective. Slugs will migrate in from
untreated areas, so the original two applications will not be
enough, even if they eliminated all slugs within the treated zone.
Fall applications are effective but not the most efficient,
because many of the adults that would otherwise overwinter usually
do not survive the winter anyway for one reason or another.
Alternative Poison Bait Products
There are basically two other poison bait
products offered for slug control - methiocarb baits and iron
phosphate baits. Methiocarb baits are generally being phased out
at this time, as they are no more effective than metaldehyde baits
and are poisonous to numerous insects as well as earthworms. Iron
phosphate baits such as Sluggo are getting variable responses from
different studies, but generally are very effective and possibly
safer to use, although this has not been proven. Recent studies
and reports are revealing that they may be just as dangerous as
metaldehyde baits in the garden. At this time they are still more
expensive than metaldehyde baits, but increasing popularity should
lead to higher production and lower prices.
Sources:
EPA RED - http://www.epa.gov/oppsrrd1/REDs/metaldehyde_red.pdf
Etoxnet quick reference - http://pmep.cce.cornell.edu/profiles/extoxnet/haloxyfop-methylparathion/metaldehyde-ext.html
MSDS - http://www.montereyagresources.com/msds/DurhamMetaldehydeGranules-m.pdf
Household Products Database - http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=19022014
Ohio State Fact Sheet - http://ohioline.osu.edu/ent-fact/pdf/0020.pdf
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