- Metaldehyde Slug Bait - How Dangerous is it in
the Garden?
by Bill Meyer
-
- 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.
To get a look at how often this happens I checked with
the EPA and found that there were 1,892 incidents reported of children
being taken to poison centers because of ingesting metaldehyde 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 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 reports of bird
poisonings in garden use.
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 can be much more difficult for
"organic" products. 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.
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 safer to use. They hold up much better to rain and
stop feeding more quickly after poisoning. At this time they are still
significantly 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
British Study - http://www.pesticides.gov.uk/uploadedfiles/Web_Assets/PSD/WIIS_2000.pdf