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, 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
British Study - http://www.pesticides.gov.uk/uploadedfiles/Web_Assets/PSD/WIIS_2000.pdf

 

 

 

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