Slug Bait – Metaldehyde vs Iron Phosphate

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Robert Pavlis

How do you get rid of slugs? I’ve looked at a number of home remedies and although some work a bit, most don’t work at all. The most effective way to get rid of slugs is to use a chemical slug bait. Slug baits contain both food for the slug and a poison that kills them.

There  are two main classes of baits; Metaldehyde and Iron Phosphate. There is a lot of controversy about which should be used. Which is safer for the environment? Which is more effective? The short answers given by many web sites are not telling the whole story.

Slug Bait - Metaldehyde vs Iron Phosphate
Slug Bait – Metaldehyde vs Iron Phosphate

Metaldehyde Slug Bait

Metaldehyde is the older slug bait and has been used for many years. It has a good track record for being effective at killing slugs and snails, but it is also toxic to cats, dogs, birds and other mammals. It can even harm children if ingested, but it does not harm bees or aquatic life.

The reported death of birds was due to eating the pellets, not the affected slugs (ref 3).

The original formulation combined metaldehyde with grain flour to make small pellets that slugs find tasty. Pellets work better than liquid products but they don’t work at low temperatures or high humidity. It is degraded by sunlight and is easily washed away with rain and irrigation.

Microbe Science for Gardeners Book, by Robert Pavlis

The active ingredient, metaldehyde, has a short half-life in soil and is not considered harmful to the environment. It should not be used in contact with vegetable crops.

Soon after eating metaldehyde, the slug becomes paralyzed and starts excreting large amounts of mucus. Over many hours they die from dehydration. In cold weather the effect can wear off before the slug dies.

Some formulations include carbaryl to increase the range of pests that are controlled. Unfortunately this also kills beneficial insects like ground beetles that eat small slugs and eggs. It is best to avoid such products.

Bitrex and Metaldehyde

In an effort to make metaldehyde safer, a chemical called bitrex is usually added to the product. According to the Guinness World Records, this is the bitterest substance know. It makes the pellets very bitter and animals tend no to eat them. Bitrex is also added to many household cleaners to prevent accidental ingestion by children.

Provided the product includes bitrex it should be safe for animals. I looked at several slug products available online and most don’t advertise the inclusion of bitrex, but the SMDS usually mentions it.

Iron Phosphate Slug Bait

In an effort to produce a safer slug bait companies started formulating iron phosphate into pellets that also contained a grain flour that attracted the slugs. Since iron and phosphate are natural chemicals found in the environment, organic farming regulators approved the product for use.

Iron phosphate is more effective than metaldehyde at lower temperatures, but is easily washed away with rain or irrigation.

This bait does not paralyze the slug, so they crawl away to a hiding place and take several days to die. Because gardeners never see dead slugs it is hard to know if the product is working.

Chelated Iron Phosphate

In an effort to make iron phosphate a more effective molluscicide, a chelated form of the product was created. The chelating chemical is usually EDTA.

Iron phosphate is quite safe for the environment, but EDTA has some issues. Most websites still claim that iron phosphate is perfectly safe since it is natural and organic, but they ignore the EDTA component which is common in many slug baits these days. EDTA Iron phosphate has harmed dogs that ate the bait (ref 2).

Which Molluscicide is More Effective?

The answer to this depends on the environmental conditions at the time of application, the species of slug and the exact formulation of the product.

In general it seems that iron phosphate is less effective than Metaldehyde, which less effective than chelated iron phosphate (ref 1, 3, 5). But either form of iron phosphate is more effective at low temperatures and in high humidity. Unlike metaldehyde, iron phosphate can be safely used around vegetable crops.

The reported effectiveness depends on how you interpret the results. For example, ref 5, compared an application rate of 1.5 g/m2 (5% metaldehyde) to a rate of 5 g/m2 (1% iron phosphate), applied as per manufacturers instruction. Metaldehyde killed more slugs, but the amount of chemical applied was also higher. Do you compare the weight of product added, or the amount of active ingredient used? Any claims that one product is better than the other needs to be closely examined.

Reference 5 looked at three different species, and found that one was unaffected by iron phosphate, but was killed by metaldehyde.

Although cost for the home gardener is not going to be a significant factor, metaldehyde is the cheapest option.

Which Molluscicide is Safer?

Iron phosphate without EDTA would be the safer product to use, but it is also the least effective one. Metaldehyde that contains betrix is almost as safe and is more effective.

Claims that iron phosphate EDTA products are natural and harmless to animals is clearly not true. They may in fact cause more harm than metaldehyde that contains betrix.

Effect on Earthworms

Neither metaldehyde nor iron phosphate affect earth worms. However, EDTA iron phosphate is toxic to them (ref 4).

The immediate reaction to this might that we should stop using EDTA and protect the earthworm which so many consider important for the garden. That may be a valid point, but remember that the earthworm, at least in North America, is an invasive species that is starting to have detrimental effects on our native woodlands. From an environmental perspective it is hard to support any idea that saves non-native earthworms.

If EDTA affects earthworms, it might also affect other native worms – we don’t really know.

References:

1) Slugs, Snails and Iron Based Baits; http://www.regional.org.au/au/asa/2001/6/c/young.htm

2) Toxicity in Dogs After Ingesting EDTA Iron Phosphate; http://www.ingentaconnect.com/content/bpl/avj/2009/00000087/00000007/art00011

3) Less Toxic Iron Phosphate Slug Bait Proves Effective; http://extension.oregonstate.edu/gardening/node/995

4) Toxicity of Metaldehyde and Iron Phospahte to Earthworms; https://www.biosci.ohio-state.edu/~soilecol/Full%20articles/2008/Crop%20Protection%2028.pdf

5) Field Tests With a Molluscicide Containing Iron Phosphate; http://www.stadtoekologie.ch/publi/speiser_cropro2002.pdf

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

34 thoughts on “Slug Bait – Metaldehyde vs Iron Phosphate”

  1. UK blackbirds feed slugs to their young chicks. They do take poisoned slugs when they find them. Whether the slugs need to be still moving I do not know. Any small amount of toxin given to a chick is inevitably going to threaten it’s life by reducing its growth, delaying maturity, and exposing it to nest predation for longer. I have just seen some UK starling fledgelings apparently eating my bright blue Fe3PO4 slug pellets. My preferred garden approach is to cover bare beds with anti-frost fleece to keep birds off and apply pellets underneath it, clear the slugs before planting.

    Reply
    • The UK ban was overturned in the High Court in June and metaldehyde slug pellets will remain legal in the UK for the forseeable future. The reason it was to be banned was due to its effects on wildlife, not due to contamination of water. It was an ill-advised decision by the then DEFRA Secretary, Michael Gove – a politician with no scientific experience who famously stated that people have “had enough of experts”. Metaldehyde is mildly toxic to mammals, basically the same as acetaldehyde into which it converts on contact with moisture. Acetaldehyde is naturally present in the environment and is produced in the human body as a breakdown product of alcohol. Acetaldehyde is rapidly broken down by soil bacteria. This is why you have to keep putting pellets down as the metaldehyde is rapidly degraded by moisture and soil bacteria into harmless byproducts. Hedgehogs are at significant risk of metaldehyde poisoning, which can be mitigated by applying the pellets as directed.

      Reply
  2. Great article.
    As a horticulturist and dog owner I would have to advise caution with metaldehyde. In Australia at least, metaldehyde has been responsible for many a dog death (an unpleasant death at that). Something about metaldehyde is very attractive to dogs. I can’t comment on whether local formulations used to contain bitrex though. For the home gardener, with a dog, or neighbours with dogs, I would always advise people to keep the packet well away from the dog, and only scatter lightly, never put in heaps. Bayer also used to sell an anti cholinesterase product called baysol, (methiocarb). Also very poisonous to dogs, but super effective, a few pellets was enough to wreak havoc on the slugs and snails. Anyway, if you have dogs, exercise caution

    Reply
  3. I’m enjoying this blog immensely-!
    It is frustrating…only two years ago I was searching for the “safe” Iron phosphate pellets, and all the stores carried was metaldehyde. Now I wish I could find metaldehyde products, but they’ve all vanished! Marketing…
    I used an iron phosphate product the past two springs. I spotted a few dead earthworms (I’d scattered the pellets under planks), and found just as many seedlings cut down by slugs…I don’t mind hand-picking slugs, but I start to wonder what the neighbors think.

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  4. ducks are a solution if you have a run that keeps them from destroying the desirable plants. Most gardeners would find even a low tunnel of fencing to be aesthetically unappealing

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  5. Dear Robert

    Thanks for this post. It speaks from my heart.
    I would like go add some comments on the article:

    In Europe (I live in Switzerland) iron phosphate pellets are about twice the price of metaldehyde pellets for the same weight of pellets. This is if you buy the pellets in a garden center. The concentration of the chemicals is regulated by the government. So there is not much you can do about this. It is 3.5% for metaldehyde and 1% for iron phosphate. The are good reasons why the general public cannot buy higher concentrated pellets. However the instructions on the label say that recommended dose of iron phosphate is 5 gramms/m2, whereas for metaldehyde it is 0.7gramms/m2. So this makes the application of iron phosphate more than 10 times more expensive. Also at these rates the total amount of chemicals used is significantly higher for the iron phosphate pellets (0.05gramms /m2 vs 0.0245gramms/m2). One may argue that this is not a huge financial burden for a home gardener. But I would call the iron phosphate pellets a rip off, especially when one considers the fact that they are less effective.

    Also I can see that you maintain the myth of that slugs can recover when there is humid and wet weather. This is not true and it is shown in these papers here.

    https://www.semanticscholar.org/paper/The-environmental-profile-of-metaldehyde-Bieri/59bdc7abd59e061e8facebd7e1c0f4dbb4c4c5f3

    https://academic.oup.com/mollus/article-pdf/64/4/467/3037091/64-4-467.pdf

    They clearly show that if the slugs have eaten enough bait, they will die. If they recover they did not eat enough bait or the bait had was already deterioated.

    This homepage of a leading metaldehyde manufacturer shows this in an easy language.
    http://axcela.lonza.com/axcela/understanding-meta

    I have used a two-fold strategy fighting the slugs in my garden slugs. It has proven rather effective. With this I must not worry about my hostas anymore.

    1st
    I noticed that when it is about to rain in summer the big brown slugs come out of their hiding places into the open. So I went and cut all the slugs I could see in two with my secateurs. I was quite surprised to see the next day that other slugs were feeding like mad on the dead bodies of their “colleagues”. They must have been attracted from rather far away, because I was quite thorough with the killing. So I cut those in half too.
    You may argue that this is brutal, but I think it is an honest way of killing. I doubt that death by pellets is more humane. It is certainly not faster. But this has helped to reduce the big brown slugs significantly in a rather short time.

    2nd
    As a preventive meaure I regularly scatter metaldehyde pellets close to the retreat, like under bushes and close to the edge of my garden, from where slugs might migrate. I do this every few weeks when the weather is wet. The amont of slime from the dying slugs is an indication of the population and allow me to adjust the regime.

    This way I got rid of the big brown Arion type slugs in my flower and my veggie garden. The small black ones which like to burry in the ground are more persistent. But they are less a thread to the plants. I do not worry so much about them.

    Thanks again for the great work

    Reply
    • Not sure what your first reference is – does not seem to be a published paper. One of the references in it says “The studies showed that metaldehyde induces severe alterations and damages in mycocytes even under low temperature and humid conditions when sufficiently high doses were applied”. That does not convince me that when the product is used in the garden the effects are not reversible.

      The second reference is done in the lab, not in the field, but it does conclude thaty “The present results show that the cellular stress level in control animals is higher at 15°C than at 5° or 10°C, and increasing temperature leads to more severe and faster pathological effects in metaldehyde-treated slugs. Thus, the effects of metaldehyde in slugs are enhanced by higher temperature”

      Reply
      • The first reference is a paper written by a scientiest (M. Bieri) of Lonza, a Swiss manufacturer of metaldehyde slug pellets. The paper is a kind of meta-study citing other papers that wer published in peer-reviewed journals. So I would give it some credit.
        But the key thing is that the slugs must eat a lethal dose of the poison. If they don’t then they may recover. And they may recover better when it is raining.
        But this principle applies to every kind of poison. If you drink copious amounts of alcohol you will die, if you drink just a lot you will get a hang over and if you drink moderately it might even be healthy according to some studies.
        So the trick is to make the baits attractive enough for the slugs and snails so that they eat enough and die. And this is why some pellets are better than others.

        Reply
  6. One of my stories about slugs is when twenty years ago we visited The Miller Garden in Seattle. Our guide casually said “We slug kill the garden in March” None of the visiting party dared ask what he meant. I imagine some of the party would have run a mile if they knew he meant they scattered slug pellets.
    I enjoyed your comparison of the two types of pellet. I expect you have written before about how gardeners apply far too many pellets and it looks like a blue mulch

    Reply
  7. After suffering from really bad slug attacks over several years the question answered itself for me. I tried almost every suggestion from the Internet and none of them worked well enough to be a solution.

    Since I live in the middle of farm land a local farmer pointed out that every field around me is dosed in slug pellets. My 20 or 30 square metres of vegetable beds would pale into insignificance compared to the hundreds of acres treated. I simply got the most powerful slug pellets I could and used them.

    I saw no obvious signs of wildlife suffering.

    A local wildlife expert where I was volunteering said he had observed birds hunting for insects and worms on the ground treated with slug pellets avoiding the pellets and explained to me that our type of birds that I have in the garden won’t eat a slug pellet when they are looking for insects and worms. They also won’t eat an ill or dead looking slug, they are in fact smarter than that.

    I see no point in using a treatment that isn’t going to be effective, that will only mean you have to use it again.

    I subscribe to, “if you are going to do something, like kill slugs, then at least do it well”…and actually kill them. If you are going to kill slugs then the moral question about killing wildlife has already been made, just go ahead and do it well. If you don’t want to risk deaths to wildlife then don’t use pesticide. I see it as a binary decision, not one of degrees of death.

    Reply

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