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.
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.
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.
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