Slugs and snails are common garden pests, and gardeners often turn to slug pellets in order to fend them off. What exactly are those blue pellets made of, though, and can they pose a risk to other animals as well as molluscs? This graphic examines the compounds used, and their potential undesired effects, as well as some of the alternatives available.
The vast majority of slug pellets contain metaldehyde as the active, slug-killing ingredient. However, this compound isn’t one that was originally developed for this purpose. In fact, it was formerly sold as a solid fuel, in the form of tablets, for use in fire starters or camp fuels. Supposedly, its ability to kill slugs was noticed completely by accident by French farmers, who noticed that dead or dying slugs could be spotted in locations at which metaldehyde tablets had been left.
This anecdote doesn’t seem to be reliably referenced anywhere, but regardless, metaldehyde is today the most widespread active ingredient in slug pellets, with an estimated 22,000kg used in the US alone each year. It actually only makes up a small percentage of slug pellets, between 4 and 6 percent. The remainder is mainly bait, along with the colourant that gives the pellets their characteristic colour. Bait, often cereal-based, is necessary because slugs are actually repelled by pure metaldehyde, and as such piling up slug pellets in large quantities can reduce their efficacy.
Slugs only need to come into contact with the metaldehyde for it to exert its effects. It causes the secretion of mucus from the slug’s mucus cells to increase greatly, leading to dehydration, convulsions, and death. It’s actually the less toxic to molluscs than another compound previously used in baits, methiocarb. This compound, however, was banned in the EU last year due to concerns over its toxicity to birds, which might also ingest the pellets.
That isn’t to say that metaldehyde doesn’t suffer from this problem as well. It’s also toxic to mammals, and pet poisonings as a result of the ingestion of metaldehyde pellets are commonplace. When ingested, it is hydrolysed in the stomach into several metabolites, the major one of which is acetaldehyde. This is also a compound formed by the metabolism of alcohol in the body. The toxic effects of metaldehyde include irritation to the skin, eyes, and mucus membranes, vomiting, diarrhoea, fever, coma, and even death if enough is ingested, due to kidney or lung failure. Acetaldehyde has been proposed as one of the main contributors to these effects, though evidence for this is still lacking.
The toxicity of metaldehyde isn’t the only issue. It’s also been detected in drinking water, albeit at harmless levels. Nonetheless, this emphasises the importance of avoiding its overuse. In the US, the presence of metaldehyde residues in crops is banned completely, which can also be a motivator to look for alternatives.
So, what alternatives are there? Aluminium and copper sulfate have both been used, though are not as effective; they have a repellent effect, but are only potent enough to kill very small slugs. Build-up of copper sulfate in the soil is also thought to affect worms, as well as slugs. Iron phosphate is another suggested alternative, though again there are concerns about its potential effects on worms.
Probably the oddest alternative, though, is caffeine. A 2002 study found that 1-2% solutions of caffeine were surprisingly effective in killing or repelling slugs. They discovered that, even at concentrations of just 0.01%, the caffeine solutions, when sprayed onto slugs, reduced their feeding by a quarter. An average cup of coffee contains around 0.05% caffeine, so could even prove effective if sloshed over your garden’s slugs – assuming you don’t mind parting with the coffee, that is!
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