Metaldehyde

The component of slug pellets
now banned in many countries.

Paul May
Bristol University

Molecule of the Month September 2024
Also available: JSMol version.

The downside of slug pellets

Slugs and pelletsKilling slugs is a good thing, isn’t it?

Yes, if you’re a gardener. For many years, and in many countries, metaldehyde was the poison of choice for gardeners wanting to rid themselves of the problem of slugs eating all their vegetables. Sprinkle a few of these slug-pellets around the base of the vegetable, and the slugs would all die, and the vegetables grow healthily and undisturbed.

How does it work?

Metaldehyde is classified as a molluscide (a poison which kills molluscs), which works when it come into contact with a slug or is eaten by one. It damages the cells that secrete mucus so that the slug’s protective slimy coating is removed. Tho counter this, the slugs generate lots of replacement, which uses up a lot of their internal water, making them dehydrated. The poisoned slugs often seek hiding places, then become inactive, and die within a few days.

metaldehyde
Metaldehyde

Cute hedgehogBut why was it banned?

There were a couple of concerns. The first was that the poison would dissolve in rainwater and then contaminate rivers and make its way into drinking water. But the main problem was that metaldehyde is toxic to many animals, especially hedgehogs, but also to pets like dogs and cats. Severe poisoning can result in a wobbly gait, muscle tremors, or seizures and even death. However, it was the perceived threat to hedgehogs that seemed to cause the most public concern. Wild hedgehogs are common visitors in many gardens, and play a beneficial environmental role by eating many beetles, caterpillars and other insects that might otherwise damage plants. Ironically, hedgehogs also eat slugs!

Are hedgehogs endangered?

No, but their populations are declining. They have been classified as vulnerable to extinction by the British Hedgehog Preservation Society (BHPS), and since the year 2000 hedgehog numbers have fallen by up to 30% in urban areas and 50% in rural areas.

And this is due to them eating slug pellets?

No, although it is believed that the poison does contribute a bit to the decline in numbers. The main reason is habitat loss and changes in farming practices, especially the removal of hedgerows. But their main predators, badgers, increased in numbers of that time, and it’s estimated that nearly one third of a million hedgehogs die on UK roads each year.

Hedgehog outrunning a car

In urban areas, hedgehogs must travel through gardens and parks to find food and mates, but barriers such as fences and walls can stop this happening.

So slug pellets were really just a small part of the problem?

That depends on your point of view. For hedgehogs, the LD50 of metaldehyde is about 500 mg/kg, which means a half-kg hedgehog would need to eat around 1,000 pellets in one go to die. This seems unlikely, as hedgehogs do not usually like to eat hard, dry things, so they are much more likely to become poisoned as a result of eating poisoned slugs. But again, the sort of doses involved would mean that a hedgehog would have to eat about 5,000 slugs for it to prove deadly. So poison by direct consumption sounds unlikely, although pathology investigations have ruled metaldehyde poisoning to be the cause of death for some hedgehogs. Moreover, pet-owners have reported drunk-like behaviour in animals that may have ingested slug pellets. Even if the risk of poisoning is small, the pellets may have wider impacts in the ecosystem. As a result, many countries have banned them, and promoted alternative means of slug control instead.

A bag of ferric phosphate pelletsSuch as?

Ferric phosphate works well for slug control. The ferric phosphate is chelated with ethylenediaminetetraacetic acid (EDTA) to make it water soluble, and then made into pellets containing some bait to attract the slugs. Once the pellets have absorbed some moisture, the slugs eat them, and the iron phosphate interferes with their calcium metabolism and damages their digestive tissue. This causes them to stop eating and slowly die, three to six days later. Because the ferric phosphate breaks down into iron and phosphate in the soil it is one of the few molluscicides approved for use in the practice of organic farming.

There are also various non-chemical slug-control measures which are reported to work - with varying degrees of success. These include sprinkling coffee grounds, egg shells or wool pellets around the base of the plants to annoy the slugs and make it difficut for them to move. People have suggested putting the plants in a tray of beer, so the slugs fall in and drown. And there is copper tape which you can buy to go around the plant pot that allegedly works because the slugs do not like the small elecrical charge in the tape and won't cross it.

Bibliography

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