Metaldehyde is used widely around the world as a molluscicide to kill slugs and snails, although it is toxic to all animals that ingest it. There is widespread concern that there has been an unacceptable number of poisoning incidents especially involving domestic pets, wild animals and birds. 

Metaldehyde is applied as a molluscicide bait for controlling slugs and snails in a very wide range of agricultural and horticultural crops, and by members of the public to control slugs and snails in their gardens and allotments. In the UK, along with methiocarb (another slug and snail killer), metaldehyde is the only pesticide approved for use on all crops. 
    In many countries, there is little information on metaldehyde usage in the public domain. In the UK, only figures from the agricultural sector are recorded. Metaldehyde, the most commonly used molluscicide, was applied across 355,465 hectares in 1998. Molluscicides in general are applied to about 8% of the area of all arable crops, and are most extensively used on potatoes and oilseed rape(1). 
    Slug and snail damage is an increasing problem in the UK due to changes in agricultural practice, such as the cessation of stubble burning, and also because of the wet, moderate climate.

The World Health Organisation (WHO) classifies metaldehyde as a class II ‘moderately hazardous’ pesticide. The acute oral LD50 (the dose required to kill half a population of laboratory animals) for rats is 283 mg/kg(2). 
    Metaldehyde is highly toxic by inhalation, moderately toxic by ingestion and slightly toxic by dermal absorption. Skin and eye irritation may result from exposure to this material. Inhalation of vapours of metaldehyde may cause severe irritation of the mucus membranes lining of the mouth, throat, sinuses and lungs. Ingestion of metaldehyde causes irritation to the stomach and intestines, and can cause kidney and liver damage. One to three hours after ingestion the following symptoms may occur: severe abdominal pain, nausea, vomiting, diarrhoea, fever, convulsions, and coma. Other symptoms of acute exposure include increased heart rate, panting, asthma, depression, drowsiness, high blood pressure, inability to control release of urine and faeces, incoordination, muscle tremors, sweating, excessive salivation, tearing cyanosis, acidosis, stupor and unconsciousness(3). 
    The US Environmental Protection Agency classifies metaldehyde as a Restricted Use Pesticide because of its potential short-term and long-term effects on wildlife. All product labels must include the following statement on the front panel: ‘this pesticide may be fatal to dogs or other pets if eaten. Keep pets out of treated areas’(4).

Poisoning cases
Metaldehyde is toxic to all organisms that ingest it, either directly or as a result of secondary poisoning from consuming poisoned prey. Poisoning results in the depression or excitement of the central nervous system(5). Molluscs (slugs and snails) are poisoned either by absorption of poison through the skin or by ingestion. As a reaction to the poison molluscs overproduce mucous which causes dehydration and an inability to move, therefore remaining open to predation(6). 
    It is dangerous to game, wild birds and animals. Some products contain proprietary cat and dog deterrent in order to avoid poisoning of these domestic animals(7).
    The Wildlife Incident Investigation Scheme (WIIS) recorded 17 metaldehyde poisoning incidents involving cats and dogs in 1998(8). Hedgehogs are very likely to eat poisoned molluscs resulting in serious internal damage caused by secondary poisoning(9). Symptoms of metaldehyde poisoning in domesticated and wild mammals include inability to stand, blindness, change in respiratory rate, excessive sweating and salivation, sudden death and seizures(10). Autopsy results from metaldehyde poisoned dogs show congestion and haemorrhages in the liver, kidneys and heart(11).

Chronic effects
Long-term repeated skin exposure to metaldehyde can result in dermatitis (inflammation of the skin) in humans. Prolonged eye exposure can cause conjunctivitis. Two-year toxicity studies and three-year toxicity reproductive studies in experimental rats found liver enzyme activity and increased liver and ovary weight at 250mg/kg in the diet. Fifty percent of female rats given this dose showed paralysis(12).
    Metaldehyde or its breakdown products may cause problems in the central nervous system through an unknown mechanism. The autopsy of a 2 1/2 -year-old boy, who lived 33 hours after ingesting one tablet of metaldehyde, had areas of collapse and congestion in the lungs, as well as cellular changes in the liver and the kidney(13).

A parasitic worm (known as a nematode) is used as a biological control agent in agriculture. In gardens, hand picking, beer traps, biological control and physical barriers are all effective if used as part of an integrated pest management system. 
    Hand picking should be carried out in the evening or early morning and can be significantly aided by erecting shelters for molluscs on the ground including; wood, metal sheeting and grapefruit shells. Beer traps can be made out of household items or more sophisticated versions, which safeguard beneficial insects, are available to buy at garden centres or hardware stores. Biological controls include taking measures to encourage natural predators such as hedgehogs and frogs(14).

Slugs and snails are an increasing pest problem both to agricultural production and in domestic gardens and allotments. The Pesticides Safety Directorate and the Health and Safety Executive should address the approved use of metaldehyde in both agricultural and domestic use with a view to restricting use and therefore reducing the risk of poisoning incidents. Restricted use in the US illustrates the unacceptable risk to wildlife posed by the use of metaldehyde.
    Integrated pest management incorporating several non-chemical controls can help to keep pest numbers under control. Natural predators, such as birds, hedgehogs and frogs, should be encouraged to establish viable populations through careful habitat management. (AW)

1. DG Garthwaite and MR Thomas, Arable Farm Crops in Great Britain 1998, Pesticide Usage Survey Group, Central Science Laboratory, Ministry of Agriculture, Fisheries and Food, Report No. 159, 1999. 
2. CDS Tomlin, The Pesticide Manual, British Crop Protection Council, 1997, p606. 
3. Metaldehyde, Extoxnet, Pesticide Information Project, National Agricultural Pesticide Impact Assessment Program, US., 09/07/01
4. Op. cit. 3.
5. Op. cit. 3.
6. Dr Symondson, Cardiff University, Slug Control,
7. R Whitehead, The UK Pesticide Guide 2001, BCPC, CABI Publishing, pp445-446.
8. MR Fletcher, K Hunter, EA Barnett, and EA Sharp, Pesticide Poisoning of Animals 1998: Investigations of Suspected Incidents in the United Kingdom, 1999, p22-25.
9. British Hedgehog preservation Society, Knowbury House, Knowbury, Ludlow, Shropshire SY8 3LQ,
10. Dr Maurice White, Cornell College of Veterinary Medicine, Metaldehyde Toxicity,
11. Op. cit. 3.
12. Op. cit. 3.
13. Op. cit. 3.
14. PAN UK, Slug and snail control – least toxic options, Pests in the home fact sheet – No. 9.

[This article first appeared in Pesticides News No. 53, September 2001, page 20]