Slug and snail control – least toxic options

The range of pesticides used to kill slugs and snails are called molluscicides and are commonly sold as pellets.  Are molluscicide pellets really the gardener’s friend?  Molluscicides not only poison slugs and snails but can also harm domestic animals and wildlife, including hedgehogs and birds, which are natural mollusc predators.  This article examines effective ways of controlling slug and snail problems safely without the hazards presented by pesticide use.
    Many gardeners are plagued by thriving slug and snail populations but molluscides could be helping your local population to either become resistant to pesticides or learn to ignore them(1).  The general climate in the UK promotes the favourably moist conditions that slugs and snails need to be active.  Gardens also provide shelter and a plentiful supply of food.

Biology 
Slugs and snails are molluscs and are closely related to shellfish, both have shells, although the very small shells of slugs are hidden beneath a fleshy mantle(1). Slugs and snails glide along the mucous trail they secrete using a single foot.  The foot is propelled by a wave of muscle contractions that run from the tail to the head.  The combination of mucous and muscle movement allows the animals to reach otherwise inaccessible  places such as sheer walls and overhangs(2). 
    Snails have two pairs of retractable tentacles, the longer pair have a single eye at the end of each one and the shorter pair are used for smell.  Their horn-like tongue is located beneath the tentacles and is used to scrape away at food.  The rest of the snail’s body is curled inside the shell.  There are widespread variations in the colour and size of snails both within and amongst species(3). 
    Slugs and snails are hermaphrodite and each individual has both male and female reproductive organs.  However, cross-fertilisation is the most common form of reproduction.  Slugs lay fewer eggs than snails, which may lay up to 100 eggs at a time, depending upon the species.  Slug eggs may remain unhatched for long periods until they receive sufficient moisture(4).

Chemical Control 
The most commonly used chemical control is in the form of slug pellets, containing the active ingredient metaldehyde, that are generally brightly coloured making them attractive to children and pets, especially dogs.  Metaldehyde works by disrupting the gastric organs. The active ingredients Methiocarb and Thiodicarb disrupt the production of cholinesterase, an essential nervous system enzyme.  Molluscicides may also contain a desiccant preventing molluscs from producing the mucous essential for survival.  Pellets remain attractive to slugs and snails for several days to prolong the length of time they are available for detection and consumption(5). 
     There have been incidents of dog poisonings resulting in the affected animals dying in agonising convulsions(6).  The Ministry of Agriculture, Fisheries and Food (MAFF) prosecuted a Cambridgeshire farming company in July 1999 after a dog came into contact with pellets and became seriously ill(7).  The Department of the Environment, Transport and the Regions (DETR) sites metaldehyde, a common active ingredient of slug baits, as being among a number of pesticide types that regularly figure in poisoning incidents(8).  Pellets should not be allowed to touch the foliage of food plants(9) as the application renders them unsafe to eat.  
     Pesticides containing the active ingredient metaldehyde must also have an animal repellent added(10).  Some repellents may also be repellent to slugs(11). 
     In 1998 pesticides were implicated in 90 poisoning incidents of pets, 38% were cats and 40% were dogs.  Five incidences of cat poisonings involved Metaldehyde and one involved Methiocarb.  Twelve incidences of dog poisonings involved Metaldehyde and five involved Methiocarb(12).  The Wildlife Incident Investigation Scheme (WIIS) investigates all suspected poisonings and incidents can be reported to them on the freephone number 0800 321600. 
     The active ingredient Aluminium Sulphate is a granular inorganic salt and is the least toxic of the chemical controls available.   

Natural Control 
Indirect Methods 
One indirect method is to reduce the preferred habitat of slugs and snails, which like moist, shady areas for resting and laying eggs(13, 14).  Ivy and succulents are favoured particularly when forming dense ground cover(15).  Whilst it is difficult to remove all such areas within the garden they should be reduced in the vicinity of vulnerable growing beds.  Young snails tend to stay close to the nest for a few months after hatching and are a good indication of where adults are laying eggs.  
     Other methods to try are liming beds annually to prevent acidity and the interplanting of herbs as slugs are thought to dislike spiky or aromatic plants(16).  Acid loving plants can be grown in an alkaline soil if planted in a sealed pit filled with acidic compost. 
     Thinning out plants and allowing air to circulate around the remainder can reduce favourable moist conditions within beds.  If weeds invade the exposed areas a thick layer of mulch unattractive to slugs can be laid as ground cover, materials with rough and jagged edges are preferable, such as broken eggshells(17). 
     A barrier of vegetation favoured by slugs, such as wilted comfrey leaves, can be placed around vulnerable vegetation as a decoy(18). 

Direct Methods 
Hand picking is an effective method of reducing the presence of slugs and snails in the garden(19).  They can be collected in a bucket of water containing a 3-5% concentration of rubbing alcohol, which anaesthetises them and so prevents escape.  The optimum time for hand picking is two hours after sunset.  Hand picking is particularly recommended in conjunction with other methods such as barrier control(20).  
     Slugs and snails will collect in traps such as under upturned flower pots if left in a shady spot overnight and can be harvested the next morning before the sun is too hot and the shelters become too dry (remember to leave a gap under pots for snails to crawl through)(21).  Other traps include wooden boards, upturned empty grapefruit halves and beer.  Beer traps are ideally made from plastic pots with tight fitting lids, they should have slots cut into the sides near the rim and buried to the level of the opening.  Lids prevent evaporation, beneficial organisms falling into the pot and also larger animals (e.g. the family dog) from drinking the beer. If dead animals are left in the traps they are more attractive to newcomers(22).  Slug traps are also commercially available and work on the same principle as beer traps(23).  Once collected the most humane way of destroying slugs and snails is by crushing but they can also be dropped into salty water(24).  
     In addition to liking moist conditions slugs also like acid substrate and hate sulphur.  Slugs can be deterred by a barrier of wood ash or pure coal soot6.  Other barriers that slugs and snails dislike crossing include wood ash, diatomaceous earth, sawdust and copper backed paper or sheeting stapled to boards around beds.  Care should be taken to prevent earth and sawdust from becoming too wet.  Copper reacts with the secreted mucous used to aid locomotion giving the slug or snail a slight electric shock.  It is necessary to ensure that vegetation does not cross the borders offering an entry route.  Once areas within the barriers are cleared it should be a matter of monitoring the state of the borders to prevent further build up of slugs and snails(25). 

Biological Controls 
Hedgehogs, ducks, chickens, frogs and toads are all predators of snails and slugs(26).  Hedgehogs eat slugs, beetles, caterpillars etc. and are an excellent addition to pest control strategies in the garden.  They can be encouraged by providing sleeping areas such as piles of leaves and garden waste and leaving out a dish of meat in the form of cat or dog food, chicken leftovers and other sources of protein and a bowl of water(27).  Piles of garden rubbish should be carefully moved and inspected before burning to detect any sheltering hedgehogs.  
    Ducks and chickens also supplement their diet with slugs(28) but care needs to be taken to decide which breed to purchase and how they will be housed.  Chickens can cause damage to valuable plants and it is a good idea to move them around in pens or let them into the garden at the end of the day when they may be looking for an alternative to plant food.  Ducks like to be near to water containers but do not necessarily need a pond, they can be moved around in pens on a rotation basis if they are trampling areas of vegetation(29). 
     Creating a wildlife pond in the garden will provide a habitat for a wide diversity of wildlife including frogs and toads.  Ponds should be deep enough in the centre not to freeze over in winter (approximately 60cm) and at least a third of the edge should be shallow and gently sloping to enhance the free movement of amphibians and other wildlife.  A sloping edge also provides an escape route for any animals falling into the pond(30) for example, Hedgehogs are good swimmers but have difficultly climbing out of a pond with steep sides, raised ponds should have an access ramp(31).  A marshy area provides a place of shelter for wildlife especially young amphibians.  Ponds do not have to be very large and are colonised very quickly.  If a newly created pond is filled with tap water allow it to stand for a couple of days to let the Chlorine evaporate and then boost it with a bucket of water from a mature pond(32). 
     Biological control in the form of a parasitic nematode is commercially available(33).  Care should be taken before introducing species into the garden, any introductions should be native, suitable for the geographical and ecological habitat, not harmful to beneficial pests and not likely to become pests themselves. 
     Rove Beetles (Ocypus olens) are native to Britain and are natural predators of slugs and snails.  They are about 2.5cm in length and have large jaws, predation of snails can be recognised by the jagged hole on the side of an empty snail shell.  They are not commercially available as biological control agents as, despite their longevity, they have a slow reproduction rate(34).

Comment
Slugs and snails can undoubtedly become significant pests in the garden.  However, a balanced ecologically based garden can accommodate pests without severe damage to plants.  Hand picking combined with other methods mentioned such as barriers, reduction of habitat and the inclusion of mixed planting and a small pond should prevent dominance of potential pest species.  It is sometimes tempting to use chemical warfare in our gardens but care must always be taken as all pesticides are toxic to living organisms and the potential hazards are very real. 

References
1.  Common-Sense Pest Control: Least-toxic solutions for your home, garden, pets and community, Olkowski, W., Daar, S. and Olkowski, H., Taunton Press, 63 South Main Street, Box 5506, Newtown, CT 06740-5506, 1991, $39.95, 592-596pp. 
2. Copley, J., Ooze Cruise, New Scientist, 11 March, 2000, Vol 165 No. 2229 pp26-29.
3. Op. cit. 1 
4. Op. cit. 1 
5. Pesticide Safety Directive, Efficacy Guideline 510, Testing of Molluscicide Products, 23 March 2000, PSD, Mallard House, Kings Pool, York YO1 7PX  http://193.133.84.30/ 
6. Burns, J., Slug pellets a fatal attraction to dogs, Farmers Weekly, 13 November, 1992, p13. 
7. MAFF, UK, Farming company fined £2,000 for Pesticide Offences, News Releases 1999, http://www.maff.gov.uk/inf/newsrel/1999/990805a.htm 
8. DETR, Wildlife Crime, 4.4 Poisoning, 4.4.12 Problem pesticide groups, 26 February 1998, http://www.wildlife-countryside.detr.gov.uk/crime/law/4_4_12.htm
9. Op. cit. 1 
10. Pesticide Safety Directorate – advice given by telephone from the helpline No. 01904 455775. 
11. Op. cit. 6 
12. Fletcher, Dr M.R., Hunter, K., Barnett, E.A., Sharp, E.A.,  Pesticide Poisoning of Animals 1998: Investigations of Suspected Incidents in the United Kingdom, 1999, p22-25. 
13. Op. cit. 1 
14. Bell, G., The Permaculture Garden, Thorsons, An Imprint of Harper Collins Publishers, 77-85 Fulham Palace Road, Hammersmith, London W6 8JB, 1995, £9.99, 170pp. 
15. Op. cit 1 
16. Op. cit. 14 
17. Op. cit 1 
18. Dudley, N., Strickland, S., G is for ecoGarden, Gaia Books Limited, 66 Charlotte Street, London W1P 1LR, 1991, £2.50, 191pp. 
19. Op. cit. 1&18 
20. Op. cit 1 
21. Op. cit. 1&18 
22. Op. cit 1 
23. British Hedgehog Preservation Society, Hedgehog House, Dhustone, Ludlow, Shropshire SY8 3PL, Tel : 01584 890 801, http://www.software-technics.com/bhps/
24. Op. cit. 18 
25. Op. cit 1 
26. Op. cit. 1, 14 & 18. 
27. Op. cit. 23 
28. Op. cit 1 
29. Op. cit 1 
30. Stickland, S., The Small Ecological Garden, The Henry Doubleday Research Association, National Centre for Organic Gardening, Ryton-on-Dunsmore, Coventry CV8 3LG, 1996, £5.95, 48pp. 
31. Op. cit. 23 
32. Op. cit. 18 
33. Op. cit. 23 
34. Op. cit 1 
35. Whitehead, R., (Editor), The UK Pesticide Guide 2000, CABI Publishing, CAB International, Wallingford, Oxon. OX10 8DE, 2000, £24.50, 601pp. 
36. A-Z of Chemicals in the Home (3rd Ed), Toxic Chemicals Committee of the Total Environment Centre, Sydney, NSW, Australia, 1996, 160pp. 

[This is an extended fact sheet that first appeared in Pesticides News No. 48, June 2000, pages 18-19]