Slugs and Snails are molluscs of the class Gastropoda which literally translated is 'stomach foot'. Gastropods form the second largest class in the animal kingdom, the largest being insects. Most terrestrial snails and slugs belong to the subclass Pulmonata, order Stylommatophora. Of the 29 species in Britain a few are carniverous, but most live on living and decaying plant tissue; usually the the plants we like to look at or eat ourselves! It's not all bad news though, the slime, or mucus they exude to ease their movement is used as a plaster to cover wounds in parts of Africa due of it's adherent properties.
Every cubic metre of garden soil could contain up to 200 slugs and they could produce 200 offspring
This is an example of their destructiveness
these three have hollowed out this swede
and created a snug home
Instead of a shell like the snails, slugs have an internal horny plate covering their breathing cavity. There are ranges of colours and sizes of slug depending on the species. The Field Slug is 2cm and grey, while the Round Back Slug can be 5-10cm, black or brown and in some cases can have an orange colour on their foot. They can have up to 27,000 teeth arranged on their tongue-like radula, which are constantly renewed in a similar way as shark's teeth with a new row moving forward to replace the worn-out teeth. They feed by rasping ragged holes in flowers, leaves, stems, roots and seeds, with young plants the leaf feeding can kill and by girdling a stem a plant will wilt or die - this can be the case with clematis. Below soil level some species attack root crops - the small Black Keeled Slug being particularly destructive, especially to potatoes and the bases of plant stems. About 95% of slugs are below ground and many species spend most or all of their life underground. Those that we see are only a small proportion of the ones that are present, some of which are only a few millimetres long.
Slugs begin to move, hatch, feed, and lay eggs when the temperature is above 5 °C. During dry and cold conditions they remain deep in the soil. The wetter weather and milder winters of late have been very beneficial, and most gardeners have noticed an increase in their numbers. They are usually nocturnal, but will venture out on dull, damp days.
As well as the milder weather increasing the numbers, an alien species has arrived recently which has a voracious appetite. It is commonly called a Spanish Slug, but it actually hails from the South of France - the proper name is the Vulgar Pest Slug (Arion vulgaris) and is widespread on the British mainland, but more localised in Ireland. Some were found in at a horticulture college in County Antrim in the 2000s. In looks it is very similar to the large brown or blackish slug found in the garden and difficult to distinguish apart from the damage it can do - one suggested method of recognition is it remains motionless when touched on the back whereas the native slug rotate from side to side. It has a thick skin and birds don't tend to find it desirable. On the Continent some countries require a certification that it is absent when selling property in a similar way to Japanese Knotweed
There are three stages in the life cycle: eggs, immature stage and adults - they can overwinter in any stage. They are hermaphrodite having both male and female organs, so every individual can lay eggs - up to 300 each in batches of 10 to 50 in moist, but not waterlogged, crevices; sometimes down the sides of pots. One individual has the potential to produce about 40,000 offspring. Eggs are gelatinous, watery, about 3 - 4mm across and usually spherical like tapioca. The period of development of the eggs varies depending on the temperature, during warmer weather they hatch after 10 days, but this can be up to 100 days in cooler conditions. After hatching the slug matures in less than a year and can live for two or more years.
The damage to brassica plants can be caused by Pigeons, particularly if it occurred suddenly and the leaves are ragged.
Slugs and snails form part of the life cycle of the lungworm parasite, Angiostrongylus vasorum, which can be lethal to dogs. The eggs are passed in the faeces of affected dogs or foxes and ingested by the slugs. A recent survey has shown that the lungworm, sometimes called French Heartworm, is found in parts of Ireland, Wales and mainly southern England, but infestations have recently occurred in Scotland. Some dogs may eat slugs, but more likely they can ingest them accidently when they drink from outside water bowls or puddles, or on toys or bones which have laid outdoors for a while. The affected slug or snail can be tiny. The eggs hatch into larvae that migrate to the lungs and develop into worms which block the airways. Some of the symptoms are listlessness, problems with breathing and weight loss. Another symptom is prolonged bleeding from minor cuts or the gums and can occur when the dog is apparently in good health - as was the case of a young dog recently in Belfast that after cutting its paw took about three days to stop bleeding. Preventive treatments are available for lungworms.
Treatments
There are many suggestions as to how kill or deter this pest, but even the most diligent collection regime has been shown to have little effect on the damage caused. The best thing is to try and cope with them by following some general hygiene methods.
Remove their hiding places and spots where they lay eggs, eg. rotting boards, logs, stones, dead leaves and other debris. Place compost heaps away from vulnerable plants. But don't be too tidy as Ground Beetles which feed on the eggs, like to hide during the day as well, large stones and pebbles will do for them. Another creature which appears to eat the eggs is the New Zealand Flatworm - 'snail caviar' perhaps.
Recent research has found that they respond to the presence of Ground Beetles. They appear to move away, which could be in response to smell as slugs are known to have a keen sense of smell. The researchers are trying to replicate the chemicals and these could be used as a deterrent.
Some other means of control:-
- use traps, eg. stones, boards, upturned pots empty grapefruit skins or wet sacking, the slugs take cover underneath and can be collected. They will also lay their eggs here so they can be destroyed as well.
- go slug hunting at dusk and chop them up or put them in hot water, this is the most effective method of reducing their numbers, but the most destructive species do not emerge often. The slimey bodies are not very pleasant to handle so use tweezers, chopsticks or skewer them to make a 'slug kebeb'.
- when on slug patrol, make a half-and-half mixture of vinegar and water in a handsprayer. Vinegar is also useful for removing slug slime. One squirt should kill them. Also a 1 : 3 mixture of household ammonia in water should work.
- Ordinary table salt is lethal to slugs, but overuse is detrimental to plants and other creatures. It is probably best to drop them into a container of this, rather than sprinkling it about.
- A tablespoonful of oats acts as an attractant when placed beside more favoured plants. The slugs can be collected and destroyed.
- attract hedgehogs into the garden with dog food (not bread it clogs their digestive system and milk is bad for young hedgehogs). Keep them there with a concealed, dry home with straw bedding and don't light bonfires without checking for sleeping 'hogs.
- cut the bottom off a plastic drinks bottle and remove the cap, to form a protective cylinder around vulnerable young plants.
- beer traps should have the entrance about 3cm above soil level to prevent Ground Beetles falling in - put a few twigs inside to help any which do fall in, to climb out. Beer cans can be quite effective, I discovered this when tidying urban gardens where late revellers use the front garden as a bin. They seldom finish the beer so the cans are already baited, the slugs crawl in and drown in a stupor, I suppose. An alternative to beer is a mixture of sugary water and yeast or milk, but the best bait is said to be stout - a bit of the black stuff. Slugs are also partial to cat and dog food, so this too makes a suitable bait.
A home-made trap can be constructed from an empty margarine or ice-cream tub with a hole cut in the side near to the top - the lid makes escape more difficult and stops the trap filling up with rain water. A stone on top should prevent the wind from blowing it about.
- researchers in Hawaii have found that a 1 to 2% solution of caffeine will kill slugs and a 0.1% solution will deter them if is sprayed on leaves. However, this latter strength solution also scorched the young leaves of cabbages. They were using purified caffeine and a 2% solution would be lethal to most things including beneficial insects. It appears to act on the nervous system. Instant coffee contains 0.05% caffeine so it might be possible to use it as a deterrent. It would probably have to be continually applied especially in wet weather and it is not known if it is harmful to other beneficial creatures. Some people already use spent coffee grounds to deter, but it is not very effective. Further work is being carried out so a suitable formulation which might be safe to use on food crops as well, may be available in the future. The spent coffee grounds can be spread around vulnerable plants and they don't like to crawl over it - any remaining caffeine could also be affecting them; it is said that the slugs are stupefied by the caffeine and perform crazy dances before slithering away!
- frogs, toads, hedgehogs, some beetles and their larvae (ground & rove), centipedes, parasitic flies, birds, chickens, and ducks are natural enemies. Attract these creatures by providing shelter like a thick hedge
- a dry mulch will reduce slug activity, this is achieved using a dutch hoe in dry weather. Alternatives are grit, sawdust, cocco shells, weathered cinders, wood ash or crushed egg shells. However their protective slime can allow them to crawl over a razor blade or sharp glass, so these methods are of limited success at best and are usually wishful thinking!
- Fito Slug Stoppa granules form a rough barrier and absorb slime so make it difficult for slugs to move over them, but are quite expensive.
- Diatomaceous Earth is a dry granular substance made up of fossilised algae. It is applied as a barrier to exclude the slugs. It loses its effectiveness when wet, but regains it after drying out. Also can be used to eliminate other insects the dry powder sticks to their exoskeleton, abrading it and leading to dehydration.
- Pine needles are said to annoy slugs probably the dry smaller ones from Norway Spruce. Tey would stick to the slimey body and "foot".
- A mulch of hair clippings are said to deter them as it sticks to their slimy underside, making movement difficult.
- a biological control is the nematode (eel worm) Phasmarhabditis hermaphrodita which occurs naturally and is used in Slugsure. They invade the slug and bacteria which they carry cause the slug to stop feeding, go underground and die. As they decompose the eel worms reproduce. I suppose it's like giving them the 'flu'. This natural control is less effective on surface dwelling species as the eel worms live in the moist conditions below ground, but they are the best treatment for the underground species which attack root vegetables. Nematodes are unaffected by wet weather. They remain active for about 6 weeks, and need a minimum temperarure of 5°C. Another brand is Nemaslug (Phasmarhabditis hermaphrodita), which is available by mail order or on-line. They can be ordered between March and October, and keep in the fridge for a few weeks after delivery.
- a slick of petroleum jelly around the top of containers prevents them crossing to the contents, but makes handling the pots difficult and it becomes covered with dust and leaves.
- Slug pellets containing metaldehyde are said to be safe by their manufacturers if sprinkled thinly around - about 15 to 20cm apart. They work by inducing excessive slime production and dehydration. They are dangerous to pets if they find the packet, 12 dogs died in 1998 after eating spilled or badly stored packets. Birds are not likely to eat them directly and they do not eat the corpses, but slightly affected slugs may be eaten and some studies show that the bird's fertility may be reduced. Also the active ingredient will be washed into the soil and will affect other organisms. Metaldehyde can also be applied as a drench. It has been found in low doses in drinking water so perhaps it should not be widely used. In December 2018 the Environment Department in the UK (DEFRA) on advice from the Expert Committee on Pesticides and the Health and Safety Executive, announced that products containing metaldehyde are to be phased out by spring 2020 due to the unacceptable risk to birds and mammals. They can be sold until summer 2019, but the ban which is for outdoor use, takes effect in spring 2020.
- Another slug bait is one containing ferric phosphate which works best in moist conditions. It is considered to be organic and although it is a bit less effective than metaldehyde,
it can be used where crops are grown and is not hazardous to pets. It degrades over time in the soil to iron and phosphate, but if spread too densely can affect the soil chemistry.
- Some species or varieties of vulnerable plants with thicker leaves are said to be resistant.
- Sacrificial plants such as lettuce planted near to vulnerable ones, may draw the slugs away and keep them interested for long enough to be picked up and removed.
- Keeping vulnerable plants such as hostas in drier conditions makes the leaves tougher and a bit less attractive to the slugs.
- There is some evidence that slugs do not tend to cross anything made of copper, it is thought that a micro-currant is created as they touch it and this is unpleasant. A continuous barrier of copper around vulnerable plants may keep them out, but some tests have shown that it is a temporary effect and hungry slugs will brave the defense to feed on inviting leaves. Check for the presence of slugs or their eggs first and destroy; there is no point in trapping them inside! Strips around pots could also be decorative. There are a number of products on the market now which use copper in different forms:
A self-adhesive copper tape is available in some garden centres or online.
A woven mesh which can be erected as a small fence, but it must be in contact with the soil along its full length otherwise they will crawl underneath.
A copper impregnated mat in various sizes designed to be placed under pots.
Copper rings in various diameters for placing around plants - can be homemade by splitting copper piping lengthwise and flattening out before bending into a ring or electrical wire stripped of the insulation.
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Most of the above measures apply to snails as well.
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