Cardiff School of Biosciences
PO Box 915
Cardiff CF10 3TL
Tel: +44 (0)29 20 875151
Fax: +44 (0)29 20 874305
Britain has few serious pests compared with the tropics where all vegetation in a whole region can be wiped out by insects such as locusts. However we do have one pest that can, under certain circumstances, destroy most of our crops. No gardener needs two guesses as to what this is; it has to be the slug.
The Black Slug
When allotment holders were asked recently to name the crops that most frequently suffered from pest attacks, brassicas and potatoes headed the list. In both cases slugs were seen as the main pest responsible, and overall they were blamed for causing the most damage to vegetables. In response to this, slug pellets lead the field as the most used pesticides.
You are probably now thinking that once again a carefully conducted survey has uncovered what we all knew anyway. But perhaps we have more to complain about than you might have realised. Britain is the slug capital of the world. Nowhere else has such ideal conditions for a broad range of slug species, for our cool, damp summers and warmish, wet winters allow slugs to be active, breeding and feeding for so much of the year. Added to this, we have recently suffered a succession of milder winters, and cooler, wetter summers. Normally icy conditions kill off a percentage of slugs and prevent the remainder from breeding and eating our crops for a while. Similarly, hot, dry weather kills slugs, which have to burrow deep into the ground to find the moist conditions that they need to survive. Meanwhile they give us a break - until we get out the hosepipe to water the beans, of course, and bring them all back to the surface!
To control any pest it is essential to know something about the beast. In this instance identification of the various types of slug is desirable, because not all of them cause problems in the garden. As explained later, overuse of slug poisons can be counter productive in that you risk killing off the predators that are helping to control your slug population. Such poisons should therefore be used, if used at all, only when and where a serious problem is seen to be developing, involving a slug species that is known to attack the crop you are growing. Once you have identified the species and can learn a little about its habits, you might well find that there are non-chemical options that are effective controls.
There are around thirty species of slug in Britain (plus some exotics in glasshouses). Only a minority pose any serious problem and four of the worst and most numerous of these that cause damage in gardens are described below. For more detailed identification of slugs you might like to refer to one of the books listed at the end of this article.
The Field Slug (Derocereas reticulatum)
The field slug is small - up to 4 cm - and usually grey/fawn in colour with darker flecks. It has a short truncated keel or ridge on the back at the tail end. The underside, or sole, is whitish with a darker zone along the centre. The mucus is colourless or white.
The field slug is a surface-feeding specialist typically found infesting lettuces and cabbages. In fact it eats almost all our crops, from carrots and beans to spinach and tobacco, from celery and tomatoes to orchids and cacti. It is a major pest of cereal crops and will even eat potato haulms.
The Garden Slug (Arion hortensis Agg.)
The garden slugs are a group of small blackish slugs up to 3 cm, with a paler side stripe. They have no keel and are distinguishable from similar species by their rounded cross section. The sole is yellow or orange; the mucus the same colour.
As burrowers and surface feeders, the garden slugs attack both leaf and root crops. Typically they attack at ground level, severing the stems of young beans or marrow plants for example. They will also climb up and attack the heads of cauliflowers and many other plants or burrow down to eat the roots of turnips or beetroot. It is a major pest of potatoes attacking both tubers and foliage and has been known to penetrate the soil to as much as a yard's depth so nothing is safe from this slug.
The Keel Slug (Tandonia budapestensis)
The keel slug is a larger species up to 6 cm. Usually dark grey/olive in colour, it has a keel, with a yellow or orange stripe along the ridge. The sole is pale, the mucus colourless. Typically it curves into a sickle shape when contracted.
A burrowing specialist, notorious for its destruction of potatoes, it will in fact attack most root crops and is difficult to control as it spends most of its time underground.
The Black Slug (Arion ater)
The black slug can he very big: a length of up to 20 cm has been recorded. Its colour is very variable - white, red, orange or grey are all common although black is most usual, often with an orange fringe. There is no keel, and the skin is coarse and granular. The sole is pale, sometimes orange, and the mucus is white. The black slug may rock from side to side when disturbed!
Familiar to all gardeners because of its spectacular size, the black slug is rarely as destructive as the three smaller species described above but can cause damage in spring to seedlings of many kinds. Later, when its preferred diet of rotting vegetation, fungi, manure and even dead animals is more readily available, it causes little damage in the garden - other than promoting heart failure if you step on one when feeding the cat at night.
SLUG CONTROL - CHEMICAL
Garden centres make a fortune selling slug pellets - more, probably, than from all the other pesticides put together. Only a fraction of the quantity sold is ever eaten by a slug, and a much smaller proportion will actually cause the death of one. Given also the fact that every year there are reports of pets and wildlife dying or being made seriously ill after eating slug pellets, it seems obvious that these poisons are not being used efficiently from the point of view either of controlling slugs or of environmental safety.
There are two different active ingredients used in slug pellets - metaldehyde and methiocarb. Both are readily available. However, most slug pellets used by gardeners are based on metaldehyde.
Metaldehyde Pellets and Spray
Originally metaldehyde was used as a solid fuel (meta-tablets). Its slug-killing properties were accidentally discovered by farmers in southern France who noticed dead and dying slugs and snails on picnic sites where meta-tablets had been left on the ground.
A Metaldedyde slug pellet contains only 3 to 6 per cent of this chemical, the rest is in fact bait, containing a cereal base with various added attractants, such as yeast. Pure metaldehyde actually repels slugs, as will concentrations of pellets, which explains why it is important to spread these thinly (see below "Using Pellets Efficiently and Safely"). The poison can affect slugs either by contact, with absorption through the skin, or through the gut when eaten. The first reaction of the slugs to the poison is the production of masses of mucus, causing dehydration. Loss of mucus also means, that the slug can no longer move around, so that dead and dying slugs are found close to the area treated with bait. The mucus cells in skin, foot and digestive tract are damaged, also preventing the slug from crawling away to feed or find a place to hide during the day. Although under wet conditions a few slugs may appear to recover temporarily most will die from a combination of poisoning and exposure to the elements.
The most rapid and effective control can be achieved during warm weather. Put down the pellets on a humid evening, when dry weather is expected the next day. Slug activity will then be very high during the night, with a good uptake of bait. By morning the slugs will lie paralysed by the poison and unable to move into shelter. The dry day, particularly if it is sunny, will finish them off. If, however, the weather will not co-operate, and cooler or wetter conditions prevail, you will certainly kill fewer but you will still prevent much of the damage to your crops. This is because slugs that have been partially poisoned are inhibited from feeding for up to a week.
Metaldehyde pellets should be scattered between plants, avoiding contact with edible parts. This chemical can also he applied in spray form, in which case it will act purely as a contact killer, so here too it is important to use it only when slugs are active. About 70 per cent of the slugs that are going to be killed will be poisoned in the first twenty-four hours of treatment. However, even heavy applications are unlikely to reduce your garden slug population, temporarily, by more than 10 per cent. Recently the shower resistant properties of metaldehyde pellets have been improved increasing the effective life of the pellets in the garden and therefore increasing slug kill rates.
Methiocarb Pellets and Spray
Methiocarb is one of a group of chemicals called carbamates, which includes herbicides, fungicides and, especially, insecticides. Again, pellets contain around 4 per cent active ingredient, plus cereals and other attractants. They tend to be more expensive and are certainly more poisonous than metaldehyde pellets. Methiocarb is less important as a contact killer, acting more as a stomach poison when eaten.
Metaldehyde is said to be more effective than metaldehyde under cooler wetter conditions, although this is disputed - recent trials have shown little difference in the overall effectiveness of the two chemicals. Slugs that have been poisoned move around for a while, then swell up with fluid and become immobile, dying shortly afterwards. In dry conditions this swelling can he reduced and a few slugs may appear to recover. Methiocarb breaks down more slowly than metaldehyde, which can be both an advantage and a disadvantage (see below).
Pros and Cons of the Two Chemicals
If you must use a chemical slug killer, choose metaldehyde pellets whenever possible. To begin with, methiocarb is also an insecticide, which means that it will kill many of the predatory beetles which are themselves helping to control your garden slugs. As these beetles take longer to recover their numbers than do the slugs, you might well be making matters worse in the long run. Methiocarb is about ten times as poisonous to mammals as metaldehyde, so it is a dangerous chemical to have around yourselves or your food. In practice, metaldehyde and methiocarb are responsible for a similar number of poisoning cases although the volume of metaldehyde products is much higher. Metaldehyde mainly poisons pets, especially dogs, when the latter have access to packets of pellets or when pellets are wrongly applied. All slug pellets should be stored in an inaccessible place and you should never leave packets unattended in the garden. Spillages should be cleaned up immediately. Methiocarb, because it is so poisonous, will frequently make an animal ill before it has had time to take in a fatal dose. Worms can be killed by it and, at least in spray form, it may affect the growth of some plants.
From a garden wildlife point of view, therefore, slug pellets of either type should be avoided where possible. Birds and hedgehogs may under certain circumstances be killed by eating pellets, although the introduction of coloured pellets (usually blue) may deter the former so if you use molluscicides avoid the cheaper, non-coloured formulations. Wildlife may also be harmed by eating the poisoned slugs, although this probably only applies to methiocarb-based pellets. The formulation of slug pellets is very like that for dog food, with added poison, so it is little wonder that they are eaten by creatures other than those at which they are aimed. Indeed, the Ministry of Agriculture once advised the use of carbamate-based slug pellets for the control of small mammals on farms. The majority of poisonings of pets seen by vets are as a consequence of slug pellet use. So the message is use metaldehyde in a real emergency where slugs are devastating a crop and when no alternative system seems to work. Otherwise choose non chemical methods.
Using Pellets Efficiently and Safely
It is an important principle of pest control that the minimum dose that will do the job should be used. Pesticide that does not reach its target will undermine the overall effectiveness of your control measures, in this case by hitting your garden "friends", the predators and parasites of slugs and other pests.
As a rule of thumb, a maximum of 100 mini pellets per square meter should be used, which results in about 10 cm of space between each one. Spread them any more densely and most will not be taken up, while the repellent effect may actually reduce the kill. That is why the use of small heaps of pellets should be avoided. Use at the recommended rate will prevent pets and wild animals from taking up a lethal dose of pellets. Confine dogs during application to prevent them from believing they are being fed and remember that curious toddlers are likely to put anything unusual (like bright blue pellets) in their mouths.
Choose the evening before a warm, humid night if possible. Confine the use of pellets to limited areas. The edges of walls, paths and lawns are the sorts of places in which slugs like to hide away during the day, where they can find damp, dark, cool refuges. When using pellets in a crop, do not throw them around wildly or you run the risk of some lodging in plant and contaminating your food. If at all practicable, protect birds and hedgehogs from the pellets by using netting. Pea guards are particularly suitable for edge situations, and are easy to manage. It is a very good idea to remove and kill any slugs around the pellets the next day because many will be paralysed but not dead. You can skim off the pellets and slugs for disposal, which will allow you to move the guards and treat another section, for if conditions were right you will have killed most of the slugs you were going to kill in that area on the first night. Such speedy treatment, and removal of chemicals, will minimise the risks to non target creatures.
There are no stated safety periods in this country between treatment and eating associated crops. However, such restrictions apply elsewhere in Europe and it would be wise to err on the side of caution and wait at least a week - even longer if you have used methiocarb spray.
Metaldehyde Tapes and Pads
Metaldehyde tapes and pads have been developed in recent years to try to retain the benefits of using this chemical while eliminating some of the disadvantages.
The tapes come in the form of coils, a little like recording tape but made of paper. In fact they consist of a sandwich of paper with metaldehyde as the filling! In order to get a dose of the filling, slugs have to eat the paper, which is impregnated with various secret ingredients to make it tasty (to slugs!). The paper is water-repellent so that it can go on working over a long period. The idea is that by the time the paper has rotted the metaldehyde too will have broken down chemically into harmless constituents and thus will not pollute the soil. Unlike pellets, the tapes do not contain a large amount of cereal and are thus not so attractive to birds and other wildlife. Predatory insects too seem to leave them alone.
The tapes must be laid out in strips alongside rows of crops, or in circles around cherished plants, such as hostas. In order to stop them blowing about weight them with stones every 12 cm or so.
The main disadvantage seems to be that birds, scratching about on the soil, will soon mess up your nicely ordered tapes, so you have to go around regularly pegging them out again. Worms also cause trouble from below, pulling the tapes down into their burrows and shredding them! This cannot be doing the worms any good and, of course, causes some of the chemical to contaminate the soil. These objections aside, the tapes are certainly an improvement on pellets in ecological terms and thus worth the additional cost.
The pads work in much the same way, but underground. These small squares of metaldehyde 'sandwich' should be buried along with your seed potatoes to give long lasting protection against keel slugs and garden slugs. They can also he used in walls, rockeries and similarly difficult locations, particularly against snails.
Aluminium Sulphate and Copper Sulphate
Copper sulphate has long been used with great success in tropical countries to clear disease-carrying snails from ditches and lakes, although it can damage water plants and fish. If you use it for slugs, keep it well away from ponds. For slugs, this chemical is used in spray form. It has been found to be effective against the small field slug (Deroceras reticulatum) when sprayed on to the soil, but less so against other species. It is a contact poison, and should thus be used only when slugs are active - as they crawl about, they pick up a dose. It is quickly washed down by rain, so repeated treatments are often necessary. However, because a build-up of copper in the soil can seriously affect worm numbers, repeated use is not recommended.
Aluminium sulphate has until recently been considered almost 'organic', and as such has been widely used. It too is effective only against very small slugs particularly field slugs, which are more susceptible than other species to contact poisons. Again, treatment should be confined to times when slugs are most active, and repeated use is often necessary. Aluminium sulphate has received a bad press since large quantities of this chemical were accidentally released into the water supply in Cornwall, causing health problems for many people. There is no evidence that the small amounts possibly absorbed by plants in treated areas of your garden would ever produce similar effects. However, there is a question mark over the connection between aluminium and Alzheimer's disease, or senile dementia. The risks can be minimised by keeping the spray off the foliage. With seedlings this is vitally important as they can be killed by direct spraying.
There are various mixtures of aluminium and copper sulphates on the market, with or without various additional salts and herbal extracts. There is little evidence that they work better than either chemical on its own.
SLUG CONTROL - NON CHEMICAL
I have deliberately not called this non-chemical slug control 'organic' because all the techniques are applicable to both organic and conventional systems. Indeed, the use of chemicals would rarely he adequate protection alone.
A number of different materials can be used to surround plants with the intention of deterring slugs. Most, like sand, ashes, broken eggshells and soot are physically difficult for slugs to get across either through being scratchy and sharp or by drying up the mucous glands that are necessary for their movement. There may sometimes be a chemically repellent effect as, for instance, in the case of ashes.
Unfortunately these substances all suffer from the ravages of wind and rain which respectively blow the stuff about or splash it with mud. In addition weeds may grow up and form bridges for slugs on the soil's surface, while the burrowers have no trouble going underneath in any case! However, with plenty of maintenance such barriers can work against surface-feeding species, particularly if other, more easily accessible food is available near by.
A surface mulch of mixed stable manure and wood chips has been found by some gardeners to be an effective slug deterrent, while others have discovered that this, and composted bark, attract slugs. It may depend again on what choice is offered to the slugs, and on weather conditions. Tannins in the bark might have a repellent effect while drying of the surface might slow them down a little. More experimental work is needed on this.
Plastic barriers can be used in various ways to create walls against slug attack, although admittedly they can he unsightly. In the simplest form, a clear plastic bottle with top and bottom cut off to form a cylinder can be used to protect small plants such as young brassicas. Otherwise, semi-rigid thick, clear plastic can be inserted, on edge, into the soil to form a wall around a whole group of plants. The above-ground wall should be at least 20 cm high, while the below-ground section should be at least 10 cm deep. The walls can, if necessary, be supported by pieces of cane or pipe. Such barriers will not be 100 per cent slug-proof, but at least they will reduce the 'slug pressure'. Of course, you must be sure that you are not trapping slugs that may already be in the plot you want to protect.
Beer or Milk Traps
Smooth glass or plastic containers, sunk into the soil and filled with beer or milk, certainly trap slugs. However there are problems. Never, for instance, sink the containers with their rims flush with the soil level. If you do, you will drown ground beetles that are important pest (including slug) controllers. The rims should be 1-2 cm above the soil's surface; slugs can crawl up and over quite easily. For this method to be an effective control, you need an awful lot of beer traps - at the very least one every meter in every direction - and an awful lot of beer or milk. The liquid must be replenished every few days, which can be quite a task. However, on a small scale, to protect a group of choice plants, the technique can work.
Night Time by Torch Light
This method of slug extermination is strictly for the non-squeamish. Fix a hat pin, or similar needle-like item, to a stick, binding it tightly with string. Then simply go round the garden spearing your slugs like the man collecting litter in the park. The slugs can be transferred to a container of salt or boiling water to finish them off. It sounds rather horrible, but at least, if you are worried, you can be sure that in this way they have a quicker death than would be the case with any of the poisons. One can become quite hard-hearted when an expensive and long-sought hosta is turned to lace the night after planting.
This technique can be surprisingly successful. Whereas a beer trap might catch a dozen or so slugs in a week, you can easily kill a couple of hundred an hour by searching. The greatest number are usually to be found on the lawn and pathways rather than on the soil itself. If your don't mind remarks about your waning sanity, you could even mow the lawn at night - you would kill slugs by the thousand! There might be lighting and safety problems, however, so take care - and warn the neighbours. Also, beware wandering amphibians which will also be hunting slugs at night.
Cultivation, Drainage, Soil Conditioning
Soil rotovation, in early spring and between crops, is one of the best of all slug controls. Three passes with a rotovator across an area of soil should reduce slug numbers to about a quarter of their previous level. This can therefore be more effective than any of the chemical treatments. Choose a time when the weather is warming up, to ensure that the slugs are at or near the soil's surface. Obviously the soil must not be too wet and claggy, or you will end up with a sticky mess. Rotovation both physically kills slugs and also exposes them and their eggs to predators and the weather. Digging by hand will have the same result to a lesser extent, and may be necessary where rotovating is inappropriate - on raised beds, for instance.
Slugs like heavy, wet soils, particularly for laying their eggs, to ensure that they do not dry out. They also like a rough surface with plenty of soil spaces to hide in. If the ground is covered with weeds as well, they think they are in paradise! So obviously it pays to avoid such conditions. Wet ground should be drained where possible, or avoided for the growing of slug-sensitive plants. Raised beds can be useful in combating the problems of heavy ground as they provide that little bit of extra height which improves drainage. In some circumstances coarse grit can help to improve a soil, as can well-rotted manure or compost (if it is not well rotted, slugs will home in from all directions). Try to obtain a goof tilth as early as possible in the season. This destroys the soil crack refuges and creates a surface that will dry out and be unattractive to slugs. Get rid of weeds such as dandelions that provide not only food but also refuges for slugs among your plants. A fine, firm seedbed is required ideally. Minimal digging systems are almost always plagued by slugs, partly because of the lack of cultivation and partly because of associated crop residues. Although certain weeds can, in the short term, divert the slugs from feeding on your plants, in the longer term you are probably storing up trouble in the forms of larger slug populations and increasing competitions from weeds.
If you have followed the advice given so far, you will have clear, smooth, weed-free beds with very little cover, particularly in spring. To encourage ground-living insect predators within the beds, you will need to provide artificial refuges. Old wooden boards are best and neatest (below), but small pieces of carpet, slates and so on will also do. The predators hide under these during the day, emerging at night to perform their duty.
I call these hiding places 'controlled refuges' because they have another important function. If you look underneath once a week, you will find slugs hiding there too, and slug eggs. These can be conveniently collected and destroyed.
SLUG PREDATORS AND PARASITES
As slugs do not normally die of starvation, there must be something that naturally prevents them from eating every plant in sight - after all, one field slug could potentially have 90,000 grandchildren and 27,000,000 great grandchildren if all were to survive. There are three main (non-human) causes of death for slugs: weather, disease and predators.
A number of creepy crawlies will eat them. These range from rare beauties like glow worms and their larvae to common centipedes. Many species of ground beetles (known as carabids) will eat slugs. These are the fast-running black beetles, found in almost all gardens. Although they will occasionally nibble a strawberry for moisture during dry weather, they otherwise do nothing but good. When not eating slugs, they will be tackling other soft-bodied pests, such as caterpillars and aphids. The devil's coach horses and related staphilinid beetles eat slugs too and are common in gardens, although less often seen.
Then there are the testacella slugs. These extraordinary creatures, found mainly in southern counties, are only distantly related to other slugs. They are carnivorous, almost entirely subterranean, have a small shell on their backs and are not slimy. They are frequently found in deep leaf mould and organically rich soil. The three species native to Britain, up to 12cm long, ranging in colour from pale cream to grey/brown with a yellow or orange sole and characteristic veining along the sides of the body. They pursue worms and burrowing slugs underground. Not only do they do no harm, these are slugs that do good too.
The most important parasites of slugs are the marsh flies (sciomyzids). The females lay 300 eggs or more, and each developing larva will kill several slugs, especially field slugs. Although called marsh flies, these insects are not confined to wet areas and can be found wherever there are slugs. You have to be an expert to identify them as they are simply anonymous looking flies, about 6 mm long, but it is very good to know that they are there.
Reptiles and amphibians may not be overwhelmingly popular, but they will do no harm to you or to your plants and include several slug eaters. Frogs are the best, the most common and the most easily encouraged. They prefer damp sites where slugs abound and hunt by ambushing anything edible that passes. A quarter of their diet frequently comprises slugs. Toads and slow worms eat slugs too and can be helpful in drier parts of the garden, particularly rockeries.
Many mammals eat slugs, including badgers and foxes but the best known slug-eating mammal must be the hedgehog. This creature forages at night, 'hoovering' up worms and slugs, particularly on lawns; unfortunately, like birds and amphibians, they will eat ground beetles too. It is to be hoped that the beetles are more successful at escaping than the slugs, and that on balance hedgehogs are doing more good than harm. I am inclined to give then the benefit of the doubt. In particular the hedgehogs will eat the larger slugs which the beetles cannot attack.
Birds are the most important of the larger predators. The long list of species known to eat slugs includes blackbirds and thrushes, robins, starlings, rooks and crows, jays, ducks, seagulls and even owls! Song thrushes are well known for snail eating. If you find a stone or paving slab with a scattering of broken snail shells around it, you know that either a song thrush or a redwing has been at work. The stone is used as an anvil to break the shell. These birds frequently eat slugs in even greater quantity, but there are no tell-tale remains to prove it.
Encouraging predators is an enormous subject and involves adopting many of the techniques advocated for creating a wildlife garden. You do not, however, have to go the whole hog, but can select elements that fit in with your personal gardening philosophy and encourage the best slug predators at the same time.
For example, if you use pesticides, such as slug pellets, you can minimise the dangers to insects and birds by avoiding insecticidal chemicals such as methiocarb, protecting the treated site with netting, and clearing always the poisons afterwards, as suggested earlier. Controlled refuges will then provide sites in which beetles can exits in the tidiest of beds.
British Gardeners tend to favour the type of plantings that create a suitable environment for predators in any case, with dense shrubberies and hedges to provide shelter, food and nesting sites for birds, and herbaceous borders that encourage many insect predators. Thrushes and starlings survive the winter by eating fruits and berries, so many of our favourite shrubs and trees, including rowan, holly, berberis, cotoneaster and crab apple, will help to persuade them to stay. Ponds are excellent, particularly with attached bog-gardens. These will provide breeding and feeding sites for frogs and toads although large fish must not be kept as they eat the tadpoles. The water will be appreciated by birds, for bathing and drinking, and with associated flowers will attract marsh flies. Nest boxes will attract more birds, particularly starlings, which will also eat the dreaded leatherjackets and other pests. They may be 'greedy' birds, but that is exactly what you want in a pest controller! Nest boxes can also be made for hedgehogs. These consist of a strong box, approximately 50 cm square with an entrance tunnel and a ventilation pipe, that can be buried in the soil or under leaf mould. Hedgehogs can then hibernate there, undisturbed and safe, ready to breed more slug hunters the following spring.
You can, where appropriate, introduce your own slug predators in the form of ducks and chickens. Hens are very destructive unless confined, but if moved over vacant ground in an ark or fold, they will not only clear the weeds, fertilise the ground and provide you with eggs but also eat any slugs and other pests that they come across. Ducks, on the other hand, do far less damage to crops, and have been used in the Far East for centuries to clear snails from rice fields. They are trained to go to flags set in the crops and clear the snails in the vicinity. If given plenty of space, and an area of grass to graze, they will not do much harm to cultivated plants, although seedlings need protection from their big feet!
Beetles can be concentrated in beds containing vulnerable plants by the construction of pitfall barriers (fig 2). These consists of a surrounding wall of smooth, corrugated lawn edging, constructed so that the soil outside the plot is flush with the top of the wall. The idea is that beetles running around the garden at night fall in and are trapped inside the bed. The smooth wall then prevents them from climbing out, and as most cannot fly, this results in a rapid increase in the density of beetles on the site. It is important to provide them with shelter, so 'controlled refuges' must be used. To get a good catch the outside soil must be flush with the top of the wall, and sand can be used to achieve this. The sand will also help deter invading slugs. Be sure to allow the beetles out again when the crop is cleared, because they will be needed again and must be allowed to find suitable breeding sites within the garden.
RESISTANT PLANT VARIETIES
Slug-resistant varieties of vegetables and ornamentals exist and can make an important contribution to reductions in damage. It must be added, however, that the potential for this has been little explored, and it is an example of an area of research that requires more support. Resistance is a very satisfying method of slug control, involving no work or side effects, and no additional costs. It is certainly worth taking the extra trouble to choose suitable varieties.
But do not imagine that resistance implies immunity - no potato is immune to keel slug attack, for example. Nevertheless, damage to potatoes is certainly greater in some varieties than others, and the list below divides cultivars into three categories.
VULNERABILITY OF POTATO VARIETIES TO SLUG ATTACK
Brassicas too have different levels of resistance according to type. In general, greater resistance can be found in spring cabbage, kale and sprouting broccoli, while summer cabbage, savoy, cauliflower and Brussels sprouts are more susceptible. It is often the sweeter, more succulent varieties that we, and the slugs, like best. I love the Fl 'Hispi' cabbage raw in salads; the slugs love it raw in the garden and seem to appear out of nowhere just as the plants approach maturity. It is the same with lettuces the sweeter they are, the more attractive they appear to be to slugs. When they taste of chicory or dandelion, they have more resistance. Such bitter-tasting elements in the leaves are often chemicals produced by the plant to deter pests. We breed out the bitterness, and should not be surprised that pest problems are correspondingly intensified.
What a slug chooses depends upon what is on offer. Slugs are major pests of both commercially-grown chrysanthemums and lettuces, but when given the choice they always choose the lettuces. So, if you grow these flowers try putting a few lettuces in among them as sacrificial crop to attract the slugs away. The slugs can thus be located and dealt with while on the lettuces. Alternatively, or in addition, you can choose resistant varieties.
Lilies come from different wild ancestors and have inherited varying degrees of ancestral resistance due to hybridisation. The roots of Lilium davidii and Lilium regale are attacked by keel slugs, while those of Lilium hennyi and Lilium tigninum are rarely affected.
Most hosta growers know that varieties with blue leaves have greater resistance to slugs than those with yellowish or variegated leaves. In this case a chemical that contributes to the colour, or one inherited with the blue colour, is effectively deterring slugs from attacking.
As new plant varieties appear regularly, you must simply be alert to any differences you notice in the garden. You might do better to replace with a more resistant alternative any variety that attracts slugs like a magnet, rather than attempting to engage in a prolonged battle of attrition with the slugs.
The practical gardener might well say that you should not only use chemicals sparingly in the garden, but that you should also spend money sparingly too! To this I fully subscribe. However, it is important to take hidden cost into your accounting. Pellets may be cheap, but if they cause the death of slug predators, the 'second wave' of slugs might well be worse than the first and be more costly to control and in terms of vegetables lost. The same principle applies to the time you spend: a method that is quick and easy in the short term might well cause you more work and trouble in the end. This does not mean that you should never use chemicals. It simply means that, when you do, use them when and where they will do greatest harm to the slugs, but the least to the predators.
Practical slug control will always be a matter of combining methods according to circumstances. You cannot use a rotovator in a rockery, or use pellets if you keep pigeons, or use a slug-resistant variety of a particular plant if it is not obtainable or if you do not like it! Preserving slug predators has long-term benefits and reduces the need for emergency action when pellets may be the only answer. Correct timing can avoid trouble too. For instance, lifting potatoes earlier than usual will avoid the period of greatest slug damage in autumn. It is at this time, when slugs are at their most numerous, that the potatoes are most attractive to them because the surface haulms, on which slugs feed, will have died off. The slugs burrow down for their food, avoiding the cooler conditions on the surface.
Use of a rotovator will clear large numbers of slugs in spring. If this is followed by the use of a contact spray (metaldehyde or aluminium sulphate, for example) the same evening, when slugs are very active, seeking new food sources and refuges, their increased activity will result in increased contact with the poison and a high percentage of those remaining will be destroyed. Having thus reduced the population of slugs considerably, you will find that further measures (controlled refuges, beer traps, barriers and so on) will be that much more effective.
Not much seems to have changed in slug control in the last twenty years. Fortunately, this state of affairs is about to alter dramatically. Some of the new developments are outlined below.
Low-dose pellets have been created, containing a detergent-like substance which increases the rate at which the metaldehyde content is absorbed by the slug's skin or gut. This allows pellets with only half the usual amount of metaldehyde to be as effective at killing slugs as conventional pellets, with obvious environmental benefits. Another new pellet, soon to be marketed, uses a chemical identical to that found in the rag worm (as used by fishermen) as part of this creature's defenses; it might thus be classed as an organic slug pellet. A further natural slug killer is found in a plant from Africa called endod or soap berry (Phytolacca dodecandra). The crushed berries are used by Africans for washing. It was discovered that in lakes and rivers where this natural soap was used, all the snails died. The related Phytolacca americana has similar properties and is a temperate plant, so perhaps one day you will be able to grow your own slug killer.
Strains of the now familiar Bacillus thuringiensis, used to control caterpillars, may be adapted to a slug-killing role, and the power of other newly discovered natural micro-organisms may be harnessed to this effect too. If these work, they may provide a simple spray-on treatment that would be harmless to everything except slugs.
My own work with beetles has revealed the existence of one species, Abax parallelepipedus, which has a proven ability to clear slugs from a crop. This beetle could be mass-produced and made available to gardeners and growers as pupa. When the pupae hatch, they could be introduced to infested crops, surrounded by plastic walls to prevent the beetles from escaping. The beetles are easily managed (they cannot fly), are harmless to people and plants, and should they escape they would in many cases simply join natural populations. This beetle is particularly effective against the field slugs. We have shown that another very common species of carabid beetle, Pterostichus melanarius, which is sometimes found in gardens, is a major predator of slugs in arable crops.
If you have read this far, you will at least have discovered that there is more to slug control than a packet of pellets. And, please, don't panic: there are options here for all situations, and new solutions on the horizon!.
New solutions on the horizon!
Since writing this chapter in 1990 there has been a major new development in slug control in the form of the slug parasitic nematode, Phasmarhabditis hermaphrodita. Although I was aware then of work to develop this biological control agent I was not free to make the information public, for reasons of commercial confidentiality. Now, however, these nematodes are available from garden centres under a variety of brand names, including 'Nemaslug'. They come bound up in clay granules, that can be simply dissolved in water and applied to slug-infested crops. Although they do not kill the slugs as rapidly as pellets, infected slugs stop feeding almost immediately and thus stop damaging your crop. The nematodes are completely safe to humans, pests and other garden wildlife, including the slug predators. This means, of course, that while slug numbers can build up again rapidly after use of a molluscicide based on methiocarb, because the predators have been removed, after applying the nematodes the predators will remain to prevent slugs re-invading, and to mop up slugs hatching from eggs in the soil.
W.S. October 1996
For further information:
A Field Guide to the Slugs of the British Isles by R. A.
D. Cameron, N. Jackson and B. Eversham (Field Studies Series, Invicta
and A Field Guide to the Land Snails of Britain and North-West Europe by MP Kerney and RAD Cameron (Collins) are both very useful.
This article was first published in The New Gardeners' World Handbook by BBC Books.