If you want to use slug pellets, do so with caution, otherwise there are alternative options
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| Garden centres make a packet from pellets
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'The majority of poisonings of pets seen by vets are as a consequence of slug-pellet use.'
'If you don't mind remarks about your waning sanity, you could mow the lawn at night – you would kill slugs by the thousand!'
Slug pellets – how bad are they?
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. Also, given 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.
Overuse of slug poisons can be counterproductive in that you risk killing off the predators that are helping to control your slug population. There are around 30 species of slug in Britain. The four most common, and problematic, are: the field slug, garden slug, keel slug and black slug. While the black slug can grow very big (up to 20cm (8in) long), it is rarely as destructive as the other three smaller species, but can cause damage in spring to seedlings of many kinds.
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| Blue pellets may deter birds
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Metaldehyde and methiocarb
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.
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. 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. 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.
Methiocarb pellets and spray
Methiocarb pellets also 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. Methiocarb breaks down more slowly than metaldehyde, which can be both an advantage and a disadvantage.
Pros and cons of metaldehyde and methiocarb
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 10 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.
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| Beer traps can work but you need a lot
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Protect garden wildlife
From a garden wildlife point of view, 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. 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. Always read the instructions on the product very carefully. Otherwise choose non-chemical methods.
When to use slug pellets
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 be 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.
How to use slug pellets
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 24 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.
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| Nemaslug biological control
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Alternative methods – Plastic barriers
Plastic barriers are used in various ways to create walls against slug attack. In the simplest form, a clear plastic bottle with top and bottom cut off forms a cylinder that 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 20cm (8in) high, while the below-ground section should be at least 10cm (4in) deep.
Beer or milk traps
Smooth glass or plastic containers, sunk into the soil and filled with beer or milk, certainly trap slugs. Never 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 1-2cm (1/2-1in) above the soil's surface; slugs can crawl up and over quite easily. You do, however, need an awful lot of beer traps – at the very least one every metre in every direction – and an awful lot of beer or milk. The liquid must be also be replenished every few days. However, on a small scale, to protect a group of choice plants, the technique can work.
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| Dilute Nemaslug in a watering can
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Night-time by torchlight
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. 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.
Mowing
The greatest numbers of slugs are usually to be found on the lawn and pathways, rather than on the soil itself. If you 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.
Soil rotavation
Soil rotavation, in early spring and between crops, is one of the best of all slug controls. Three passes with a rotavator 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.
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| Pour the solution onto the ground
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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. Rotavation 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 rotavating is inappropriate – on raised beds, for instance.
Biological control
A major development in slug control comes in the form of the slug parasitic nematode, Phasmarhabditis hermaphrodita. 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. So, while slug numbers can build up again rapidly after use of a molluscicide based on methiocarb (because slug predators have also been removed), after applying the nematodes, however, the predators will remain to prevent slugs re-invading, and to mop up slugs hatching from eggs in the soil.
*Dr Bill Symondson is based at the Cardiff School of Biosciences. Text adapted by Sarah Brocklehurst.
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