SLUGS AND SNAILS By Howard Drury
INTRODUCTION
It is rather disconcerting to realise that when a gardener grows succulent plants protected from drought, frost and winds, he is actively encouraging his traditional enemies, the slugs and snails. To what extent must he learn to live with this, and to what extent can he control the pests? Slugs and snails are certainly not as easily dealt with as other pests such as aphids but, through a knowledge of the slug’s life history and good gardening practice, things can be improved. The RHS in 2024 suggested we should encourage slugs and snails as part of a healthy garden environment
LIFE HISTORY
Slugs and snails are Molluscs, most of which (whelks, limpets etc.) live entirely in water. This is the key to their life on land, and a key to one of the ways they can be controlled. A typical slug has a pair of long tentacles carrying two eyes above a mouth with a rasping, toothed tongue. Breathing is via a simple lung open to the air through a small hole in the animal's side. A flat muscular pad, or foot pours out a slimy mucus to smooth the path and it is this that alerts the gardener to the slug's presence. They will happily eat any vegetation in the garden, weeds and rotting compost as well as your vegetables and, as an active slug will eat up to three quarters of its body weight each day, this can soon lead to serious damage to any garden crop. Succulent plants are most at risk. The ones commonly attacked include brassica seedlings, lettuces, celery, carrots, French and runner bean transplants, pansies, sweet peas, primulas and French marigolds. Tulip, iris and lily bulbs may be attacked and ripening fruit, especially strawberries. One of the problem facing the gardener is the ease and speed with which slugs and snails breed. This is because they are hermaphrodites. That means each individual is capable of laying eggs. These are pale, spherical and translucent, laid in batches in cavities in protected places either in the soil or under stones, where the temperature and humidity do not change too much (otherwise fatalities result). A Field slug (see below) will typically lay about 200 eggs in a batch of which there may be 30 in its six months of adult life. Most are laid in May and September in soil with 75% moisture content. In drier soils the eggs are laid deeper but if the moisture content drops to 10% no eggs are laid. Hatching takes 1 3 weeks. At one or two slugs and snails per square metre, they do little damage. At 50 60 per square metre they reach nuisance level and after long periods of warm damp weather when numbers build up to 100 200 per square metre, you will probably wish you had read this fact sheet earlier.
IDENTIFYING THE SPECIES
Note space and time does not permit me to list and detail all recognised 43 species here there following links are invaluable:
Slug Identification guide John Innes Institute
Identifying Land Snails Wildlife Trust
Identifying Land Snails Blogspot
The NHBS Guide to UK Slug Identification
Identification of common British & Irish garden molluscs
There are 43 recognised species of slugs in the British Isles, but only seven of them are garden pests. Of the nearly 100 species of snails, only eight regularly cause garden damage. Snails are generally restricted to chalky soils because of the lime needed for their shells.
1) The Garden Slug (Arion hortensis ) is the most serious pest because of its large numbers. It grows to 4 cm (1.5") and is identified by the orange or yellow colour of its foot. It attacks plants both above and below the soil and is a serious pest of maincrop potatoes.
2) Bourguinat’s Slug (Arion fasciatus). Similar to the garden slug but its foot colour is porcelain white. It is common in the Midlands. Differs from Derocerus in that it lays its eggs in early summer and its clutch size is approximately 30 with a total lay of about 300 eggs per slug. The species is semi‑subterranean and is partial to root systems of young seedlings and shoots.
3) The Black Slug (Arion ater) is much larger at 20cm (8" ) and is not always black. There are grey, reddish, yellowish and albino forms. Young specimens up to 2.5 cm (l") are always straw coloured with distinctive dark eye tentacles. They are not as numerous as other slugs, but a large slug has a large plant based appetite.
4) The Field Slug (Deroceras reticulatum). This is common, again growing to 4cm (1.5"), light brown in colour with darker matting. A surface dweller causing damage above or just below soil level. When prodded it exudes a characteristic chalk white slime. The most frequently seen slug species generally has an annual life span. Mating occurs on the surface of the soil in moist, damp conditions during early to late autumn and each slug moves down into the soil and begins laying egg clusters of about 35 each. Over the following weeks it continues laying clusters up to a total of some 500 eggs. The eggs hatch, subject to soil humidity and temperature, between 8 and 15 days after laying and the adult slug, after laying its final clutch, dies. By this time, there are tens of thousands of young hatchlings in the soil, steadily eating and growing rapidly and is very difficult to kill with baits and pellets.
5) Keeled Slugs (three species of Milax) grow slightly larger than the above species, up to 7 cm (2.5"). They are recognised by a ridge of skin (the keel) running along the middle of their backs and by their covering of very tacky slime. Most are brown or black in colour. They live below ground, damaging the underground parts of plants, bulb and root crops ‑ especially potatoes. It is a generally subterranean slug which causes most of the damage to potatoes and root crops. Unlike the previous two species, one generation mates and lays eggs in late Autumn while a parallel generation lays its eggs in early Spring. The autumn generation of eggs will only hatch in the autumn if the soil temperature and humidity are ideal. Otherwise, where an early winter sets in, the eggs will remain dormant until the spring when they will join the Spring hatchlings to give a double attack on root crops and potatoes. Milax lays comparable egg clusters and total numbers of eggs to Arion.
6) The Garden Snail (Helix aspersa) is the largest (shell up 3 cm or 1½" diameter) and most widely distributed snail.
7) The Banded Snails (Cepaea nemoralis C. hortensis) have smaller, (2.5 cm or l" ) more variably coloured shells.
8) The Strawberry Snail (Trichea striolata) has an even smaller (15mm or ⅝") diameter flattened, pale brown shell.
Arion vulgaris (Spanish Slug)
Hit the news in 2012 when allegedly first reported! I believe I saw it in the late 1970s.
I photographed this pest in the 1980s and saw many references to it on websites in 2004
There is some confusion with Arion flagellus 1946?
Arion vulgare is listed in Pests, Diseases, Disorders 1981
Lays twice as many eggs as other species
Tolerates hotter drier conditions hence thought it originated from Spain.
They are thought to be omnivores eating other animals and causing huge amounts of damage to crops and individual plants.
It is thought there no or few natural predators to control them
A grant has been given to Dr. Ian Bedford of John Innes to research further the problem they are causing.
CONTROL WITHOUT CHEMICALS
The gardener is unfortunately his own worst enemy here because he often goes out of his way to provide ideal conditions for slugs and snails. The whole of their life cycle relies upon wetness and warmth, so when we water plants, protect them from the wind and frost or add compost and manure to improve the soil's water retention, we are also providing ideal conditions for the slugs and snails.
The most effective measures you can take are those that expose the slugs and snails (and their eggs) to drying or freezing conditions. So, where you can, try to clear your garden of shady damp spots and areas that will shelter slugs such as piles of old wood, rubble etc. Obviously, your compost heap, water garden or shade garden will attract slugs and snails, but you will just have to live with it. There is no point in having a slug free concrete desert for a garden.
Dry conditions stop slugs and snails from freely moving around and if dry weather is prolonged it will kill many by desiccation. Capitalise on this by keeping the soil surface weed free by light hoeing and generally removing anything that can act as a refuge. If, however, the only edible things in your garden are flowers and vegetables, these will receive all the slugs' attention. It is a question of a sensible balance. Digging beds just before frosty weather will affect the surface and shallow living slugs and their eggs. However, recovery can be quick for these surface dwellers (each adult laying 200 eggs which become mature slugs within six months ) and Keeled slugs living deeper in the soil are largely protected from the extremes of our climate.
Hand picking after dark has been tried by many people, but in the long run the sheer number of slugs defeat the hand picker. One-man hand picked 180,000 slugs over four years from his garden before giving in. The number problem also means that trapping them in the traditional way under grapefruit skins or in beer traps will not have a great effect. Trapping is a useful exercise, however, because it gives warning of numbers building up and when slugs are becoming active on the surface and so susceptible to baiting. Homemade beer traps are often advocated as an "organic" method of control. The main drawback is that these are non-selective and often kill as many helpful insects as slugs. Ground beetles are common victims of these slug traps. This reduces the numbers of useful predators in the garden. Finally, certain plant varieties are less susceptible to slug damage than others. 'Maris Piper ' and ' Pentland Crown' are potatoes liable to be badly damaged, while `Stormont Enterprise', 'Pentland Ivory' and 'Pentland Dell' are less severely attacked.
A number of the old-fashioned controls such as pouring salt on slugs are, strictly speaking, no longer legal, as they have no government approval, and we are therefore unable to recommend them. Crushed eggshells are said to be a deterrent - but how many eggs are needed to provide a reasonably thick layer of material around susceptible plants? The gardener's cholesterol levels would be dangerously high to make it an effect method of control!
In summary then, slugs cannot be effectively controlled without chemicals. But damage can be kept to a minimum by digging before frosty weather, frequent summer hoeing, and keeping the garden realistically free of damp spots and plant remains. Trapping will not remove many slugs or snails, and will kill useful insects, but it will tell you when action is necessary to control numbers.
BIOLOGICAL CONTROL
Non-chemical slug control has now been developed and these native nematodes (a microscopic natural slug parasite which has been isolated from soils in the UK) have been shown to be effective and environmentally safe. The parasite, in large numbers, is an extremely effective slug control specific to molluscs without posing a risk to other wildlife, pets, children etc. The parasite enter under the slug's mantle releasing bacteria which causes it to stop eating within a few days. Infected slugs burrow into the soil and die, out of sight, releasing more parasites into the soil. There is no danger to warm blooded creatures as the parasites die rapidly at temperatures above 35C. Nematodes are being sold by many different suppliers, but most will originate from one producer. A web search will reveal a list of suppliers including: Nematodes Direct, Ladybird Plant Care, Gardening Naturally and Garden Organic
ENCOURAGING PREDATORS
Encourage predators of slugs and snails
Ground and Rove Beetles
The devil's coach horses and related staphilinid beetles
Frogs and Toads
Sloworms
Hedgehogs
Centipedes
Birds, including ducks and chickens, wild birds
Testacella slugs
Badgers and Foxes
Plus many others now being thought to be useful predators (up to 70 subjects being investigated)
OTHER CONTROLS
Caffeine from coffee
Beer cans
Comfrey plant
Lime or talcum powder
Wormwood Tea
Horseradish & Scented Geranium spray
Beer and flour batter
Honey, Yeast and cooking oil
Budweiser Beer
CHEMICAL CONTROL
Chemicals have been, until now, the most effective method of control, although their effectiveness is still poor in comparison with other chemical pest controls such as insecticides. Traditional materials such as lime and soot spread around plants deter attack but soon lose their
effectiveness. All sorts of lethal killers have been tried, e.g. copper sulphate, calcium cyanamide, copper aceto‑arsenide. These are not recommended nowadays as they lead to toxicity in the soil, damage plants or are dangerous to handle. Because slugs and snails remain hidden for most of the time, they cannot be sprayed with poison but need to be lured to a bait.
Metaldehyde – Banned 2022
Methiocarb – Banned 2014
Aluminium Sulphate - There is an entirely slug and snail specific chemical (aluminium sulphate) This is claimed to kill slugs and snails by contact as they crawl over treated surfaces. The chemical causes a shrinkage of the slug's slime forming organs. However, questions are now being raised about the advisability of using products containing aluminium, which can cause a build-up in the soil.
Essential Oils – these are often based on oil extracted from common lantana (Lantana camara) and mint (Mentha). Humans and mammals are not endangered as these materials deter the slugs and snails; they are not killed but merely go away from the small to feed.
Ferric Phosphate - Slug pellets with the active ingredient ferric phosphate are available and are safe for garden wildlife, small children and the environment. After ingestion of this agent, the snails stop feeding, retreat to their hiding places without forming slime and die there. Thus, the dead animals do not lie around in the beds. The iron III phosphate is dissolved by microorganisms and organic acid and does not harm the soil but is used as a plant nutrient. There is some concern that in order to make the Ferric Phosphate more soluble other ingredients are added and these may be more of a danger to our environment. More research is needed.
Effective Use of Pellets - Pellets gradually lose their effectiveness after a few days. Slugs and snails can only be poisoned when they are active on the surface and correct timing is essential. Slugs and snails are most active on warm, damp nights with little wind, so apply your baits in the evening of a suitable night. Repeat treatment will be necessary both after heavy rain, which makes the pellets ineffective, and after a few days. Plants are most vulnerable at the seedling stage, so treat before and during this stage.
NOT ALL SLUGS ARE PLANT EATERS!
We must also remember that not all slugs eat living plant material. Some are vital in aiding the decomposition of rotting organic matter. Others are purely carnivorous, unfortunately one of this groups staple diet is that of garden worms, while some carnivorous slugs will eat other slugs. While only a few species of slug only feed on living plant material they are often found in exceedingly high numbers while predatory carnivorous slugs are only found in very small number across around 8 species. This is difficult to confirm as some slugs live amongst detritus material helping it decompose, they can also be carnivores. The leopard slug (Limax maximus) has a tendency to cannibalistic behaviour and is said to prey on other live slugs.
SAFETY - PETS AND WILDLIFE
Slug killers are poisons so treat them with respect. Always wash your hands after handling and keep them well away from pets and children. All amateur gardeners should therefore take care to ensure that they use only currently approved, amateur products in their original containers.
If you have hedgehogs in your garden, you will want to avoid harming them as they eat slugs and snails. Slug pellets are not responsible for the widespread death of hedgehogs. Figures from government sources show that 360 pellets per kilogram of body weight would be needed to cause a problem. That's about 260 slug pellets to kill a hedgehog - in the unlikely event it would or could eat the pellets. Once the slug has eaten a pellet, it is broken down in the gut to release the compounds, which cause the slug to dehydrate and die. It does not pass on any toxic effect to hedgehogs, birds etc. which may eat it, although already dead slugs are not usually eaten. Pellets remaining on the soil gradually break down physically and chemically with weathering over several weeks to form harmless gasses that occur naturally in the air.
So, it's really up to you. Some plant loss is inevitable if you want to leave it entirely up to the hedgehogs. If you are losing significant numbers of valuable plants or food crops, some action will have to be taken. When using any baits, be certain to site them wisely, read the label on the container and don't use too much. The easiest organic method of control the gardener has is his boot (for small numbers at least)! Squashing the slugs or snails underfoot is quick, painless and chemical free. The trouble is when I did it on a recent television programme, I had letters of complaint from people claiming that I was being cruel to defenceless creatures and such horrific pictures should not be screened during the day! I still have the letters for those who don't believe me. It seems we can't win - but action does have to be taken if we want attractive and productive gardens.
PRODUCTS CURRENTLY AVAILABLE
NB - When using ANY garden chemical or product, always read the label thoroughly and follow the manufacturers' instructions precisely. Store in a safe, cool place well away from children and pets. The RHS states on its website that slug pellets (even organic ones) have been shown to have negative effects on wildlife in the garden. It’s better to encourage natural predators and use the cultural and biological control methods
Doff 650g Power Up Slug & Snail Killer
Doff Slugs Be Gone Defence Gel Organic
Doff Slugs Be Gone Wool Pellets
Doff Slug & Snail Adhesive Copper Tape
Growing Success Organic Slug Stop Pellet Barrier
LIST OF LESS PALATABLE PLANTS
This list is based on my personal observation and photography over the last many years.
Herbaceous plants
Acanthus mollis (bear’s breeches)
Achillea filipendulina
Aconitum
Agapanthus hybrids and cultivars
Ajuga
Alchemilla mollis (lady’s mantle)
Alyssum alpine forms
Anemone × hybrida (Japanese anemone), A. hupehensis (Japanese anemone)
Antirrhinum majus (snapdragon)
Arabis
Aquilegia species
Armeria species
Artemisia
Aster amellus, A.× frikartii, A. novae-angliae (Michaelmas daisies)
Astilbe × arendsii
Astrantia major
Bergenia (elephant’s ears)
Centaurea dealbata, C. montana
Corydalis lutea
Cosmos
Crocrosmia
Cynara cardunculus (globe artichoke)
Dianthus
Dicentra spectabilis (bleeding heart)
Digitalis purpurea (foxglove)
Echium
Eryngium species
Euphorbia species (spurges)
Foeniculum vulgare (fennel)
Fuchsia cultivars
Gaillardia aristata
Geranium species
Geum chiloense
Helleborus
Hemerocallis cultivars (day lilies)
Heuchera
Iris many
Knautia Macedonia
Lavateris
Maclea
Nepeta
Papaver nudicaule (Iceland poppy)
Peony
Pelargonium
Phlox paniculata
Physostegia virginiana (obedient plant)
Polemonium foliosissimum
Polygonum species
Potentilla hybrids and cultivars
Pulmonaria species (lungwort)
Rudbeckia fulgida
Salvia × superba
Saxifraga × urbium (London pride)
Scabiosa caucasica (scabious)
Sedum spectabile (ice plant)
Sempervivum species (houseleeks)
Sisyrinchium species
Solidago species (golden rod)
Stachys macrantha
Tanacetum coccineum (pyrethrum)
Thalictrum aquilegiifolium
Tradescantia virginiana
Tropaeolum species (nasturtium)
Verbascum species (mullein)
Slug Resistant Hostas
Hosta ‘Above The Clouds’
Hosta ‘American Halo’
Hosta ‘Blue Elf’
Hosta ‘Devon Dream’
Hosta ‘Etched Glass’
Hosta ‘Firm Line’
Hosta ‘Halcylon’
Hosta ‘June’
Hosta ‘Justine’
Hosta ‘Lipstick Sunset’
Hosta ‘Party Pooer’
Hosta ‘Paul Vernooij’
Hosta ‘Regal Splendour’
Hosta ‘Sister Act’
Hosta ‘Stand By Me’
‘Sun Mouse’
Hosta ‘Touch of Class’
Shrubs
Ceanothus
Choisya
Fuchsia hardy
Hydrangea
Lavender
Rosemary
Santolina
Vinca
Slug and Snail Tolerant Potatoes
Good resistance
Good Resistance
‘Estima’
‘Charlotte’
‘Golden Wonder’
‘Kestrel’
‘Pentland Dell’
‘Pentland Ivory’
‘Pentland Falcon’
‘Pentland Squire’
‘Romano’
‘Sante’
‘Sarpo Mira’
‘Sarpo Axona
‘Stemster’
‘Wilja’
Intermediate Resistance
‘Desiree’
‘Majestic’
‘King Edward’
‘Pentland Crown’
‘Pentland Hawk’
‘Record’
‘Romano’
Low Resistance
‘Cara’
‘Kingston’
‘Maris Bard’
‘Maris Piper’
‘Pentland Crown’
‘Sutton’s Foremost’
Personally, I would avoid Maris Piper as I feel it is a martyr to molluscs, closely followed by Suttons Foremost.
THREE THINGS NATURALLY KILL SLUGS
Weather
Cold dry
Hot dry
Predators
Many including Dogs, birds, rats,
Nematodes
Diseases
Slugs and snails carry diseases
Killed by diseases carried by other animals
Natural old age (some only live a season others up to five years)
SLUG AND SNAIL FUNNIES
Slug blood is blue
Fastest slug is the tramp slug (Deroceras invadens), which has been recorded travelling at a breakneck speed of 17.6 metres per hour (or 29.4 centimetres per minute)
The ash-black slug (Limax cinereoniger), a woodland dweller, has been known to reach 30cm in length
Slugs are right-handed
Slugs poo from their heads
Slugs are made up of over 70% water
It's not slime its mucus to prevent drying out
A slug is really one big foot
Slugs don’t have any bone
Their nose is on top of their heads
They have more teeth than sharks
FURTHER READING
Field Guide to the Snails of Britain and North West Europe by Michael Kerney
RHS Trials on Slug Control using various methods
Slugs of Britain and Ireland Identification, Understanding and Control
A web search will bring up the latest books on this subject along with details of the supplier. It is also worth searching through second hand bookstores online as some of the offer incredible value, but you must remember some of these may contain outdated information especially regarding varieties and the use of chemicals. Two of my favourites are Abe Books and World of Books for which I receive no commission in any form.
The information given in this Fact Sheet is provided in good faith. It is however of necessity general information and advice on the topic. Howard Drury will not be under any liability in respect of the provision of such advice and information, and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.
(C) 2025. This material has been produced by Howard Drury and must not be reproduced in part or full physically or electronically without the written consent of Howard Drury, Kings Heath Birmingham, B13 0SJ. The only exception be to print a single copy for personal use from a downloaded file as part of www.thedrurys.com website. Visit www.thedrurys.com or email
JHD/23/03/2025
