Garden Pests – How to Control Snails and Slugs Without Toxic Chemicals

Snails and slugs are one of the most destructive garden pests, causing extensive damage to seeds, seedlings, leaves, fruit and tubers. They go about unseen because they only come out at night or on rainy days to feed, and hide during the day. The damage is usually discovered the next morning, but there’s usually no sign of them other than the tell-tale glistening slime trails they leave behind.

By better understanding how these pest operate, we can develop effective pest control strategies to limit their destruction in the garden.

About Snails and Slugs

Snails and slugs aren’t actually insects, they’re molluscs, as they belong to the taxonomical phylum Mollusca. Within this phylum is a sub-category, the class known as Gastropoda, the gastropods, which includes all slugs and snails; not only terrestrial ones, but also aquatic species from freshwater and marine environments. The term gastropod means ‘stomach-foot’ in Ancient Greek, and is an apt description for these creatures which are essentially crawling stomachs on a large slimy foot.

Incidentally, octopuses, squid, cuttlefish and nautilus are molluscs too, but they’re cephalopods. Mussels, oysters, clams and scallops are also molluscs, but they’re bivalves! It would be fair to say that snails and slugs have many interesting distant relatives.

It’s estimated that around 248 million years ago, close to the beginning of the Mesozoic Era, otherwise known as the ‘age of the dinosaurs’, many gastropods developed lungs instead of gills, moved from their marine environment and adapted to life on land.

Being aquatic creatures, they had to take their water along with them in the form of a coating of water-absorbent mucus, or ‘slime’ to keep their bodies moist. This is why snails and slugs prefer cool, moist, shady conditions, and take advantage of cooler evenings when the air is more damp to come out and feed. On bright sunny days when the air is hot and dry, they seek out a cooler, shady hiding place to rest. They will emerge from their hiding spots in the evening after it has been raining, or a garden bed has been watered, in response to increased moisture levels and humidity.

Even though they move very slowly, with a speeds of 0.28cm/sec – 1.3cm/sec (10.08 m/hour – 46.8 m/hour), snails and slugs can cover a lot of ground during the night, to reach most parts of the garden, and they’re able to climb vertically up plants, fences and walls. Small snails can competently climb to the top of young citrus trees to devour the tender new growth at the top which they are very fond of!

How Do Snails Find Plants to Eat?

Snails and slugs have one to two pairs of retractable tentacles extending from their heads.

The upper pair of tentacles are longer, and have the eyes at the tips which look like shiny black dots. Even though these eyes have a cornea, a lens, and a retina, they’re not able to focus, so they’re not used for vision but to distinguish light from dark. If a shadow suddenly falls over a snail, it quickly retreats into its shell defensively to avoid possible predators. These eyes are also used to maintain circadian rhythms, establishing a natural 24 hour cycle.

The other pair of tentacles are shorter and closer to the ground. These tentacles contain chemoreceptors, which detect chemicals, and function as olfactory organs, providing a sense of smell. Since snails and slugs can’t see very well, they depend on a strong sense of smell instead, and are able to locate food several metres away.

Both pairs of tentacles, the upper and lower ones, also contain tactile receptors, giving snails and slugs a sense of touch and the ability to feel around their environment.

How to Identify Snail and Slug Damage

Snails and slugs feed on a both living plants and decaying plant matter. They cut holes in the middle of plant leaves, eat young seedlings completely to the ground, damage fruit located close to the soil surface such as strawberries, attack  low-hanging fruit such as such as tomatoes. The foliage and fruit of some trees is also eaten by snails and slugs. and they are known to have a preference for young leaves of citrus trees.

There’s a simple way to distinguish snail damage from caterpillar damage.

• Caterpillars only eat leaves from the edges, and gradually chew their way to the middle.
• Snails and slugs have a rasping tongue known as a radula, which is covered in rows of teeth made of chitin (the hard material beetle shells are made of), which they use to scrape off particles of food to eat, and which can also cut holes in the middle of a leaf.

Large holes in the middle of leaves usually indicates snail or slug damage. Other insects such as earwigs or beetles may make many small holes in the centre of leaves, but not large holes as they don’t eat anywhere near as much as snails can. Snails can eat a lot of plant material relative to their body weight. The large Roman snail, Burgundy snail (Helix pomatia), which is also known as an edible snail or escargot, can eat up to 6 grams of plant material each day when fully grown.

Another tell-tale signs of snail and slug damage is the shiny silver slime trails they leave behind.

Characteristic snail damage on cabbage leaves. Unlike caterpillars which start chewing leaves from the edges, molluscs have a rasping tongue and are able to eat holes in the middle of a leaf.

How to Controls Snails and Slugs

An integrated pest management (IPM) approach is a scientific and strategic way to control pests using a combination of techniques to limit pest populations and the damage they cause, while eliminating or drastically reducing the use of pesticides to minimise risks to people and the environment.

Snail and Slug Physical Controls – Manual Methods

Snails and slugs are commonly found hiding under dense groundcover plants, amongst strappy leafed plants, under debris in the garden. and inside upturned flower pots. They can also be found moving around in large numbers, making their way to their favourite plants in the early evening after sunset during or after light rain.

Hand picking can reduce their numbers, drop them into a bucket of soapy water to kill them, or save them as food for ducks. I use a small bucket, quarter-filled with water, with two squirts of organic liquid hand soap. Leave the bucket overnight with a cover over the top, such as a plastic pot saucer. Next morning, pour the contents into the compost. Organic liquid soap is Castile soap, it’s a natural soap made from olive oil, it’s almost identical to insecticidal soap used by horticulturists and is biodegradable. Synthetic ‘soaps’ and dishwashing liquids are not soaps, but detergents, and shouldn’t go into the compost or soil!

Snail and Slug Physical Controls – Barriers

An effective way to control snails and slugs is by using a physical barrier to prevent them reaching the plants they want to eat.

1. Plastic Bottle Snail and Slug Barrier

A simple plastic bottle barrier can be made by cutting the bottom off a soft drink bottle, removing the lid, placing it over a vulnerable seedling or small plant, and pushing it slightly into the soil so it won’t blow over.

This barrier is a really a garden cloche cover, which were traditionally made of glass, and it will protect plants from many other pests too. It will also act like a mini-greenhouse, so it’s important to never leave the bottle lid on, as the temperatures will rise on a warm day and cook the plant.

If temperatures get very high, and direct sun is striking the plant, lift off the covers, and replace them in the evening, or use 50%-rated shade cloth supported some distance away or above the plants (at least 30cm (12”) to reduce the sun’s intensity. The surface of shade cloth gets hot, so the space is needed for air circulation!

2. Snail and Slug Ring Barrier

A ring barrier can be formed around plants using crushed eggshells, coffee grounds, sawdust or wood shavings to protect them from snails and slugs. These materials stick to their slime layer, making it difficult for them to crawl over the loose particle.

Sometimes materials such as garden lime or wood ash are also suggested for this purpose, but they would make the soil highly alkaline around the seedling, which is undesirable, as we don’t want to alter the soil pH.

Rinse, dry and crush eggshells, then form a reasonably thick circle around the base of the plant, a few centimetres away from the stem, so the snails and slugs can’t reach up and over the barrier. There needs to be a decent enough barrier so they get tangled up in the bits of eggshell.

3. Snail and Slug Copper Tape Barrier

One of the best physical barriers is copper banding around plants or pots. It’s usually sold as a thin, self-adhesive copper tape which is is taped across the top of pots, or as a copper foil which can form a low fence on the soil around the stem of the plant. When snails make contact with copper, it gives them a bit of an electric shock, and pull back quickly.

4. Snail and Slug Coffee Repellent Spray

A coffee repellent spray can be used to keep snails and slugs away. To make this spray, combine 10 parts water to one part espresso coffee (because instant coffee is too weak). Spray this on the leaves of plants and over the soil surface. The caffeine in coffee is toxic to snails and slugs, they absorb it as they crawl over it and it poisons them. It needs to be re-applied after heavy rain, which will wash it away.

5. Snail and Slug Repellent Mulches

Mulching around gardens with strong smelling herbs such wormwood, mint, tansy or lemon balm serves as a repellent to snails and slugs. Chop up the plants using secateurs, mulcher, or mower so they release more of their aromatic oils.

Snail and Slug Physical Controls – Traps

Traps are another effective physical control for managing snail and slug pest problems.

The easiest way to trap snails and slugs is to use:

1. Inverted flower pots propped up on one side with a rock. Terracotta clay pots are heavy enough not to be blown over by the wind.
2. Wooden boards raised slightly off the ground, high enough for the pests to crawl underneath.
3. Citrus halves laid on the soil, with the open end down

With these traps, leave them overnight to do their work. Snails and slugs roving around the garden at night will find the traps, and hide there before the sun rises.

Here’s an inverted pot trap. The shorter terracotta pots work best because they sit much lower and don’t topple over.

Boards propped up off the ground can be used as snail and slug traps. The wider the better, as they provide better cover, but even recycled fence palings work reasonably well.

How to Make a Beer Trap for Snails and Slugs

Beer traps can work, it’s the fermented materials that attracts snails and slugs, so a mixture of sugar-water and yeast mixture can be used instead of beer. These traps can be purchased as pre-made plastic traps, or you can make your own as shown in the steps below.

To make a snail and slug beer trap:

1. Take a reasonable deep plastic container with fairly vertical sides set into the soil, with the rim extending a bit above the ground to prevent soil and debris falling into the container.
2. To keep the rain out, and create an attractive hiding place for snails and slugs, cover the trap with a plastic or terracotta pot saucer, which is raised above the ground using 3 or 4 rocks evenly spaced around the container. The gap allows snails and slugs to crawl underneath. If a plastic pot saucer is used, place a flat rock on the top to stop it being blown away by the wind.
3. To attract and trap slugs and snails, make the fermenting attractant bait solution as follows. Place some beer or baking yeast in a 5% sugar-water solution (one teaspoon of sugar into 100ml container, fill to 100ml mark with water). Some gardeners just use straight beer.
4. Half-fill the container with beer-sugar water or yeast-sugar-water bait solution or just straight beer. The liquid will evaporate, especially on hot days, and needs to be topped up.

These these traps are quite messy, and  aren’t very effective for the amount work involved. Beer is quite expensive to be used as snail bait and beer traps only attract slugs and snails within an area of a few feet, but if you wish to give them a try, give it a go and see what happens!

Snail and Slug Cultural Controls

These controls are practices which disrupt the pest’s environment of the pest, reducing its establishment, reproduction, dispersal, and survival.

Here are some simple changes we can make in our gardens to help control snails and slugs:

1. Eliminate any hiding spots near gardens, such as tall grass and weeds along fences, rotting logs and wood piles, where possible.
2. Create a less favourable habitat by using drip irrigation rather than sprinklers to reduce humidity and moist surfaces.
3. Grow sacrificial plants such as horseradish as a decoy, to direct the snails away from other plants. They will eat the horseradish in preference to other plants, saving them, and the horseradish is a tough plant that will grow back and recover.

Snail and Slug Chemical Controls – Which Snail Baits are Safe?

In integrated pest management (IPM), chemical controls such as snail and slug baits are used as the last resort, and should be used in conjunction with the IPM cultural controls detailed above for maximum effectiveness.

Snail baits can be natural or synthetic formulations, and only some natural formulations are permitted in certified organic gardening systems.

In the section below, we’ll examine the main types of snail baits, learn about the ingredients in them, and how safe they are to use in the garden.

1. Elemental Iron Snail Baits

The safest snail baits are made of elemental iron, which simply means they contain the element of iron, in other words, plain iron!

These formulations are relatively new, quite expensive, but are non-toxic and permitted in certified organic systems, because iron is a natural plant micronutrient that is found in soil.

2. Chelated Iron Snail Baits

Most simple compounds of iron (iron salts) can’t be used as snail baits because they’re insoluble (don’t dissolve in water) and just pass straight through the pest without entering the blood stream, or are repulsive to them, so they won’t eat them.

A solution to this problem was discovered In 1994 and 1995, independently by two scientists, who found that some chelated iron compounds could be used in a grain based bait to produce effective snail and slug pellet baits.

Chelated iron is elemental iron bonded to a carbon-containing molecule, making it easier to be absorbed by living things. Iron chelates are used in horticulture as iron supplements for plants showing symptoms of iron deficiency, and some are used in iron supplements for humans.

These snail baits, and iron supplements for plants typically contain and iron-EDTA complex, which consists of iron bonded to an ethylenediaminetetraacetic acid (EDTA) molecule, with the structure shown below.

The chemical structure of iron-EDTA chelate, where Fe is the chemical symbol for iron, and the rest is the EDTA molecule, the lines represent chains of carbon atoms bonded together.

Snail baits based on chelated iron/ Iron EDTA complex are considered safe to use around children, pets such as dogs and cats, and wildlife. The active ingredient is rated less toxic than common salt, but if consumed in large quantities it may be toxic. If snails and slugs consume even a small amount of this bait though, they will stop feeding and die several days later.

Chelated iron snail baits are not permitted in certified organic systems, because the EDTA portion is a synthetic carbon compound.

Here’s a summary of the chemical compound, and its environmental impact.

• Iron EDTA– (pellets are red/brown), a chelated aminopolycarboxylic acid compound, used as a  molluscicide, and a trace element supplement for plants.
  The environmental impact can be summarised as highly mobile in soil (potential to be carried by water move off the site of application) and highly persistent, toxicity to fish, mammals and other non-target wildlife is low. Iron chelate compounds are  toxic to crustaceans, but there is little chance of snail pellets getting into aquatic environments.

Why is Iron Toxic to Snails and Slugs?

The reason that iron compounds are toxic to snails and slugs is because their blood doesn’t use iron to carry oxygen like our blood does, it uses copper instead. Under certain circumstances, the iron replaces the copper in their blood, resulting in suffocation and death.

We use iron-based haemoglobin in our blood to carry oxygen, which makes it red, like rusted iron!

Molluscs, which include octopus, squid, nautilus, horseshoe crabs, snails and slugs, use copper-based haemocyaninin in their blood to carry oxygen, which their blood blue in colour! Incidentally, it’s the same in crustaceans, they have blue blood too.

Which Snail Baits are Toxic and Unsafe to Use?

Most commercial snail and slug baits sold are based on either methiocarb or metaldehyde.

Here’s a summary of these two chemical compounds, and their respective environmental impact.

• Methicarb – (pellets are coloured blue), a carbamate compound used as a  insecticide, molluscicide, acaricide, bird repellent.
  The environmental impact can be summarised as moderately mobile in soil (potential to be carried by water move off the site of application) and highly persistent, highly toxic to bees, fish, birds and mammals.
• Metaldehyde – (pellets are coloured green), an aldehyde compound  used as a molluscicide, and as a solid fuel in small heating systems, such as camping stoves and lamps.
  The environmental impact can be summarised as highly mobile in soil (potential to be carried by water move off the site of application) and highly persistent, moderately toxic to fish and mammals. Note – If cool, wet weather follows after applying this bait, snails and slugs can recover if they ingest a sub-lethal dose.

Both of these snail bait ingredients are considered a toxicity risk to domestic animals, wildlife and children, and are best avoided.

When buying snail pellet baits, look at the ingredients on the label, and if they list either methiocarb or metaldehyde, move on, it’s not worth the risk.

Is It Okay to Use Salt for Snail and Slug Control in The Garden?

Most kids know that pouring common table salt on slugs and snails causes them to melt, resulting in a gooey mess!

Don’t ever use salt to melt snails in the garden, as salt is really bad for the soil, it causes soil salinity, which is harmful to most plants.

As soils salinity increases, the ability of plants to draw water from the soil decreases. When soil salinity levels are high enough, water in pulled out of the roots back into the soil. When this happens, plants no longer are able take in enough water to grow. With very high salt concentration in the soil, plants will wilt, dry out and die, no matter how much water is applied.

If you want to melt snails, here’s a formula that I invented when I was much younger!

How to Make a DIY Slug and Snail Spray

It’s really easy to make a garden spray that dissolves slugs and snails without affecting your plants, and what’s really great is that it costs almost nothing!

This spray simply consists of 1 part ammonia to 4 parts water in a spray bottle, with a dash of liquid soap to make it stick better on the pests.

You will need:

• Plastic spray bottle
• 1 teaspoon (5ml) of pure liquid soap (use pure organic Castile soap, made from olive oil if you want to keep it all natural)
• 100ml of Cloudy Ammonia
• 400ml of water

Here’s the process for brewing up your own DIY Slug and Snail Spray:

1. Use a 500ml spray bottle with measurement marks on the side, this makes the process really easy.
2. Pour 100ml of Cloudy Ammonia into the spray bottle
3. Add 400ml of water
4. Add a dash of liquid soap to make the mixture stick better to pests.
5. Shake bottle lightly, and you’re ready to go small-game hunting for garden molluscs!

This formulation works really well, just one or two squirts on snails and slugs and they dissolve into a foaming green sludge. Slugs dissolve almost instantly because they don’t have a protective shell. Snails might need a second squirt to soak them properly with the spray.

It’s best to set the nozzle of the spray bottle to a narrow jet, as this allows you to target them more precisely, and get more of the spray on target.

Best time to hunt these critters is just when it gets dark, or after it has just rained, which is when they crawl out for a feed. Grab a flashlight and search for them, if you see them, spray them. Do this over several nights and it puts a serious dent in their population.

In case you’re wondering, the ammonia feeds into the soil’s nitrogen cycle, and the liquefied molluscs are returned to the soil as fertiliser.

Spraying snails may not be everyone’s idea of a a good way to spend an evening, most people would be more content to scatter some snail pellets and call it a night!

How to Use Snail Pellet Baits Effectively

If we decide to use snail pellet baits in the garden, it’s important to use them properly for maximum effectiveness. Here are a few handy tips:

• Never lay down snail pellet baits in a pile, that just makes them more likely to be eaten by pets and children! When snail pellet baits are scattered over a wider area, they’re more likely to be eaten by snails and slugs, and therefore work more effectively.
• The best time to apply snail pellet baits is in the late afternoon or evening, when snails and slugs are active and feeding, They can be encouraged to come out and eat by watering the garden before putting the bait down.
• Don’t apply apply snail pellet baits during the hottest and driest, or coldest times of the year, as snails and slugs aren’t active at these times.
• The best places to scatter snail pellet baits are around the spots where snails and slugs hide, along the areas they have to cross to travel from their hiding places to the garden, and in the garden around the plants that need to be protected from them.

Snails and the Ecological Balance of Nature

It‘s easy to think of snails and slugs as horrible pest, because they can seriously mess up a garden overnight, but it’s worth keeping in mind that the indiscriminate use of snail baits can affect all species of snails.

Some species of snails and slugs damage living plants, while others feed on fungi or decaying plant material. There are even predatory species that feed on other snails and slugs!

Predatory snail species include the Rosy wolfsnail, Euglandina rosea, which has a shell around 4-7cm long, and can cruise along the ground at a speed three times faster than other snails, following their slime trails to track them and hunt them down. They eat both slugs and snails, but have a preference for snails!

There’s also a predatory slug, the Leopard Slug, Limax maximus, which can grow up to 16cm long when mature. These giant slugs feed on fungi and decomposing plants, but also eat other slugs and their eggs.

Snails and slugs also serve as a food source for many other species, such as frogs and toads, lizards, birds (especially ducks and geese), mammals (such as hedgehogs, moles and raccoons) and insects (such as ground beetles).

It’s best to respond to snail and slug pest problems with a well thought out control strategy, rather than resorting to toxic chemicals that are extremely harmful to the environment. By using a combination of the various control measures outlined in the article, it’s much easier to strike the right balance.

Gardening tasks are always easier when we work with nature, and exercise a little creative thinking in our problem solving, as this witty little quote from one of the permaculture founders reminds us…

“You don’t have a snail problem, you have a duck deficiency.”

— Bill Mollison

References

• The University of Maine, Home and Garden IPM from Cooperative Extension – Slugs and Snails
• University of Minnesota Extension – Slugs in home gardens
• University of Arizona Cooperative Extension – Snails and Slugs
• University of California Statewide IPM Program, How to Manage Pests, Pests in Gardens and Landscapes – Snails and Slugs
• University of California Statewide IPM Program, How to Manage Pests, Pests in Gardens and Landscapes: Quick Tips- Snails and Slugs
• University of Maryland, College of Agriculture and Natural Resources, Home and Garden Information Center – Slugs and Snails
• Oregon State University, PACIFIC NORTHWEST NURSERY IPM – Snails/Slugs
• Government of Western Australia, Department of Primary Industries and Regional Development, Agriculture and Food Division – Snail and slug control
• University of California Division of Agriculture and Natural Resources Statewide Integrated Pest Management Program, Pests in Gardens and Landscapes – Snails and Slugs
• Texas A&M AgriLife Extension Service- Managing Soil Salinity, by Tony Provin and J.L. Pitt