Using scientific evidence to control the most successful plant destroyers
If you have ever gardened, you have met them. Not only have you encountered their slimy tentacled bodies, but you have very likely also cursed them. Chances are you have secretly thrown them in your neighbour’s gardens, or in the hedge. If your living, like theirs, depends on your garden plants, you have probably spent some of your chilly spring nights religiously picking them before they did any damage. The least lucky among them have faced the brutality of your rage and ended up in a bucket, under your sole, or poisoned. Perhaps, however, you proudly feed them to ducks and hedgehogs, thus relieving some pain from your dirty conscience.
As strange as it may sound, when it comes to food, slugs are amongst our worst competitors. According to the Royal Horticultural Society, they…
…are persistent and widespread pests which can cause havoc in the garden, eating holes in leaves, stems, flowers, tubers and bulbs. They can cause damage throughout the year on a wide range of plants¹.
To me, both as a vegetable grower and as a scientist, the most amusing aspect of our centuries-long interaction with slugs is our apparent inability to deal with them. According to the vast majority of gardeners, farmers and scientists around the globe, there isn’t an economic or effective, let alone sustainable or regenerative way to control slugs. But what makes them so devastating and uncontrollable? What are our best chances to get along with them without competing for the same food? In this article, I will try to answer these and more questions, by addressing:
A few fundamental facts about slugs;
All the control methods scientists have devised and compared (including garlic, beer, chemical pellets, copper tape, etc.);
A reflection on how to avoid slugs without generating more work for yourself or more trouble to your ecosystems.
If you have read any of my previous posts, you won’t be surprised to discover that this is a non-technical, yet fully-referenced article.
1. A short primer in sluggology
Perhaps you feel like you could do with a short primer in slug life habits and resilient strategies before delving into ways of controlling them. If that is the case, you will find all you need to know about slugs in the first article of this series.
Otherwise, if you know more than you dare to let others know about slugs, let’s dive into the science of dealing with them!
There are three aspects worth noting that are fundamental to get along with slugs:
they need moisture, both in the soil and in the air, to live. They only come out of their mucus-lined holes in the ground if it is either dark, raining or really damp. They and their eggs can hide away during dry spells and survive for weeks until moist conditions return. Therefore, involuntarily providing them hiding places such as stones, wooden fences, hedges, bushy vegetation might not play in your favour;
they are voracious but picky eaters. Usually, they prefer to eat dead plant matter, thus helping us in recycling fertility into the soil. However, if numbers get large, they come out and attack living plants. They love specific species and really do not dig others; however, within their menu, they will change preferences in accordance with what they haven’t eaten in a while;
Slugs are attracted to beer and other fermenting solutions;
unlike snails, they don’t have a “homing instinct”, so they won’t be able to go straight back to their feast if you throw them far enough.
2. You shall not pass! The science of Slug control
Slugs are major pests of many agricultural crops, including oilseed crops and cereals¹ and horticultural crops including vegetables and ornamentals². They can dramatically affect yields by damaging seeds and killing seedlings, eating stems and growing points and sensibly reduce the leaf area³. Sometimes a seeded crop is so severely attacked that entire fields need to be resown⁴. They also make harvested plants messy by mixing in their bodies, eggs, faeces or slime, which can lead to deterioration in the quality of products and eventually in financial loss⁵.
The history of slug control is as old as agriculture. Scientists started to approach the challenge with field experiments as early as 1890s⁶. However, gardeners and farmers have always been reluctant to update their control practices. In the last few decades, several products have appeared on the market, some of which respond to the need for sustainable alternatives to toxic chemicals. At the same time, new creative solutions have been devised and studied (such as the use of ash or beer).
Control methods come under four headings:
Chemical measures make use of synthetic substances that can be applied either to plants or to the soil and kill or deter slugs;
Cultural methods are cultivation or prevention practices that deter or completely remove the presence of slugs from a garden or farm;
Physical approaches consist of using barriers that prevent slugs from getting in contact with plants;
Biological solutions rely on the introduction of other organisms (usually insects or microbes) that can either kill or modify the behaviour of the feared pest.
Metaldehyde and carbamate pellets: blue death
Traditionally, chemical control of slugs relied on the use of chemical bait pellets which contained metaldehyde or carbamates (mainly methiocarb). The molluscs run into this (usually blue) pellets and either eat them (food uptake) or touch them (dermal contact). The chemicals then act either as stomach or contact poisons⁷; they paralyse them, make them shrivel and die.
However, killing slugs by contact poisons is not as easy as with many other pests, because of their mucus which prevents chemicals from coming in direct contact with the skin.
Moreover, there are application and environmental problems with both these types of molluscicide. Methiocarb is toxic to beneficial invertebrates such as earthworms and carabid beetles⁸ and both methiocarb and metaldehyde be toxic to several non-target animals such as birds, frogs, hedgehogs and other mammals⁹. In response to the concerns raised by many, not least by wildlife conservation organisations, the European Union has banned first methiocarb (in 201⁴¹⁰), and more recently metaldehyde (in 202⁰¹¹). Some farmer associations (in the UK, in particular) have protested against the ban as they feel concerned for the impact this might have on their crops, in the absence of any cheap alternative to synthetic chemicals to control slugs¹².
Iron phosphate pellets
An alternative slug pellet solution containing iron phosphate was registered in 2001 in Europe¹³. The way iron phosphate works is quite cruel: when eaten, it causes the liver and pancreas of the slug to degenerate, thus leading to feeding inhibition and death¹⁴. Because it is slower than metaldehyde, slugs eventually dry out and decompose while they are underground. Iron phosphate’s effectiveness has been demonstrated under laboratory, glasshouse and field conditions¹⁴. Interestingly, the same chemical compound occurs naturally in nature, and it is commonly used as a human nutritional supplement and as a fertilizer. Bacteria can decompose it and cycle it back into the soil ecosystem, where it can become a plant nutrient. One is, therefore, inclined to trust the optimistic claims of environmental agencies and commercial distributors, that it has little toxicity towards mammals¹⁵.
However, earthworm¹⁶ and dog¹⁷ poisonings have been reported in the scientific literature, and iron toxicity is not rare in humans either¹⁸. Moreover, although the studies cited above do show that metaldehyde and iron phosphate pellets had a very similar impact on slug populations and leaf damage in field trials, many gardeners have not been impressed by iron phosphate’s ability to control slugs¹⁹. This might have to do with the species that are more susceptible to liver and pancreas disease, which may not coincide with the ones tested in the trials. It might also be wise not to liberally add phosphorous and iron to your garden beds, as this can affect the chemistry and microbiology of your soil; this would imply that iron phosphate applications should be limited.
An additional disadvantage of chemical approaches is that their effectiveness can be altered by the size of the slugs²⁰ or their interaction with other organisms living in the same habitat. Earthworms, as already mentioned, can consume pellets, thus harming themselves and reducing the impact on target molluscs. Moreover, earthworms can trigger chemical defence alerts in plants, which consequently respond by making themselves less attractive for slugs²¹. Another fact to take into consideration is soil moisture. In a recent study, metaldehyde was more effective under less frequent watering while iron phosphate was unaffected by watering. Interestingly, “earthworm activity was reduced with less frequent watering but did not interact with slug control”²².
As many container gardeners have been keen to believe, copper barriers create an electrical current when they react to snail or slug secretions²⁴. Some slug warriors have been seen arching their flexible bodies to overcome the metallic obstacles and reach their meals. Unfortunately, no scientific studies have been carried out on other energy-expensive methods that take advantage of the electrically conductive slime slugs are surrounded by, such as 12V battery-powered circuits.
Wool pellets have not been studied in the peer-reviewed scientific literature, although there is consensus among gardeners about their scarce effectiveness.
The advantage of physical barriers is that they don’t kill, but just repel slugs. Some gardeners and farmers don’t see this as a real solution, especially on large scales, because the slugs will be eating something else which is not protected by the barrier, rather than die.
As I discussed in my previous piece on slugs’ life habits, slugs are really fond of beer. This peculiar weakness is used to trap and drown them²⁵ and has been studied over the last decade. Although scientists are still trying to figure out which beer slugs prefer, what we know is that anything fermented or fermenting is hugely attractive to slugs, be eat a solution of yeast and sugar or a pint of cheap beer. As a consequence of this, beer traps have risen to the attention of many horticulture lovers, who have taken to their gardens in the hope of recycling some cheap old beer in exchange for wondrous slug control effects. A comparative study of how effective these traps are haven’t been published yet, but my experience teaches me that a solution of water, yeast, sugar and flour can drown hordes of slugs in a single night. They are attracted by the smell of the fermenting solution, they fall in the trap, and they are unable to escape it. Then you are left with the mess.
Powdery barriers: ash, lime, sawdust
Very dry and alkaline barriers, such as continuous lines of sawdust or ash, provide a dry surface which slugs avoid²⁴. Other natural repellents include wood ash, hydrated lime, diatomaceous earth, sawdust, wood ash. Some research shows that hydrated lime has the best contact efficacy on slugs both individually and in combination with other substances:
Slugs were observed producing large amounts of mucus when the animals came into contact with the hydrated lime. Satisfactory contact efficacy was also achieved by applying wood ash. The effect of wood ash is similar to that of hydrated lime, as both substances cause dehydration of cuticle and blocking of airways²⁴.
Salt is obviously an effective way to desiccate slugs (rather than repelling them), but it can unbalance soil life and dehydrate plants as well⁴⁸. Similarly, bran has great desiccating properties but can attract pests such as ants.
Unfortunately, despite many claims that they deter slugs, the effectiveness of coffee grounds has not been addressed by any scientific test.
However, the effectiveness of these barriers obviously decreases once they become wet.
Essential oils: spearmint and thyme
An alternative option that has been tested in indoor settings is the use of essential oils. When slugs are added to containers planted with annual plants and the containers are sprayed with essential oil emulsions, 97.5% mortality of slugs is observed, a result comparable to metaldehyde. In particular, thyme and spearmint oil emulsions were shown to be lethal. Fortunately, the oils do not affect the plants at all, just the slugs!²⁶
Garlic and chemical repellents
One of the most effective repellents for slugs has proven to be Garlic, which is used by many greenhouse growers. In a comparative study of copper, aluminium, mulch, chemical repellents and garlic, the results showed that:
Garlic, ureaformaldehyde and cinnamamide (two chemical repellents) have the highest reduction of damage, barrier efficacy and mortality rates in experiments carried out in the laboratory.²⁷
Cinnamamide is used as a seed coating for its antifeedant effect²⁸. Ureaformaldehyde pellets are used as a slow-release nitrogen fertiliser and a pest repellent; unfortunately, it is toxic²⁹. Garlic, amongst the three, is undoubtedly the most promising control measure.
Biological alternatives: larvae, beetles or nematodes?
Biological control methods involve introducing a predator or another organism that can displace or kill the unwanted pest. In the early days of biological slug control, ducks and chickens were used in the morning to sweep the gardens clean of slugs¹³. Unfortunately, they were often responsible for more damage than the pests they were supposed to control!
Encouraging frogs into gardens is also a common suggestion because they feed on slugs⁴⁹; however, they don’t feed exclusively on the mighty pests, and usually can’t keep control large populations in control.
In the 1980s, Reidenbach et al.³⁰ explored the possibilities of using Sciomyzidae (a fly larva) as biological control agents for crop pest molluscs with some success. Similarly, a 1989 study reported that carabid beetles reduced slug population by up to 80%³¹. Apparently, rates of 6 beetles and 30 slugs per square meter showed significant results. Unfortunately, if alternative prey species (such as fly larvae) are present, the beetles leave slugs undisturbed³².
However, the rising stars of the biological control jet-set are…
A recent addition to the slug hater’s toolkit is Phasmarhabditis hermaphrodita, a nematode that attacks several slug and snail species²⁰ and is available as a biological control agent called Nemaslug in the UK³³. Nematodes are very small worms, from 0.1 to 2.5mm long, which are present in nearly every ecosystem: “from marine to freshwater, soils, from the polar regions to the tropics, as well as the highest to the lowest of elevations.”³⁴ Nematodes are important members of the soil food web, although different species can have very different roles³⁵. Some of them are pests, while others are really useful in controlling a large variety of other pests.
Slug controlling nematodes are applied as a spray and as they land on plants or soil, they start to search for slugs, using mucus and faeces as a trail. When they finally enter through the shell cavity at the back of the gastropod³⁵, they kill it in 4 to 21 days. The nematodes feed on bacteria present on the decaying slug and eventually produce new infective juveniles³⁶.
Interestingly, slugs are able to detect and avoid areas where P. hermaphrodita has been applied³⁷, whereas some snails are resistant and able to encapsulate and kill nematodes using their shell³⁸.
Even more curiously, scientists have discovered that already-infected slugs stopped avoiding exposure to areas with a high density of nematodes³⁶. Imagine being a slug who diligently sniffs around trying to avoid lethal contacts with nematodes. Eventually, one of those little worms manages to enter your body, and your immune system starts to fight the impostor; but suddenly you start moving towards areas with concentrations of nematodes, until you are infested with parasites. Researchers have found that the nematode manipulates parasite avoidance behaviour in host slugs by interfering with its serotonin levels. Essentially, the slug gets less anxious about protecting itself, and that is fatal.
Using this strategy, nematodes can really decimate slugs in a few weeks, and a lot of studies have focussed on trialling their performance on different plants:
P. hermaphrodita has been used successfully to protect a range of crops against slug damage, including oilseed rape, winter wheat, strawberries, asparagus and Brussels sprouts and the use in floriculture has been demonstrated in orchids and hostas³⁶.
Unfortunately, despite their promising results, nematodes fail to appeal gardeners and farmers around the world. They are much more expensive than chemical pellets, especially because they die in the absence of moisture, so if you apply them right before a dry spell, you have wasted your time and money and need to re-spray. Novel approaches are being devised to reduce the number of nematodes needed, and therefore the cost of controlling slugs by their application, but they have obtained only partial results³⁶.
Cultural solutions: my personal answer
Despite not many scientific studies have been done on the subject, whenever someone asks me how I manage to keep slugs away from my vegetables, I answer: with tidiness. It will not come as a surprise to hear that the best way to deal with a problem is to avoid its occurrence.
Several measures can help keep slugs away, either from the garden, or from the need and temptation to devour your precious food or flowers.
First of all, you can avoid creating a convenient shelter for your garden’s resident slugs⁴⁰. You should avoid placing your beds near hedges or fences, and be really vigilant if you use raised beds with wooden sides, as slugs tend to hide between soil and wood. Wood chips in the paths can help, but are not as effective as wood ash, as we saw in the previous section. Keeping the grass relatively short also removes hiding places. It is wise to avoid leaving objects and large stones on the soil, where slugs could comfortably rest in the day while they plan their next attack⁴¹.
We know that slugs eat mostly dead organic matter, and love hiding under stuff: put the two together and you have a potential disaster. Hay mulch and dead decaying leaves from brassicas, closely spaced border plants or weeds are exactly that: slug heaven. If you live in a wet climate, avoid mulching with decaying plant material and keep your plants and garden tidy: things will improve massively, in my experience.
Use shelter to your advantage
This tendency to hide from dry air and sunlight can be used as a strategy to trap them as well: at night, you can leave a plastic sheet on the soil and turn it over in the morning, to monitor and reduce your slug population. Repeating this for a few weeks will very likely have a big impact on large slugs population.
Hand-picking: nights out in the garden
Hand-picking⁴², although not strictly a prevention measure nor a cultural control method, is also very effective. In spring, as days become longer and nights warmer, most slugs around are youngsters. If you get in the garden with a torch and a good amount of determination, you can pick a lot of mini-gastropods, thus stopping the population rise. Doing so for two weeks does wonders, and is a great way to enjoy the garden at night, with its sounds and visitors.
Monitor plant health
Healthy plants have the best chances of surviving the attack of a slug. First, because their turgor (inner pressure, that gives them toughness or crunchiness) makes them harder to nibble at; secondly, because they have plenty of energy to recover from the bite of a passing slug. Research shows that boosted silicon (Si) and calcium (Ca) levels in wheat seedlings can reduce slug grazing⁴³, and many experienced growers will confirm that weak plants will not withstand much slug disturbance⁴⁴.
Plant seasonal crops and look after young seedlings
Young seedlings are very susceptible to slug damage and don’t have many resources to recover after an attack. They also have tender tissue, which slugs love. If your plants have been stressed during transplanting, either because of root disturbance or because of the temperature shock, their days in slug-land are counted. Planting seasonal cultivars, that are suited to your outdoor conditions will help avoid problems⁴⁵.
Choose immune plants
It is useful to know which plants are more prone to becoming slug sweets.
On the slugs menu, the most appreciated foods are ground elder (Aegopodium podagraria), bean leaves (Phaseolus spp.), potato tubers, strawberry fruits (Fragaria spp.), beets (Beta spp.), rapeseed (Brassica napus.), cabbages and other brassicaceae including kale, Brussel sprouts, pak choi and other oriental greens as well as many common horticultural weeds⁴⁶.
There are, however, some plants slugs systematically avoid, such as those containing tannins and phenolic compounds: onions and garlic (Allium spp.), beech trees (Fagus spp.), most berry plants (Rubus spp.) and aromatic herbs (Origanum, Rosmarinus, Lavandula, etc.), grasses (Poa spp.), tomato and potato leaves (Solanum spp.), Ranunculus, Geranium, Potentilla and Verbascum⁴⁷.
Slugs are fascinating creatures and play an important role in the soil ecosystem. We can learn how to live with them without necessarily having to compete for the same food. Scientific trials have given and continue to give us interesting data that we can use in our gardens and on our farms. However, ecosystems are complex, so are our brains and the slug’s physiology. We are on a learning journey, and as the mighty slug teaches, even slow journeys can turn out to be full of bounty.
The scientific articles and all the other references corresponding to the numbered footnotes can be opened by clicking on the link below. They are in a google doc format. I suggest that you open them in a separate tab (by pressing CTRL+left-click) so that you might refer back to them while you read the article, without the need to scroll back and forth.