For years, slugs and snails have been the arch enemy of gardeners, and this rainy and difficult growing year has been particularly ‘bad’ from a gardener’s point of view. But in his new book, The Good Slug Guide, Jo Kirby sets out to dispel some of the myths around slug control - and why much of the advice around them is outdated and misguided.
A retired academic and lifelong gardener, Jo is passionate about the environment. He has been on a 30-year quest to understand the ecological processes at work in gardens, and how they might be adapted and used to help create beautiful places in which pests could never become a major issue.
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In The Good Slug Guide, you will not find the usual methods of control such as flinging slugs over the garden fence, smearing your pots with salted Vaseline or snipping a slug in half. Instead, he gives advice on adding more plants and more organic matter, pruning and digging a little less and working towards accommodating the many natural enemies of the most problematic slugs in your garden.
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Here, he outlines 12 myths about slugs that are actually hindering the way in which we deal with them.
12 myths about slugs that you need to stop believing
Myth: The average garden has 200 slugs
Reality: Most gardeners will realise that this figure is far too low, but it was cited erroneously in a popular guide some 20 years ago and is still being repeated in the media today. The true figure is up to about 200 per square metre, which means that the average garden in the UK of 174 square metres might contain – and please prepare yourself, this is going to come as a shock - as many as 35,000 slugs.
Myth: All slugs are equally bad for the garden
Reality: In the average garden there are actually only five big five mollusc pests: the common garden snail (Cornu aspersum), the grey field slug (Deroceras reticulatum), the Budapest keeled slug (Tandonia budapestensis), the blue-black soil slug (Arion hortensis) and the Spanish slug (Arion vulgaris). It is worth researching these, so that you know your enemies.
Many garden slugs are harmless and actually provide ecosystem services, processing much decaying material. The two cellar slugs, green and yellow, eat detritus and nothing else, as does the hedgehog slug. The large and alarming-looking leopard slug – big with blackish stripes and spots on a yellowish background – not only does not eat green plant material but behaves aggressively towards other slugs, driving them away, and may even predate some. All of these species are distinctive, easily identified, and firm friends of the gardener.
Myth: Slugs all look similar and are hard to identify
Reality: The more common slugs in gardens are easy to sort out, although it is true to say that young slugs and some of the less common ones are tricky. With a little practice, it should be possible to learn the half a dozen or so most frequently met, and the more difficult ones can easily be sorted into their groups if not exactly to species. For example, the round-backed slugs (the genus Arion) all have their breathing pore in the front half of the mantle – the smooth region just behind the head – but in all the other British slugs the breathing pore is in the back half. When taking a photo for identification purposes, be sure to include the right hand side of the slug because that is where the breathing pore will be found, and try to take the shot when the breathing pore is open, because the colour of the pore rim can be important too.
Myth: Slugs love the rain
Reality: Slugs actually hate the rain and will take shelter from it. They are active when it is damp, after rain.
Myth: Slugs are vegetarians
Reality: The data are now in and are conclusive: there is a shortage of protein in the diet of slugs and they will go out of their way to satisfy that need. Many scavenge on dead animals and faeces, and some even hunt live food such as earthworms. One or two are even thought to be predatory on other slugs and snails. Many slugs, including the pest species, also eat a lot of detritus because decaying leaves can have as high a protein content as living leaves, due to the protein held within the fungi that decompose them. Some have taken it one step further to feed solely on fungi and lichens. As a result of these findings, scientists no longer describe slugs and snails as 'herbivores' but as 'generalist herbivore-detritivores'.
Myth: Remove all damaged, dying and dead leaves and shoots to avoid attracting slugs
Reality: Probably the most commonly given piece of advice in conventional slug (and snail) control, the idea here is that by removing dead plant material, we are denying them food. Unfortunately, what we now know about the dietary requirements of slugs, and what they really eat, means that tidying up is almost certainly counterproductive in the long run. It will work as part of a conventional control programme, if you are prepared to put in long and regular hours, but otherwise it doesn't. The usual advice to remove damaged, dying and dead vegetation – often their favourite food – will leave slugs with no choice other than to eat our precious green plants.
Myth: Deny slugs and snails shelter
Reality: Another old idea designed to make the garden a hostile place for slugs, this also discourages the natural predators of slugs and snails such as ground and rove beetles and leaves the gardener with no option other than to clear up his or her mollusc problem alone without any help from their natural enemies in the garden. Adding shelter in the form of pieces of wooden board placed on the soil surface gives your enemy somewhere to roost, and you can keep a close eye on numbers.
Myth: Encourage hedgehogs, frogs and toads in the garden to combat slugs
Reality: This advice is found everywhere in the media, but the recent research suggests that hedgehogs and toads are not the gardener's best friend after all. Not only do they not eat that many slugs – toads might eat just one slug a week, for example - but they also eat a lot of ground beetles. The new research indicates that these beetles would have eaten more slugs than the hedgehogs, had they not become hedgehog snacks. This is not to say that you should not have hedgehogs in your garden, but that you need to provide your smaller friends with more nooks and crannies where they can avoid predation.
Myth: Use barriers and copper against slugs
Reality: Some gardeners swear by using physical barriers such as cinders, grit, crushed eggshells, coffee grounds, wool, pine bark and a combination of salt and Vaseline against slugs. The Royal Horticultural Society (RHS) has conducted trials and is adamant that they do not work. Copper tape or bands are said to act by passing a small electric current through the slug as it touches the metal, but the RHS is equally adamant that copper does not work either.
Myth: Organic slug pellets are a good bet
Reality: Iron phosphate slug pellets (which replaced those made from metaldehyde, now banned) were mostly trialled on the grey field slug, as it is the worst slug pest in agriculture. Gardeners have more species to contend with, such as the Spanish slug, which is less susceptible to iron phosphate pellets. One Spanish slug can polish off up to 20 pellets before it stops eating, which means that about 200 pellets per sq m (10 sq ft) of ground must be applied to achieve control, well above typical recommended application rates. In applying these pellets we might remove other slugs and hand the Spanish slug a competitive advantage.
Myth: Slugs have few natural enemies in the garden
Reality: Recent research tells us that something like 75 natural enemies of slugs can live in our gardens, and it is up to us to provide a home for them so that they can help us out. You have many more friends than you might think, so give them a home. Plant more densely, and allow a leaf litter layer to develop either naturally or through the addition of compost, top-dressings and mulches.
Easily the biggest group within the natural enemies of molluscs are the 39 beetles commonly found in gardens – and of those, 32 are ground beetles. The larger ones such as the black clock and the violet ground beetle are especially good, so it's well worth providing them with refuges and hibernation sites. Slow worms are also well worth having - slugs make up about 30 per cent of their diet.
Garden birds including the song thrush, blackbirds, starling and blackcap rarely eat slugs but will eat snails - encourage them into your garden by creating providing food plants and shelter for the insects that they feed on. Also provide places to roost and nest, such as mature shrubs and hedges.
The wood mouse is a predator of the common garden snail and shrews eat many slugs. Even the despised brown rat may help. Harvestmen, flies, worms and even spiders do their bit. They might only play a small role, but together their activities add up to a significant cumulative effect.
Myth: Slugs are primitive
Reality: Far from it; slugs have evolved from snails. In fact, so useful is the slug form – it confers the ability to squeeze into tiny spaces, and little need for the calcium that is such a major component of snail shells - that the slug form has evolved independently several times among different groups of snails that are not at all closely related. So, to lump all slugs together is rather like saying that the hyena and the walrus are the same; it really doesn't help us to understand what is going on. Different slugs can have very different needs depending on their evolutionary history, and that includes the things they eat.
Gastropods might look simple – a `stomach-foot` and little else – but biochemically they are very sophisticated indeed. Their digestive systems are capable of processing many materials that others creatures cannot. They can, for example, break down chitin, the notoriously tough structural component of both fungi and insects.
Slug slime is particularly complex. The discovery of hyaluronic acid in their slime came as a surprise to many; it plays important roles in the human body, where it is for example a component of synovial fluid, the low-friction liquid that helps your joints move easily. Hyaluronic acid is now a miracle ingredient in popular cosmetic creams, but its use in medicine and cosmetics is not new; sluggy folk remedies for skin conditions go all the way back to Hippocrates in classical Greece. Mollusc mucus also contains novel antiviral and antibacterial compounds that are being assessed for their potential usefulness, with positive results so far.