Most gardeners and allotmenteers at some point in their gardening life will have a problem with slugs!
Well that is probably an understatement as we probably have a problem with slugs most of the time! I know that I do! 2016 was the second to worst year that I have seen (in my short tenure of growing vegetables) for slugs and snails and the worst was 2011 when, 2 years into my gardening journey, everything and I mean everything was decimated by them. Even the plants they apparently don’t like to eat!
Over the last two years I have been trying to be much more wild-life friendly in my gardening and have been turning to more natural gardening techniques (I dislike the word organic). My go-to slug and snail remedy was metaldehyde slug pellets until last year when I found out that bioaccumulation in birds and hedgehogs were having detrimental effects. So I got rid of the pellets!
So what does this mean for the slug and snail population? Am I doomed to be feeding an army of hungry, greedy slimy little gastropods? Well I say ‘No More!’
I am declaring an all out war on slugs and snails! They are banned from the buffet table that is my allotment – on pain of death!
Over the next few months I am going to explore the more natural and biological ways to prevent slug damage and control their population (from henceforth these will be referred to as ‘battle plans’!) and hopefully keep the metaldehyde on the shop shelves and out of the allotment!
As I start drawing up the battle plans, there is one thing a good general knows – her enemy! So I though it best to learn what I could about this slimy menace!
What are slugs?
Slugs are single shelled gastropod molluscs. There shells have evolved to be internal rather than external like snails. There are hundreds of species of slug across several slug families but in the UK we only have 30 (or 44 depending on what article you read) species. Out of these 30 (44) species, 95% of which live underground, only four, yes four, cause nearly all the damage that we have come to hate slugs for. They cost, on average, £8 million in damage to the agricultural industry alone! The other species in the UK have diets that consists of decaying vegetation and fungi or are carnivorous and eat other slugs and even sometimes carrion!
And slugs are everywhere, in vast numbers! The average UK garden can contain approximately 20,000 slugs with an average population density of 200 slugs per cubic metre. Makes London look spacious! By my calculations that is 150,000 slugs across my three allotment plots alone!
It is good to know that I am in fact only protecting my plants from four species and not hundreds and that some of those other species may actually be useful but I wonder how many of those possible 150,000 slugs in my allotment belong to those four species?
So who are these four UK species? We have:
- Nettled or Grey field slug; can be variable in colour but is often creamy or light coffee/light brown slug with darker veins and patches behind the mantle. It can grow up to 4-6cm in length and has an omnivorous diet yet does like to sample the local horticulture! The maximum life cycle is only a year and they die at first frosts laying eggs in the ground to start the next generation the following spring.
- Common garden slug: brown or grey to bluish black in colour and can grow up to 3-4cm in length. The slug’s foot is often orange and is a serious pest of strawberries, lettuces and seedlings.
- Common keeled slug: dark brownish grey with a keel that is slightly lighter in colour. It can grow to 6 cm in length and it is usually found where ever horticulture or agriculture takes place. It is a particular predator of potatoes and root crops. They are quite hard to control as they can live underground.
- Large red or black slug: Orange-red or black in colour, although they are mainly found to be black in colour with an orange fringe and can reach up to 12cm in length. It is the main culprit behind vanishing rows of seedlings in the spring. This slug sometimes has the ability to self-fertilise its own eggs!
I have memorised the pictures so I know what to look out for (although I recognise all of these as being present on my allotment) but now that I have identified my enemy and have an estimate of their numbers, I need to know more about their strengths and weakness (anatomy and physiology) if I am to crush them in battle.
Slug bodies are made up of the head, which contains two sets of tentacles (I hate the word tentacles!). One set, the longer top set are for light sensing and smell and if the slug was to lose these, they have the ability to regrow them. The smaller, bottom set of tentacles are for feeling and tasting. These tentacles are retractable. Behind the head is the mantle which contains the opening from which a slug breathes and contains a single lung. It also contains the male and female reproductive organs. Oh yes! Slugs have both! More on slug mating later…
The rest of the slug is called the tail and the underneath of the slug is called the foot. The foot is a large muscle which enables the slug to move by large rhythmic contractions. At the same time the foot produces mucus which enables to slug to pretty much climb/slide over anything.
Slug mucus and slime trails!
The mucus that a slug produces is hygroscopic, meaning that it will absorb water. This mucus will cover the slug’s entire body and help it to prevent desiccation. The slug must produce mucus in order to survive! A slug’s body is mostly made up of water and it has to protect itself from losing that water. Therefore, slugs mainly feed at night when it is damper and cooler or are generally more active when it is raining and it is why we also find them in damp, dark spots underneath various pots and objects in the garden. The tell-tale slime trail that slugs often leave behind have two purposes; it acts as a navigation tool for them helping them to find their way back to those lovely damp spots they like so much and it helps them to find a mate. Carnivorous slugs also use these slime trails to find their prey. Good on them! The slime is generally used as a repellent to predators too as it is often unpalatable. This is why you may sometimes see a bird wiping a slug on the grass before eating it!
As I said before, slugs have both female and male reproductive organs so that they can pretty much mate with any snail of their species that they come across (some can self-fertilise in the absence of a mate). Once slugs have mated they will produced, depending on the species, anywhere between 20-100 eggs each time and this can happen several times a year! The eggs are laid in a hole in the ground or in damp dark places. Reproduction mainly occurs in the Spring and Autumn. The eggs lie dormant until conditions are right or them to hatch.
Slugs have many natural predators too which is good for me! Virtually every major vertebrate group contain a predator of slugs. Frogs and toads are known for the slug eating habits and are welcomed into most gardens and allotments. Some lizards and snakes will also eat slugs. Birds are a major predator including blackbirds, rooks, starlings, owls and ducks. Foxes, badgers and hedgehogs are also a fan of having a plate of slugs for tea!
So now I have identified my enemy, I have a rough estimate of numbers and I know their strengths and weaknesses. All knowledge I can use to my advantage!
However, we do have a wild card! The Spanish Slug!
It is a brute of a slug which has invaded our dear shores. There are quite invasive and are known to dominate and area and push other slugs out. They are harder to control for as they can survive in hotter and dryer environments. They have extensive omnivorous diets which do include eating your precious plants and they can lay up to twice the number of eggs as UK species also having the ability to self-fertilise! The slime they produce is extremely unpalatable so they have even less/virtually no natural predators in the UK.
Be especially on the look-out for Spanish slugs and when you find them maybe a good salt bath is in order!
There are a number of chemical and biological control methods and I was given a little book ’50 ways to kill a slug’ for Christmas a few years ago. It is a book I intend to put to good use.
Some of the methods of control include:
- Metaldehyde pellets – lethal and can bioaccumulate in predators which is not good for the environment
- Iron phosphate pellets – less toxic to slug predators but just as effective at killing slugs.
- Parasitic nematodes – they enter slugs and infect them with a bacteria which kills them. Very effective especially on large scale.
- Beer traps – invites them to a drunken doom! Effective in small places like a garden
- Crushed eggshell/wool pellets/diatomaceous earth/ copper bands – are supposed to create barriers that the slug’s delicate bodies can’t cross.
- Salt – will draw the moisture out of a slug’s body via osmosis. You will need to catch them first though!
So with this all in mind – let the battles commence! I will be starting to put my battle plans in action and I’ll let you know how it goes!