Slug Wars: Episode 2 – Attack of the Nematodes!

I have been formulating my battle plans against the slug army and as a first wave of defense I will be deploying my infantry into battle – the nematodes!

So what are the nematodes?

The nematodes that I will be using are phasmarhabditis hermaphrodita, the nematode that you can buy as part of the ‘Nemasys’ range.

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Microscopic image of nematodes.

It is described as a facultative parasitic nematode. This basically means that the nematode itself does not absolutely rely on a host to complete it’s life cycle. It can also live on rotting vegetation and dead invertebrates which coincidentally is probably where you will find a lot of slugs! It’s life cycle is quite simple; it finds a slug host to infect and once inside, it release bacteria which kills the slug. The nematodes then feed off the decomposing corpse whilst reproducing and producing the next generation to go off in search of more slugs.

The nematodes are already present in my soil but their numbers are generally insufficient to control the estimated slug population in my allotment. Adding more nematodes to the soil bolsters their numbers and helps to reduce the slug population. The nematodes will die back to their natural levels again when there are less slugs to feed off.

The benefits of using these minuscule worms is that they are biological control, so no nasty chemicals are used which could adversely affect other organisms and they are slug specific. This means that slugs (and snails) are the only organisms that will die! They also don’t accumulate up the food chain and have nasty effects on slug predators such as frogs and hedgehogs!

So my first batch of Nemasys slug killer has arrived and it has been sat in the fridge for the last couple of weeks (you should store it in the fridge upon receipt). This weekend , I went down to the allotment, mixed up the nematodes into the correct amount of water and applied this to the beds I think are most likely going to see slug damage.

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Making up a stock solution!

The first bed was around the pond. I have lost many a plant there due to slugs and my hosta is having a hard time sprouting leaves as the slugs eat them before they can fully open! I also have two more hostas I want to plant out but am unwilling until I see a reduction in the number of slugs!

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Hosta damage!
The second bed I targeted is the main strawberry bed on plot 2. With flowers starting to show on my strawberries it won’t be long before the fruit start developing and if I act now that should give the nematodes time to do their job! I also have watered the nematodes into the Jerusalem artichoke bed as I lost all but one of the shoots that grew last year to slugs!

I have another packet of nematodes arriving from my supplier slightly later on in the season. This packet will mainly be used on the potato beds and if I have enough left, the brassica beds. Slug damage is a big issue for my potatoes as last year I threw away a third of my maincrops away due to these slimy pests! Hopefully these nematodes will reduce the amount of damage I see this year.

So I have released my foot soldiers out to do battle with the gastropodic enemy! Only time will tell who the victor will be!

nematode burns

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SLUG WARS: Episode 1 -The Slimy Menace

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!

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Slug wars

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:

  1. 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.
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Grey field slug Image credit: T.E.R.R.A.I.N

 

 

  1. 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.
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Common garden slug Image credit:Slug watch
  1. 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.
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Common keeled slug Image Credit: Slug watch
  1. 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!
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Large black slug – Wikipedia
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Large Red Slug – Wikipedia

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 anatomy

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…

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Slug anatomy – Wikipedia

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!

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Tell-tale slug slime (and a load of dead slugs after using metaldehyde pellets!)

Slug mating

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.

Predators

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!

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.

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Spanish slug Image Credit: Slug watch

Be especially on the look-out for Spanish slugs and when you find them maybe a good salt bath is in order!

Control methods

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:

Chemical

  • 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.

Biological:

  • 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!

References:

https://en.wikipedia.org/wiki/Slug

http://www.slugwatch.co.uk/?page_id=13

http://www.slugoff.co.uk/slug-facts/facts

https://www.rhs.org.uk/advice/profile?pid=228