Slugs and home made slug killing soup with nematodes
I was made aware of a recent press article in the Daily Telegraph about a home made slug killing soup!
The article recommended making a homemade slug killing ‘soup’ by collecting 10-20 slugs and putting them into a bucket of water, add leaves and seal them in the bucket with a concrete slab on top of the bucket.”With the slugs safely inside, place a concrete slab (or any firm cover) over the top to seal them in. The bucket is the perfect environment for the nematodes and bacteria to breed. Nematodes spread in water, so check regularly, giving the slugs a stir with a stick. The idea isn’t to drown them but to keep them moist so the nematodes can hunt them out” This, it states, now makes the ‘perfect environment’ for any slug killing nematodes and bacteria to breed and pour the soup around your vulnerable plants after two weeks and like the version I am about to describe, this ‘soup’ will give up to 6 weeks of slug protection.
Many years ago I heard of a similar home made slug killing ‘soup’, made by drowning slugs in a bucket of water and pouring the resulting ‘soup’ onto your plants. The idea was that the slugs would taste/smell their dead ‘relatives’ and this would deter them from eating your plants! For me and many others, it did n’t work, a classic case of pseudoscience! Live healthy slugs will even eat other slugs that are still alive after being poisoned by slug pellets containing metaldehyde, as will predatory beetles. This new recipe is a little more advanced but just as worthy of the title ‘pseudoscience’.
The article refers to nematode worms that live inside their host for part of their life cycle before killing its host, (the slug), feeding, breeding and then dispersing into the soil to find a suitable and healthy slug host. One such species of nematode worm (Phasmarhabdatis hermaphrodita) is a bacterial feeding nematode and is indigenous to Europe including the UK. It is sold to the horticultural trade and gardeners as a biocontrol of slugs, using the trade name Nemaslug. They do not attack or kill animals, birds, earthworms, etc., or plants and live naturally in our soils in low numbers. Robbie et al (2005) used the same nematode in experiments and found that it had no effect upon 5 species of earthworm (Eisenia fetida, E. andrei, E. hortensis, all used for worm composting!, Dendrodrilus rubidus and our garden common earthworm Lumbricus terrestris). Unfortunately nor did affect the earthworm predator the imported New Zealand flatworm Arthurdendyus triangulatus
Like life and nature, of course it is more complicated than simply pouring nematode worms onto the soil to kill slugs. By themselves alone these nematode worms will not kill slugs. To be effective against slugs researchers found that they HAVE to be at a certain stage in their life cycle ( dauer larvae) AND infected with its associated bacteria (Moraxella osloensis). Neither of these partners in slug crime, live in sufficient numbers to make a significant difference to slugs in our gardens. Thus they are both mass cultured. In fact the associated bacteria was one of many that was tested to see if the combination of these particular nematodes and this particular bacteria were successful together in effective slug control and both were capable of commercial reproductivity. So why are n’t the nematodes effective against slugs by themselves you may ask? The bacteria have to be inside the nematode. The nematode has to be at the right stage in its life cycle and has to be inside a slug! Then the worms release the bacteria, which breed and release a toxin, which kills the slug. This toxin is particularly effective against the grey field slug, whilst results with other slug species, larger slugs, adults of certain slug species and snail species are more variable.
A tall order ?
Lets compare the above soup with the real product. Mass producing nematodes needs specialist equipment, treatment and aftercare in sterile and bacteria free conditions. They are produced in specially designed air lift fermentors, which minimises mechanical damage/injury to living organisms, in huge 1000-20,00 litre capacity vats on a three dimensional matrix of foam chips impregnated with the required foods and nutrients and later inoculated with the symbiotic bacteria, (Moraxella osloensis). The two organisms, have to be kept at ideal temperatures and conditions throughout the whole process. In particular the nematode worms have to be ‘nursed’ in order for them to undergo two-three generations before they form the infected juvenile form, the only form that has proved lethal to slugs. So whilst this process is going on somebody has to check which stage the nematodes are at in their life cycle. If the conditions etc., are not to the likings of the nematodes their numbers will be affected. Once at the required stage in their life, the nematodes are harvested by centrifugation of the medium, mixed with an inert carrier such as powdered clay or vermiculite. Then the water content of the medium has to be adjusted so that the nematodes become partly dehydrated and immobile. This conserves their energy, as at this stage of their life, the nematodes do not eat and live off fats deposited within their bodies. To keep them viable, they have to be kept at 5˚C/40˚F, not allowed to freeze and kept out of sunlight. Then they are packaged in high density polythene bags which have been designed to allow air exchange but retain moisture. Under refrigeration, the nematodes can survive up to 6 months in this state. Would the ‘soup’ be satisfactory to ensure the above conditions?
Application of nematodes.
As mentioned, they can be stored or open the bag, add to clean water and stir! Then the nematodes become free and active and are applied to the soil surface using a watering can, conventional hydraulic spraying and irrigation systems. To be effective, the product has to be applied as manufacturer’s instructions dictate, i.e. use simple clean non infected water which separates the nematodes from the soap formulation in which they are carried, to apply them when conditions are suitable, ( refers to soil temperature and moist conditions) water as directed to the bare soil and not on foliage. A typical package contains enough to treat a stated 25 square metre area, (12 million infected nematodes), not just a few!, to give them a better chance to find a slug. The varying temperatures, soils, soil moisture retention properties, are all important for survival and dispersal of the nematode worms. Failure by the gardener in any of the above instructions could affect the nematodes making them ineffective. These products are produced in high quality laboratories using state of the art equipment, by experts. The commercial product that is used by gardeners was a teamwork effort of bacteriologists, nematologists, entomologists, malacologists, biochemists, molecular biologists and industry with the aim to integrate their efforts to understand and exploit the peculiar attributes of symbioses.
It would also appear that the infected nematode causes death of many species of slugs and snails, but not all. The grey field slug (as above) certainly is killed, but for example the leopard slug (Limax maximus) is not. It is a larger slug than the grey field slug and scientists found that this slug was able to trap invading nematodes by increasing its shell size. (Although not visible to us, slugs are really snails and do have a “shell” hidden underneath the slug’s mantle. It is made of a fragile membrane of calcium carbonate and is hardly noticeable).
Predatory beetles, infected slugs and nematodes
The life cycle of these nematodes requires them to feed upon and breed from the slug that they have infected. Whilst at this stage the slug itself and therefore the nematodes are likely to be found by predatory ground beetles, such as Pterostichus melanarius and preyed upon. The nematodes themselves would become prey. Researchers experimented with this aspect and found that slugs infected by the nematodes actually deterred the beetles from consuming the infected slug. Even though the nematode cannot infect an insect host. The nematodes and bacteria mixture in effect stops the slug and therefore the nematodes, being preyed upon.
So in essence does this ‘soup’ work? Have a go yourself, you never know !!
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Refs…. Besides my own experiences, observations and knowledge….
Anon, (2008) Material Safety Data Sheet, Becker and Underwood, Littlehampton, UK.
Baker G M, (Editor) (2004) “Natural enemies of terrestrial molluscs” Cabi Publishing, Landcare Research, Hamilton, New Zealand
Grewall et al, (2003), “Parasitism of molluscs by nematodes: Types of association and evolutionary trends” Journal of Nematology 35 (2) : 146-156, The Society of Nematologists
Grewal, S (editor) et al, ( 2005), “ Nematodes as biocontrol agents” CABI Publishing, Wallingford, Oxfordshire, UK.
with thanks to Roy and Marie for the woodmouse/leopard slug photograph…… http://www.moorhen.me.uk
and Gareth Martin, Field Development Technician Becker Underwood Ltd for his input