Thoughts/discussion on nematodes

wild edgeswild edges The north west of south east WalesPosts: 4,071
Just dragging this discussion out from another thread as I'm hoping to get some better advice and thoughts from people.
I've been cautious of nematodes for a while because of their broad range of target species and their potential impact on soil health due to killing non-target organisms which perform valuable roles in gardens. In theory there should already be populations of nematodes in a healthy garden and you're just boosting the numbers by using commercial drenches. How long do the boosted numbers last though as the nematodes don't always need prey to survive apparently unlike your natural garden predators.
For example slug nematodes target all species of slugs and snails not just the main pests and this will surely have a knock on effect in terms of food for other wildlife such as thrushes and hedgehogs as well as removing the gastropods that are doing good work breaking down rotting plant matter all over your garden. It also targets the slugs that eat other slugs. There are over 30 species of slug and over 100 species of snail and I imagine only 10 or so of these are garden pests.
Vine Weevil nematodes http://http//  again have a broad range of target species and it's hard to find information on exactly what will be affected and if there are any impacts on general soil health. Things that worry me are quotes like:
"Parasitic nematodes seek out insects harmful to garden plants, shrubs and trees in their soil-borne stages and destroy them from the inside out. Present in soils throughout the world, these microscopic, non-segmented worms destroy over 200 types of insects that mature in the ground"
"Nematodes do not prey on ladybugs, earthworms or most other beneficial insects."
A target of one or two pests and 198 other insects are effected and it's considered safe for use? This just seems very poor marketing to me as most companies don't mention these crucial facts and Google has an annoying habit of prioritising commercial marketing over scientific research results. However I'm faced with a minor plague of vine weevils at the moment and they're causing a lot of problems and I'm losing plants. For physical control I have to empty every pot and strip plants that are affected back to bare root and even them I've found grubs hidden in root cavities. One adult can lay several hundred eggs and I've found pots with up to 40 grubs. The fact that they're only a problem in pots shows how important soil ecology is though surely?


  • LynLyn DevonPosts: 15,015
    I’ve just put this on the other thread, I’ve used both.

    If nematodes for slugs just keep multiplying, why do you have to re do them every few months, they don’t last forever.
    Vine weevil nematodes need to be applied twice a year. 
    They never upset our compost, not did they get put back on the ground next year.  If you can’t use them until the weather is warmer, how do they survive in the compost bin. 

    Gardening on the wild, windy west side of Dartmoor. 

  • B3B3 Posts: 10,894
    I don't like the idea of biological warfare but I do use slug pellets (chemical warfare) but I've never had weevils in the numbers that you mention. I think if I did, I'd use a chemical. It's all very well saying that one predator is only interested in one kind of prey but living things can mutate and adapt.
    I'm not saying that other gardeners shouldn't use nematodes, just that I don't. In the same way, I don't use weedkiller, but then I haven't got marestail or that other pernicious weed that I can't remember the name of.

    In London. Keen but lazy.
  • AnniDAnniD Posts: 5,197
    I use vine weevil nematodes in pots as l stated previously. I appreciate wild edges concerns, and have read the articles in the links provided. As in  types of chemical use, such as slug pellets, l feel it is up to the gardener concerned to decide as to whether or not to use them. There is so much information out there these days, both for and against,  and obviously the manufacturers have a vested interest. It may well be that in years to come, nematodes will be regarded in the same way as we now look at DDT and the like. 
  • raisingirlraisingirl East Devon, on the Edge of Exmoor.Posts: 3,697
    It's a difficult one, I think. There have been numerous studies in various parts of the world and none have found any significant risks to non-target species. This one seems to answer your question regarding persistence.
    With any 'scholarly' articles, there are always two problems; first it's generally hard to know who paid for the study and second, absence of proof is not proof of absence. The laws of unintended consequences seem to apply very often and though scientists haven't yet found a problem, that doesn't mean there won't be one.
    Rather like with GM food, you can believe the science and the fact that no harm has been found, or you can trust your instinct and believe that that's just because they haven't looked in the right place yet. It took a couple of generations for the harms of margarine to become apparent.

    I have used nematodes for slugs in the raised beds in my veg patch but not in my garden. Though the numbers of slugs eating my veg decreased, I saw no reduction in the numbers elsewhere in the garden or in the numbers of toads and slug eating birds around, so anecdotally, in my garden, I haven't seen a problem. I've applied them twice in 10 years. The fact that the numbers did begin to increase again a few years later does suggest they haven't persisted beyond that time in significant numbers.

    But of course, when antibiotics were first discovered, the idea of superbug resistance wasn't considered. We may be creating super-weevils. Certainly you are disrupting the natural ecology and it would be far better to use plants, mulch and encouraging native predators as an approach rather than any 'artificial' introduction, if you can.

    Maybe you just shouldn't have plants in pots.
    On and on the rain will fall, like tears from the star
    On and on the rain will say, how fragile we are
  • AnniDAnniD Posts: 5,197
    Trouble is raisingirl, it's not that simple sadly. I am trying to reduce the number of pots l have (not very successfully if l'm honest !) , because of the vine weevil problem,  but others may have no choice but to garden in pots. As you say, we could be storing up trouble for the future, we just don't know. 
  • DaveGreigDaveGreig West Fife, ScotlandPosts: 41
    My garden has suffered badly from Spanish slug infestation in the last few years. I’ve used beer traps, pellets etc.. to no avail and these huge slimey creatures carpeted my paths with the smaller ones scything down any new growth in my vegetable and flower beds. 

    I spent a season removing them manually which is pretty sickening so i tried using nematodes in specific areas with poor results. For the last 2 seasons I’ve treated the whole garden and there has been a significant decrease in slug damage. The problem is that my considerable frog population has more than halved. There is very little spawn in the pond this year compared to previously and it’s impossible to say if this is the cause or there are other factors at work. There’s also a huge decrease in hedgehog activity. The runs in my grass caused by their movements have grown over and disappeared  and I only saw 2 last autumn. A decrease from an almost nightly occurrence. I cannot say with any certainty if the nematode use is the cause but it’s  a bit of a coincidence.

    Im not going to use nematodes any more and see what happens.
  • wild edgeswild edges The north west of south east WalesPosts: 4,071
    It's a difficult one, I think. There have been numerous studies in various parts of the world and none have found any significant risks to non-target species. This one seems to answer your question regarding persistence.
    Thanks, that's quite helpful but still leaves a few questions about methodology of testing and what they were hoping to find. persistance in a damp climate would be higher than in a dry one for example and is their test on them spreading to nearby locations based on their own movement or is human activity allowed for? They say intial mortality is high due to UV radiation and dehydration but a well prepared soil and an evening application would surely prevent that?
    I've read sources that say that slug nematodes can live in the bodies of slugs that are immune to the bacteria they use to kill their target species so they are carried a good distance by the slug and kept alive. Other sources say they can persist in damp soil without access to prey for long periods.
    It sounds like they quickly become part of the soil ecology, such as it is in the fairly artificial and damaged environment of our gardens.

    To answer Lyn's questions I imagine that the repeat application is required partly because they want to sell more of their products but also because it will give faster results or toreplace nematodes lost to the summer weather if it was hot and dry. With vine weevils you can probably get a massive reinfestation from only 2 or 3 adults coming from a pot you missed or a neighbour's garden. And compost heaps seem like the ideal warm, damp, prey-rich environment for them to thrive and survive the winter. My issue is that none of these questions are really very adequately answered or tested for as far as I can see.
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