Ordered by post and released, not attracted with planting. Here's when to use slug nematodes, what it needs, and how to tell if it worked.
How to use it
Availability: Sold refrigerated in some garden centres or by mail order, arriving live in a sachet suspended in a paste. It's a perishable product: best used straight away, though it can be kept in the fridge, never frozen, for up to a few weeks if there's a delay. Expensive relative to other slug controls, and effectiveness varies by what's growing: good on daffodils, only intermediate on lettuces, and less effective on hostas and where beans are mulched.
Conditions: Moist soil, 5-20C (RHS). Water into the soil; light sandy soils let the nematodes move freely and give the best results, while heavy, poorly drained soil can give poor results. Snails are less likely to come into contact with it than slugs, since they spend more time above ground. Keep the soil moist and warm for at least a few weeks after applying, to keep the nematodes active once they're in. A single application only works for around six to eight weeks, since a naturally-occurring predatory fungus kills the nematodes off in the soil within that six-to-eight-week window (Garden Organic). Apply on several occasions between spring and autumn: that means reapplying once the six-to-eight-week window closes, not stacking treatments on top of each other while the first is still active.
How to tell it's working: Infected slugs don't die out in the open: the nematodes can shift a slug's behaviour so it stays below the soil surface before death, so finding slug corpses on top of the soil isn't the sign to look for. Less feeding damage over the following weeks, and fewer slugs found when you dig or turn soil over, are the real indicators, and that effect fades after six to eight weeks as the treatment dies off, which is when a repeat application is worth timing.
What it helps control
Myths about slug nematodes
5 common claims about slug nematodes, checked: 5 busted0 disputed0 untested0 hold up
“A slug nematode treatment controls snails as well as slugs, since they're the same kind of pest”
Slugs and snails are both molluscs and the nematode can infect either, but in practice it barely touches snails. Slugs spend most of their life in and on the soil, where the nematodes live and hunt, while snails spend most of their time above the surface on plants, fences and walls, so the two rarely come into contact.
Our read The distinction is about habitat, not biology: the nematode isn't choosy between the two molluscs, snails just aren't often down in the soil layer where it operates.
What to do: Treat a nematode application as slug control, not a general mollusc control. Keep using barriers, torchlight picking or other snail-specific tactics for snails, especially on plants they favour like hostas grown up on walls or fences.
2 sources
- RHS: Slugs and snailsThe nematode is unlikely to affect snails, as they rarely come into contact with the soil-dwelling nematodes.
- RHS: Biological control in the gardenSnails can be affected by the nematode but are less likely to come into contact with it, as they spend most of their lives on or above the soil surface.
“Slug nematodes can be applied any time the ground isn't frozen”
The nematodes need moist soil consistently above 5C, not merely above freezing, and that threshold has to hold overnight as well as during the day for the treatment to keep working. The soil needs to stay above 5C even at night, and the working range is 5-20C, with spring to early autumn as the practical application window.
What to do: Check soil temperature rather than air temperature or the calendar before applying, and look at overnight readings specifically, not just the daytime high. A soil thermometer pushed a few centimetres in gives a truer read than guessing from the weather.
In Scotland: Scottish soil often reads well above 5C by mid-afternoon in March while still dropping below it overnight, which is exactly the gap this threshold falls into. Waiting until night soil temperatures hold above 5C, generally later in spring than the first mild daytime spell suggests, gives an application a better chance of working from the start rather than sitting dormant.
2 sources
- Garden Organic: NematodesThe soil needs to be above 5C, and remain so, even at night, for the nematodes to work.
- RHS: Slugs and snailsEffective in moist, warm soil at temperatures of 5-20C, with spring to early autumn as the application window.
“Slug nematode treatments also kill earthworms and other beneficial soil life”
The slug nematode is specific to molluscs, with no adverse effect on other types of animal. Earthworms are annelids, a different group entirely from the slugs and snails the nematode is built to infect, so they fall outside what it targets.
Our read RHS's own biocontrol overview page is more cautious in general terms, noting that nematode products can affect non-target animals within the same broad group they're aimed at (so a mollusc-specific nematode could in theory affect other molluscs, not unrelated animals like earthworms). The two pages aren't in tension once read together, but the general caution is easy to over-read as covering more ground than it does.
What to do: Don't hold off on nematodes out of concern for earthworm populations; the real trade-off with this product is that it also kills beneficial slug species, not that it disturbs the wider soil food web.
2 sources
- RHS: Slugs and snailsThe nematode is specific to molluscs, with no adverse effect on other types of animals.
- RHS: Biological control in the gardenNematode biological controls are usually either insect or mollusc specific, and have the potential to infect non-target animals within those same groups.
“Leftover slug nematodes can be frozen to save them for the next application”
The nematodes are shipped alive in a paste and are a living product, not a shelf-stable powder. They can be kept in the fridge for up to a few weeks if there's a delay, but must never be frozen.
What to do: Buy nematodes close to when the soil is warm enough to use them (moist and consistently above 5C), use the sachet promptly, and treat a short fridge stint as the only fallback if plans change, not freezing.
1 source
- Garden Organic: NematodesThe product arrives live in a sachet suspended in a paste, best used straight away, though it can be kept in the fridge (never frozen) for up to a few weeks if needed.
“It doesn't matter what time of day slug nematodes are watered in”
Apply very early morning or at dusk because the nematodes are sensitive to light. Watering them in under strong midday sun risks losing more of them before they reach the soil where they need to be.
What to do: Apply in the early morning or evening rather than the middle of the day, and water them straight into moist soil with a coarse rose on the can so the sachet's contents aren't strained out or left drying on the surface.
1 source
- Garden Organic: NematodesApply in the very early morning or at dusk because the nematodes are sensitive to light, using a coarse rose on the watering can to avoid clogging.
Frequently asked questions
Where do I get slug nematodes?
Ordered by post from a biological control supplier, not bought off a shelf. See "How to use it" above for timing and conditions.
How do I know if slug nematodes are actually helping?
You won't see slug nematodes themselves working: they're too small to spot. Watch the pest population instead: fewer slugs and snails over the following weeks is the real sign it's working.
Worth knowing
RHS files slugs under biodiversity, calling them 'a gardener's friend' and advising tolerance, yet it also endorses a nematode that kills every slug species, including the ones doing good in the garden. The most effective slug control sits in real tension with that framing.
Sources
3 sources, recorded with what each was used for
- RHS: For Slugs and Snails: RHS endorses Phasmarhabditis hermaphrodita as effective when conditions are right: moist soil at 5-20C, most effective if applied on several occasions between spring and autumn. Cautions it affects ALL slug species, including beneficial ones. Available from refrigerated cabinets in some garden centres or by mail order.
- Garden Organic: For Slugs and Snails: Kills slugs, particularly small soil-dwelling ones; a single application only works once, for around six to eight weeks, since the nematodes themselves are killed off in the soil by a naturally-occurring predatory fungus within that window; effectiveness varies by crop, good on daffodils, only INTERMEDIATE on lettuces, less effective on hostas and beans with mulch; expensive relative to other slug controls. The nematodes can also modify an infected slug's behaviour so it stays below the soil surface before dying, rather than dying somewhere visible. Soil needs to stay moist and warm for at least a few weeks after application to keep the nematodes active. The product itself arrives live in a sachet suspended in a paste, best used straight away, though it can be kept in the fridge (never frozen) for up to a few weeks if needed.
- RHS: RHS's dedicated biological control page: soil type affects how freely the nematode moves (light sandy soil gives the best results, heavy poorly-drained soil can give poor results), and snails are less likely to come into contact with it than slugs.