🐞✗ Tested and false“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. Garden Organic is specific that the soil needs to stay above 5C "even at night", and RHS gives 5-20C as the working range, 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.
