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). RHS's advice to apply on several occasions between spring and autumn means reapplying once that 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
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.