Fungus Gnat & Sciarid Fly Nematodes | Indoor Plant Protection

If you've ever lifted a houseplant pot or seed tray and seen a small cloud of tiny dark flies rise up — those are sciarid flies (also called fungus gnats), and they're the most common UK indoor and propagation pest. Fungus Gnat Nematodes are the proper biological control: microscopic parasitic nematodes (Steinernema feltiae and Steinernema carpocapsae) that hunt and kill the larvae in your compost before they can damage seedlings, cuttings or houseplant roots. Safe for plants, pets, bees and edibles. Properly the indoor gardener's nematode.

From Ladybird Plant Care, our trusted partner specialising in biological pest control. Live nematodes are despatched fresh from their facility direct to your door — not held in stock; ordered in fresh for each customer.

About sciarid flies and fungus gnats

Sciarid flies (also called fungus gnats; species in the Sciaridae family) are the small, dark, slightly fluffy flies you see hovering around houseplants, sitting on compost surfaces, or rising up when you water a seed tray. They're properly familiar to anyone who grows plants indoors. The adults themselves are mostly an unsightly nuisance — they don't bite, don't transmit disease — but they're the visible sign of a more serious problem below.

The real damage is done by the larvae, not the adults:

• Small white maggot-like larvae live in the top 3–5cm of compost, feeding on organic matter and tender roots
• Damage to seedlings and cuttings is properly severe — the larvae sever fine roots and feed on the stem base, killing young plants outright
• Established houseplant roots can also be damaged; larger plants show stunted growth and yellowing leaves
• Propagation losses can be catastrophic — entire seed trays of carefully raised seedlings can be lost to a serious infestation
• Larvae also feed on fungal mycelia — hence the name "fungus gnat." Damp, organic-rich compost is their ideal habitat
• Larvae come to the surface to feed at night and in low-light conditions, where they're sometimes visible as tiny pale wrigglers

If you sow seeds indoors, raise cuttings under cover, grow houseplants, manage a greenhouse, or grow microgreens, fungus gnats are almost certainly part of your pest landscape. The nematode treatment is the proper biological answer.

How the nematode treatment works

The active species are Steinernema feltiae and Steinernema carpocapsae — a multi-species mix that handles fungus gnat larvae alongside a wider range of soil and surface pests. The biology is properly elegant:

• The nematodes are mixed with water and applied as a soil drench (watering can, spray bottle, or compost-drench)
• Once in the compost, they actively swim through the thin water films around soil particles, searching for prey
• They detect their hosts (sciarid larvae) by sensing carbon dioxide and metabolic waste products from the larvae's breathing
• Once they find a larva, they enter through natural openings (breathing pores, mouth)
• Inside, they release symbiotic Xenorhabdus bacteria that liquefy the host within hours
• The nematodes feed and reproduce inside the liquefied host, then emerge to seek further prey
• When pest populations drop, nematode numbers naturally fall back to background levels — no long-term environmental impact

The whole process is completely invisible to the gardener: you water in the nematodes, the population of adults declines over the following 2–3 weeks, and the seedlings, cuttings or houseplants are protected from larval damage.

The complete control approach — combine with sticky traps

For the best results, biological control of fungus gnats benefits from a two-pronged approach:

• Yellow sticky traps catch the adult flies, reducing the visible nuisance and the breeding population
• Nematodes kill the larvae in the compost, which is where the actual plant damage happens

Sticky traps alone reduce visible adults but don't stop the larvae underground; nematodes alone deal with the larvae but adults already present continue laying eggs. The combination tackles both lifecycle stages and gives proper rapid control. A pack of yellow sticky traps from any garden centre is the natural pairing for this product.

Better suited to higher pest populations

One genuinely useful point: nematodes work better when there's a proper pest population to feed on. A few isolated fungus gnats can be hard to clear with nematodes alone — the predators don't have enough prey to establish. For light infestations, sticky traps and good compost hygiene (allowing soil to dry between waterings, removing organic surface debris) may be sufficient.

For established or severe infestations — properly noticeable clouds of flies, seed trays losing seedlings, multiple houseplants affected — nematodes are the right escalation. The biological control works most effectively when there's pest pressure to maintain the predator population.

How to apply

• Mix with water according to the packet instructions (typically dissolving the powder in water)
• Apply as a soil drench using a watering can or hand sprayer — cover all the soil surface in affected pots and trays
• Soil must be moist before application — the nematodes need water films to move through. Water plants gently the day before if compost is dry
• Apply in evening or low light — nematodes are killed by direct UV. Less critical indoors than outdoors, but still avoid bright sunshine through windows
• Don't let compost dry out for 2–3 weeks after application — keep slightly moist for the nematodes to establish and hunt
• Soil temperature should be 10°C or above — warm indoor conditions are typically ideal
• A second application 2–3 weeks later may be needed for severe infestations or to catch any newly hatched larvae
• Don't combine with chemical sprays — chemicals kill nematodes alongside pests. Biological control requires chemical-free conditions

Detailed application instructions are in the official instruction PDF — worth reading before first use.

Pack sizes — choose by your indoor garden scale

Each pack treats up to 60 square metres of compost or growing surface:

• 1 x 60 sqm pack — Plenty for a small collection of houseplants, a single seed tray rota, or a small greenhouse propagation area. The right starting point for the home gardener
• 2 x 60 sqm packs —  For larger houseplant collections, conservatory growers, or full greenhouse propagation
• 3 x 60 sqm packs — For multiple greenhouses, commercial propagation, or substantial collections requiring multiple application rounds

For most home gardeners growing houseplants, raising seedlings indoors, or running a small greenhouse, the single 60sqm pack is sufficient for several treatment rounds.

The seed grower's natural pairing

If you sow seeds indoors — flower seeds in spring, vegetable seedlings, herb propagation, microgreens through the year — this is one of the genuinely useful protective tools to have on hand. Indoor seed propagation creates exactly the conditions fungus gnats thrive in: moist sterile compost, organic matter, warm temperatures, no natural predators. A single fungus gnat infestation in a seedling tray can wipe out a season's seed-sowing investment.

For Bishy customers raising our flower seeds, vegetable seeds or microgreen seeds indoors, having fungus gnat nematodes on hand is properly the right preventative approach. The £9.00 single pack is genuinely cheap insurance against losing a season's careful work to sciarid larvae.

Where this fits in our biological control range

This is the indoor / houseplant / propagation member of our Ladybird Plant Care biological control range:

• Fungus Gnat Nematodes (this) — indoor and propagation focus
• Fruit & Veg Protection Nematodes — same active mix; outdoor fruit and vegetable focus
• Codling Moth Nematodes — dedicated apple-tree codling moth
• Chafer Bug Nematodes — lawn chafer grub control
• Ant Control Nematodes — pavement and lawn ant management
• Fuchsia Gall Mite Sachets — predatory mites for fuchsia gall mite and red spider mite prevention

Different problems need different biological solutions. The Fungus Gnat Nematodes are properly the right product for indoor pest problems where the other nematodes are aimed at outdoor pests.

Why nematodes are the right answer for indoor growing

• Completely safe for indoor use — no fumes, no airborne residue, no chemical worries near cooking surfaces or sleeping spaces
• No harm to plants — the nematodes only target the pest larvae; ornamentals, houseplants, edibles all unaffected
• Pet-safe — cats, dogs, rabbits, birds, fish all completely safe (relevant since indoor pests are often near pets)
• Child-safe — properly safe to use in homes with children
• Edible-safe — vegetables, herbs and microgreens can be harvested and eaten the same day. No withdrawal period
• Works in confined spaces — conservatories, greenhouses, propagation areas, indoor seed-sowing setups all benefit
• No resistance buildup — biological controls remain effective indefinitely (chemical insecticides build pest resistance)

Important supply note

Nematodes are live biological products that don't sit on shelves. Each order is sourced fresh from Ladybird Plant Care's specialist facility:

• Order by 10am Monday for same-week despatch
• Orders after this typically ship the following week
• Once delivered, store cool (refrigerator works well) and use as soon as practical
• Application life is typically 2–4 weeks from despatch — don't keep packets stored indefinitely

This is properly a fresh biological product delivered at peak viability, not a same-day shipping item.

Specifications

• Supplier: Ladybird Plant Care (UK biological pest control specialists)
• Active species: Steinernema feltiae + Steinernema carpocapsae multi-species mix
• Pack sizes: 1 x 60 sqm (£9); 2 x 60 sqm (£16); 3 x 60 sqm (£23)
• Coverage per pack: Up to 60 square metres of compost / growing surface
• Soil temperature: Minimum 10°C for full effectiveness (typical indoor conditions)
• Application: Soil drench via watering can or hand sprayer; soil must be moist; water in immediately
• Reapplication: A second application 2–3 weeks later may be needed for severe infestations
• Primary target: Sciarid fly / fungus gnat larvae
• Combine with: Yellow sticky traps for the adult flies (the complete control approach)
• Safety: Safe for bees, ladybirds, pets, children, edibles, houseplants
• Withdrawal period: None — harvest and eat same day
• Despatch: Fresh from Ladybird Plant Care; order by 10am Monday for same-week despatch

A small thought: there's a particular frustration in lifting a watering can over a carefully tended seed tray and watching a cloud of sciarid flies rise up like a small grey mist — the visible sign that your careful work is being undermined by something invisible underground. The nematode treatment is the proper response: invisible to you, devastating to the pest larvae, completely safe for the seedlings and everyone in the house. The kind of small intervention that quietly saves a season's worth of careful sowing. Order in early spring before the indoor sowing rush; keep on the shelf as the insurance every indoor gardener eventually needs.

Variant-dependent:
- 1 x 60sqm pack of fungus gnat nematodes; OR
- 2 x 60sqm packs; OR
- 3 x 60sqm packs
Plus application instructions (PDF)

- On arrival, store in refrigerator (not freezer) until use
- Use within 2-4 weeks of despatch for best viability
- Soil must be moist before application
- Apply in evening or low-light conditions
- Water in immediately after application
- Keep compost slightly moist for 2-3 weeks after treatment
- Don't combine with chemical sprays