How to get rid of fungus gnats on tomatoes

What fungus gnats look like on tomatoes

Adult fungus gnats around indoor tomato seedlings; mild stalling in propagation trays. Established tomatoes rarely show damage.

For the full set of fungus gnats damage symptoms across host crops, see our Fungus gnats identification page.

Why tomatoes attracts fungus gnats

Tomato seedlings raised in damp propagation mix indoors share the same risk profile as any seedling. The pest does not move with the tomato to the garden bed.

Severity for this combo: Low — occasional. When to act: Indoor sowing through transplant; once outdoors, fungus gnats stop being a tomato problem.

Step-by-step control protocol

This is the integrated-pest-management protocol — non-chemical control first, biological and organic options second, conventional sprays only as a labelled last resort. Total cycle: about three weeks for most home cases.

1. Confirm the pest. Inspect the plant — look for tiny dark flies running across soil or flying around the plant base. If unsure, photograph the affected area and open Growli for instant species ID.
2. Isolate where possible. Move container-grown tomatoes away from healthy plants. For outdoor beds, mark the affected row so you can monitor it daily.
3. Apply non-chemical control first. Let the top 2-3 cm of mix dry fully between waterings
4. Add biological or organic spray. Same as for seedlings — bottom-water, top-dress with grit, drench with Bti if larvae are visible in the top inch of mix.
5. Repeat on schedule. Most fungus gnats protocols need repeating every 5-7 days for three weeks to catch each new hatch. Egg-to-adult in 3-4 weeks at room temperature. Larvae feed on fungi and organic matter in the top 1-2 inches of damp mix, which is why letting the surface dry breaks the cycle.
6. Monitor and prevent recurrence. Inspect tomatoes weekly for the rest of the season. Bottom-water instead of top-water for seedling trays

Best biological control for fungus gnats on tomatoes

For greenhouse, polytunnel, conservatory, and indoor production on tomatoes, biological control gives long-term suppression without the residue or pollinator harm of synthetic sprays:

• Bti (Bacillus thuringiensis israelensis) — soil drench or Mosquito Bits, kills first-instar larvae specifically
• Steinernema feltiae beneficial nematodes — drench the mix, parasitises larvae
• Hypoaspis miles (Stratiolaelaps scimitus) — predatory soil mite for greenhouse propagation benches

Organic spray options

Pyrethrin soil drenches knock back adults but rarely solve the problem alone — larvae are the damaging stage. Bti is the safest targeted larvicide for indoor and seedling-tray use. Avoid neonicotinoid soil granules on edible crops.

Prevention going forward

• Let the top 2-3 cm of mix dry fully between waterings
• Bottom-water instead of top-water for seedling trays
• Top-dress with 1 cm of horticultural sand, grit, or perlite to block egg-laying
• Yellow sticky cards laid flat on soil catch egg-laying adults
• Switch to a sterile, peat-light seed-starting mix; avoid bagged compost that has sat damp

Common mistakes when treating fungus gnats on tomatoes

• Spraying once and walking away. Egg-to-adult in 3-4 weeks at room temperature. Larvae feed on fungi and organic matter in the top 1-2 inches of damp mix, which is why letting the surface dry breaks the cycle. A single spray misses everything that hatches afterwards — plan a 3-week protocol.
• Confusing the species. Tomatoes hosts several similar-looking pests. Confirm before treating; the wrong protocol wastes weeks. Open Growli for a confirmed ID.
• Spraying in midday heat. Insecticidal soap and horticultural oil burn leaves above 30 degC and on drought-stressed plants. Apply at dawn or dusk.
• Mixing biological control with broad-spectrum sprays. Pyrethroids and neonicotinoids wipe out predator releases. Use one strategy at a time.

Frequently asked questions

How do I get rid of fungus gnats on tomatoes?

Same as for seedlings — bottom-water, top-dress with grit, drench with Bti if larvae are visible in the top inch of mix. Egg-to-adult in 3-4 weeks at room temperature. Larvae feed on fungi and organic matter in the top 1-2 inches of damp mix, which is why letting the surface dry breaks the cycle.

What do fungus gnats look like on tomatoes?

Adult fungus gnats around indoor tomato seedlings; mild stalling in propagation trays. Established tomatoes rarely show damage.

When should I treat fungus gnats on tomatoes?

Indoor sowing through transplant; once outdoors, fungus gnats stop being a tomato problem.

Why are fungus gnats attracted to tomatoes?

Tomato seedlings raised in damp propagation mix indoors share the same risk profile as any seedling. The pest does not move with the tomato to the garden bed.

What is the best biological control for fungus gnats on tomatoes?

Bti (Bacillus thuringiensis israelensis) — soil drench or Mosquito Bits, kills first-instar larvae specifically. Same as for seedlings — bottom-water, top-dress with grit, drench with Bti if larvae are visible in the top inch of mix.

Will fungus gnats on tomatoes spread to other plants?

Yes. Fungus gnats from tomatoes typically migrate to nearby susceptible hosts — see the affected-crops list on the main fungus gnats page. Quarantine, sticky traps, and weekly inspections of neighbouring plants are essential.

Are pesticides safe to use on tomatoes?

Insecticidal soap, horticultural oil, neem oil, and Bti are the safest options for edible and indoor tomatoes. Always read the product label and follow manufacturer's PPE, dosage, and re-entry guidance. Pesticide approvals change — confirm via the UK HSE pesticide register or US EPA before use.

Sources

Combo-specific guidance sourced from US Cooperative Extension publications (UC IPM, NC State, UMD, UMN, Penn State, CSU, UF/IFAS EDIS), Clemson HGIC fact sheets, Royal Horticultural Society guidance, and Cornell NYS IPM Biocontrol fact sheets. Reviewed by the Growli editorial team in May 2026.

Keep going

• All crops affected by fungus gnats
• Fungus gnats — full kill protocol (article)
• How to grow tomatoes — full guide
• Tomatoes plant-care reference
• Tomatoes companion plants
• All 8 garden pests covered in this guide
• Garden pest identification — complete article