Fungus gnats (sciarid flies)

Small black flies that breed in constantly wet medium. Adults are a nuisance; the larvae can damage seedling and young roots. The fix starts with watering.

Symptoms

• Small black flies running and hopping across the surface of wet medium, or flying up when you water.
• Tiny white larvae with black heads in the top layer of medium.
• Weak, slow or wilting seedlings and cuttings in bad cases, from larvae feeding on roots.

Quick checks

• Lift test: is the container heavy for days after watering?
• Hang a yellow sticky trap flat near the medium for a few days and count.
• Look closely at the top 2–3 cm of medium for larvae.
• Check saucers and trays for standing water.

How to fix it

1. 1
   Environment

   Water less often

   Let the top layer dry between waterings (use the lift test), empty saucers and make sure containers drain freely. In coco, keep it evenly moist but not soggy.

2. 2
   Physical

   Trap the adults

   Place yellow sticky traps near the medium surface to catch adults and track numbers. A layer of sand or grit on top of the medium makes it harder for them to lay eggs.

3. 3
   Biological

   Target the larvae

   Water Steinernema feltiae nematodes into the medium (keep it moist so they can move, and apply as the supplier says), or add the predatory mite Stratiolaelaps scimitus (Hypoaspis miles), which lives in moist medium above about 15 °C. Use one or the other at a time, or follow the supplier's advice.

4. 4
   Safe treatment

   Replace badly infested medium

   For houseplant-scale problems, repotting into fresh medium and discarding the old removes eggs and larvae.

Likely causes

• Medium that stays wet all the time (the classic sign of over-watering).
• Saucers and trays left full of water.
• Organic matter and debris on the surface.
• Gnats arriving in bags of compost or on houseplants.

What to avoid

• Don't keep topping up water on already wet medium.
• Don't leave dropped leaves or spilt medium on the floor.
• Don't use sprays: they don't reach the larvae and harm beneficials.

Prevention

• Water by weight, not by the clock.
• Store bags of medium sealed.
• Keep yellow traps in as an early warning.
• Use containers sized to the plant, so the medium dries at a sensible rate.

Monkey products that can help

Root Shoot

Designed to support a healthy root zone and faster root growth while young roots recover. Fix the watering first.

Follow the feed chart: 2 ml per 10 L in growth weeks 1–2, then 3 ml per 10 L from growth week 3 to flower week 5.

View product →

Products support a healthy plant. They don't replace fixing the cause.

When to get help

If seedlings or cuttings keep collapsing even after you've corrected watering, check for damping off or root rot, or email info@monkeynutrients.co.uk with photos.

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