# Tiny clear larvae with black heads in the medium: likely causes and fixes

> Tiny clear larvae with black heads in the medium on cannabis is most often Fungus gnats or Overwatering (roots short of air). How to tell them apart, the fix for each and what recovery looks like.

Also called: worms in soil, larvae in coco, maggots in pot, small white worms with black heads.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=larvae-in-medium

## Likely causes, most likely first

### Fungus gnats (Likely)

- Kind: Pest; urgency: Low urgency
- Fix: Fix the watering first: let the top 2-3 cm dry. Then Bti drenches (larvicide), Steinernema feltiae nematodes or Stratiolaelaps mites; yellow cards for adults. Larvae chew seedling and clone roots and can open the door to Pythium.
- Recovery: Card catches fall within 2-3 weeks; no larvae under potato slices.
- Confirm: Sticky cards for 1-2 weeks: Hang yellow cards at canopy height and lay one flat on the medium surface; add blue cards for thrips. Count catches every 3-4 days. Rising counts of small dark flies at the medium mean fungus gnats; slender insects mean thrips; white wedge-shaped fliers mean whiteflies; winged aphids mean a colony nearby.
- Confirm: Potato slice test for larvae in the medium: Press 2-3 raw potato slices (1 cm thick) cut side down onto the medium surface. Lift after 2-4 days and look underneath: translucent larvae with shiny black heads are fungus gnat larvae.
- Article: [Fungus gnats: the watering problem with wings](https://genetixbay.com/knowledge/pests/fungus-gnats.md)

### Overwatering (roots short of air) (Less likely)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Water only when the pot is clearly lighter (about half the water used), then water to 10-20% runoff. Seedlings in big pots: water a small ring near the stem only. Add air: fabric pots, 25-40% perlite, no saucer standing water. Let a soaked pot dry back before the next watering.
- Recovery: Leaves lift within a day of drying back; colour and growth recover within 5-7 days.
- Confirm: Lift the pot and check moisture at depth: Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Confirm: Smell test of roots, medium and reservoir: Smell the roots, the bottom of the root ball, the drain water or the reservoir up close. Healthy roots smell earthy or of nothing. A sour, rotten, swampy or sewage smell means anaerobic conditions and rot. In hydro, a slimy feel on the reservoir walls or tubing adds to the case.
- Article: [Drooping cannabis: overwatering vs underwatering](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.