One of the things many folks don’t understand is that if they see small fungus gnats flying around their plants, these insects do have larval forms (really, really small worms) that can burrow into the roots and below-ground stems (in cuttings) of plants.
To make matters worse, it turns out these small larvae also carry Botrytis, Pythium, Fusarium, Phoma and Verticillium spores to infect your plants.
A quick organic control is to cover yellow sticky cards with Tanglefoot or other long-acting horticultural glue and lay them “horizontally” near the soil surface. The adults will land on the traps and be stuck (thus no egg-laying will happen)
If you have a basic hand lens, fungus gnat larvae are wormlike with a black head capsule and a white to transparent body.
There are three ways to easily control them
Hang yellow sticky traps over the seedling trays.
If The Plant or Cutting Is Well-Rooted
Dry the cuttings out *once they have rooted* so the soil dries out but doesn’t wilt the cutting.
And the same goes for mature plants – dry them out more because the adults are likely feeding on microscopic soil algae.
If The Plant Is Not Well Rooted
And the second is to flood the cutting with insecticidal soap. I’ve never killed a cutting with this flooding but your results may vary depending on what you’re propagating.
You can get more plant propagation techniques right here.