How to prevent (further) damage?
You can prevent further damage by removing dead plant debris. Also make sure not to water your plants too much, because fungus gnats like to lay their eggs in moist soil. Another tip is to add a thin layer of sand on top of your soil. Since fungus gnats can’t cross this it stops them from laying their eggs underground. Still found larvae in the soil? Then you can combat them using nematodes, the natural predator of the fungus gnat. This parasite kills any larva it comes across. You can also use a foliar spray such as C-No-Insects
to create and electrostatic field around the plant which will discharge, eliminating all pest in contact with the plant.
About fungus gnats
Fungus gnats are small, dark gnats with long antennae and legs. They are between three and five millimetres long. Their preferred conditions are warm, damp and near plants. As such, in greenhouses they can be found all year long. After mating, the females lay between 50 and 200 eggs that hatch within two to three days.