Adult fungus gnats are grey with longs legs and are very tiny. Females lay eggs at the rate of approximately two hundred per week at the base of the cannabis. These little buggers attack in both the adult and larval (maggot) stage. Fungus gnat maggots are microscopic, sporting black heads and transparent bodies. Fungus gnats are found at the soil level and initially feed on fungus. Once they run out of soil ick, they attack the root systems, damaging larger roots and eating the smaller root hairs. This retards the plants' growth, discolors leaves and causes malformation of the stems and branches. As their name denotes, they render the marijuana plant susceptible to various fungal diseases.
Fungus gnats are particularly damaging to cannabis due to affecting the drainage properties of the soil by their droppings. Marijuana needs well drained soil so as to keep the root system from becoming too wet and inviting pests who thrive in wet conditions. Taking care to not over water your crop is good preventative measure against infestation; they are drawn to moist conditions.
There are a couple of simple ways to detect whether or not you have a gnat problem. One is to purchase yellow sticky cards from your garden center and place them in the soil around your plants. After a few days, remove the cards and see what has stuck to them. If you see more than a few gnats, uh oh!
To check for fungus gnat larvae, place a half inch thick piece of potato on the ground. After four to eight hours, check for maggots. If there are several dinner guests on the potato, it's time to send them on their way.
Placing yellow sticky cards or yellow sticky tape (gnats love the color yellow!) throughout the garden will trap the adult gnats. Fungus gnat larvae are a different matter. Begin by letting the top few layers of the soil dry completely. This means delaying the watering schedule for a few days. Once the soil is dry mix one part pure 3% hydrogen peroxide with four parts water. Water the top layer of soil thoroughly. This will kill the larvae. Peroxide will not hurt your plants. It will break down into oxygen and water molecules which actually benefits the soil. Going through this exercise periodically will ensure the gnats go away and stay away and is actually a healthy treat for the soil.