For more information, visit: This video show you the close up or macro footage of Marijuana Fungus Gnats infesting Cannabis plants. The larvae of these pests feed on the root of your weed. The grower is using fly traps to get rid of these flies. One strategy used to get rid of them is to let the soil dry between watering. Doing this will decrease the survival of the laid eggs and hatched larvae. The dry soil will also deter female gnats from laying their eggs.