Fungus Gnats

Marijuana Horticulture

by Jorge Cervantes

Identify

Maggots, larvae, grow to 4-5 mm long and have translucent bodies with black heads. Winged adult gnats are gray to black, 2-4 mm long, with long legs. Look for them around the base of plants in soil and soilless gardens. They love the moist, dank environments in rockwool and the environment created in NFT-type hydroponic gardens. Adult females lay about 200 eggs every week to ten days.

Damage

Infests growing medium and roots near the surface. They eat fine root hairs and scar larger roots, causing plants to lose vigor and foliage to pale. Rot wounds invite wilt fungi like Fusarium or Pythium especially if plants are nutrient-stressed and growing in soggy conditions. Maggots prefer to consume dead or decaying, soggy plant material; they also eat green algae growing in soggy conditions. Adults and larvae can get out of control quickly, especially in hydroponic systems with very mist growing mediums. The adult gnats stick to resinous buds like flypaper! The gnats are very difficult to clean from the buds.

Controls

The easiest way to control these pests is with Vectobac, Gnatrol, and Bactimos, all contain Bacillus thuringiensis var. israelensis (BT-i). This strain of Bt controls the maggots; unfortunately, it is available only in large one-gallon containers. Difficult to find at garden centers, check hydroponic stores.

Cultural and physical control 

Do not over water, and keep ambient humidity low. Do not let growing medium remain soggy. Cover growing medium so green algae won’t grow. Yellow sticky traps placed horizontally 1-2 inches over growing medium catches adults.

Biological

The aforementioned Bt-i works best. Alternatives include the predatory soil mite (Hypoapsis Geolaelapumites) and the nematode (Steinernema feltiae).

Sprays

Apply neem or insecticidal soap as a soil-drench.