Fungus Gnats

Identification & Appearance:

Fungus Gnats (Orfelia and Bradysia) are gray to black, mosquito-like insects that prefer warm, humid conditions. Due to this preference, fungus gnats are a prevalent pest in greenhouse and indoor growing settings as well as in outdoor potted plants. Common egg-laying sites include moist organic debris/potting soil, decaying wood (mulch), and ground cover plants. Adults measure 1/8 to 1/10 inches long (about the size of a common gnat) and they are distinguished from another greenhouse pest, the shore fly, by their long antennae. Fungus gnat larvae, the damaging life stage, are worm-like and clear to white in color. In large infestations the fungus gnat larvae leave slime trails on the growing medium's surface similar to that of slugs & snails.

Fungus Gnat Life Cycle:

Fungus gnats mature through four life stages: egg, larva, pupa, and adult. Eggs are laid in damp media where they hatch within 3 days at 75°F. Hatched larvae feed on a variety of organic matter: decaying material in the media, fungi, or root hairs. Feeding occurs for about 10 days after which larvae pupate. Adults emerge after 4 days and reproduce shortly thereafter.

Damage Symptoms:

Symptoms of fungus gnat infestations are sudden wilting, poor growth, chlorosis (yellowing), and foliage loss. After hatching, fungus gnat larvae promptly begin feeding. Their feeding can cause significant root damage and severely inhibit nutrient uptake. Additionally, these feeding areas can become entrances for soil-borne diseases. The resulting damage commonly shows up as nutrient and/or water deficiency brought about by decreased root absorption. Adult fungus gnats do not damage plants; however, they can vector diseases like Pythium, Fusarium and Verticilium.

Controlling Fungus Gnats:

  • Monitor your growing area for presence of adult fungus gnats using yellow sticky traps. Adding Melissa oil to the traps has shown improved results in attracting fungus gnats.
  • Control soil moisture by using soil mixtures with proper drainage. Avoid overwatering to minimize breeding habitat.
  • Preventive and corrective applications of Bacillus thuringiensis israelensis products kills larvae before they can pupate. Aquabac, Mosquito Bits and Mosquito Beater WSP can all be used similarly to control and/or suppress fungus gnat populations.
  • Apply Nemattack, Sf to the growing media. The beneficial nematodes will parasitize existing fungus gnat larvae, slow their feeding and kill them.
  • S. scimitus (H. miles) helps break up the life cycle and eventually controls fungus gnat infestations by feeding on larvae and pupae. This small mite also feeds on thrips pupae and springtails.
  • Beauveria bassiana is a beneficial fungus that infects and kills a number of insect species. Applications should be made to growing media for fungus gnat control.
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