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The Life Cycle of Fungus Gnats

November 28, 2020

  • Eggs
  • Larvae
  • Pupae
  • Adults
  • Cultural Controls
  • Chemical Controls

Female Fungus Gnat Adult

Image of a female black fungus gnat from Wikiquote by Eric Burton.

There are almost 2,000 different species of fungus gnats and many of them share many characteristics. Fungus gnats are tiny, black, short-lived gnats that belong to the Family Sciaroidea. They are commonly found in greenhouses affecting indoor plants because they are attracted to the moisture in the growing medium. Fungus gnats also go by the name of Sciarid Flies and adults can measure to about 2.5-4mm in length which can make early detection difficult. Typically you can find fungus gnats where there is enough moisture for algae and fungi to grow. In indoor cannabis cultivation, fungus gnat infestations could be due to poor growing medium sourcing, but mainly it is due to overwatering plants which leaves the medium too wet for too long.


Adult female fungus gnats can lay around 200-300 eggs in their lifetime. Eggs are laid on organic matter, typically on the grow medium just below the surface. The eggs are yellowish-white in color, oval shaped, and are approximately 0.1-0.25mm (0.009 inches) in length. In ideal conditions, eggs hatch within a few days (48 - 72 hours) revealing larvae. Their life cycle is typically completed in 3 weeks from this point, depending on environmental conditions.


Larvae hatch from the eggs and are legless, with white or clear bodies and black heads. They typically feed on dead plant roots and other decaying plant material, but larvae can also damage plant roots and stunt plant growth when present in exceedingly large numbers. Larvae are especially harmful to seedlings and young cuttings causing them to wilt and eventually die. Besides direct damage, fungus gnats are known to be vectors for fungal plant diseases such as Pythium root rot which will negatively affect cannabis plants.

Fungus gnat life cycle

The life cycle of fungus gnats.


Pupae are similar to larvae in appearance, but they are distinctly different from larvae with a smaller body size and inactivity. When larvae are ready to undergo metamorphosis, they begin to change color from white to yellow or brown as they slowly change into adults. This process is known as pupation. During this process fungus gnat pupa do not eat again until they are in the imago or adult stage of their life cycle.


Adult fungus gnats have dark, slender bodies that are similar in appearance to mosquitoes. They have long antennae, long legs, and wings with clear venation. Cannabis growers commonly confuse fungus gnats and root aphids, but adult root aphids have short, stocky bodies with wings much longer than their body. Fully grown fungus gnats are about 2.5-4mm in length. Fungus gnats can reach maturity from eggs in approximately two weeks and live for about one week. Although adult fungus gnats do not cause damage to plants directly, they are important for completing the life cycle by dispersing eggs. Additionally, adult fungus gnats can indirectly damage cannabis plants by being a vector for fungal diseases such as Pythium root rot from diseased plants to healthy plants.

The fungus gnat can complete its life cycle in three to four weeks at 77°F or higher.

Cultural Controls

Cultural controls in Integrated Pest Management (IPM) is the manipulation of abiotic and biotic conditions to control the environment to make it less favorable for pests. If you catch the fungus gnat infestation early enough, you can let your growing medium dry out to minimize larvae growth. Then for your next waterings it is recommended that plants are given less water than previous feedings to prevent overwatering. It is also important to measure the volume of water given to plants and record the data for analyzing. Based on the age of cannabis plants, strain variety, growing medium and size, and environmental conditions, the amount of volume and nutrients required will change for growers. Inspect plants and record their symptoms and then the volume can be adjusted based on the outcomes.

Chemical Controls

Chemical controls are pesticides approved for use on cannabis plants either in residential or commercial settings. Typically these pesticides are OMRI listed and safe for use in the environment. Neem oil should be used as a foliar spray to combat fungus gnat adults, while azadirachtin products should be used as a root drench applied every 7-10 days to combat fungus gnat larvae and pupae. Rotate neem oil sprays with insecticidal soap sprays to minimize pesticide resistance in fungus gnats.


  1. Bethke, J.A. and Dreistadt, S.H. (2013). “How to Manage Pests in Gardens and Landscapes”. University of California Agriculture and Natural Resources.

  2. (2020). “Fungus gnats (sciarid flies)”. The Royal Horticultural Society.

  3. Smith, Tina. (2015). “Fungus Gnats and Shore Flies. University of Massachusetts, Amherst. The Center for agriculture, food, and the environment.

  4. Burton, Eric. (2009). “Female black fungus gnat”. Wikiquote.

  5. Cranshaw, W.S. and Cloyd, R.A. (2009). “Fungus Gnats as Houseplant and Indoor Pests”. Colorado State University Extension.

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