Have you ever noticed those little black flies that hang out near the base of your plants that come out in masses whenever you try to water your plants? Those are fungus gnats, and they are one of the most common and annoying pests in the indoor garden.


What are Fungus Gnats?

Fungus gnats are the common name for multiple different species of insect from the families Mycetophilidae and Sciaridae. They are small and mosquito-like, but fortunately for us, they don’t bite.

The adult stage of these insects are harmless, and no more than a nuisance, but the larval stage that lives in the soil or root zone can become a problem for your plants as they eat away all of the precious root hairs.

The Life Cycle of a Fungus Gnat

Fungus gnats begin as an egg, of which an adult fungus gnat can lay up to 300 in their short lifespan. These eggs hatch in the matter of just a few days (4-6), which then release a tiny larvae. This larvae then begin to feed on the root hairs of your plants, fungal hyphae, and other organic matter in the soil for the next 2 weeks. 

After this they enter into a pupal stage which lasts just 4-6 days, before becoming adults and leaving the soil. These adults only live about a week, but they tend to keep busy laying around 300 eggs each.

The whole life cycle can be as fast as 3 weeks, or as long as 6 depending on the temperature.

Fungus Gnat Damage

Fungus gnats are not an immediate danger to your crop, and are generally very easily noticed and corrected long before any significant damage has occurred. The adult fungus gnats always buzzing around the root zone makes them an easy pest to spot. 

If allowed to continue for long enough however, damage such as a sudden loss of vigor, yellowing leaves, and wilting can occur as the larvae munch away all of the root hairs of your plants.

Controlling Fungus Gnats

Fungus gnats thrive in damp conditions. Potted plants that need to remain moist for long periods of time, such as african violets, orchids, or poinsettias are particularly vulnerable to fungus gnat infestation. Poor drainage, or simply overwatering can also promote fungus gnats.

Knowing the lifecycle of these insects can help a lot in regards to control. Their lifecycle takes a few weeks, and many generations each at different life stages can all be present on your plants. This means treatment will need to be repeated over the course of 3-6 weeks.

The main methods of control for these insects include:

1. Yellow sticky strips

Yellow sticky strips are used to draw the attention of (the yellow color), and trap  adult fungus gnats (the sticky part). This keeps them from being able to reproduce and lay eggs, but should not be the sole means of control for large fungus gnat infestations. They do however, make a great preventative treatment however, and makes it easy to see if fungus gnats have begun entering your garden early on.

2. Pyrethrum sprays

Pyrethrum is an insecticide that has been extracted from chrysanthemum or feverfew plants. It is a natural chemical that degrades quickly in the presence of oxygen or UV light. It is completely safe to use on food crops, and will not harm your plants. It also just so happens to be the best treatment for fungus gnats. Simply spray the pyrethrum over the soil of your plants or on the roots if you are growing hydroponically. This treatment should be repeated about twice a week for 3-6 weeks to ensure all the hatched larvae have been killed.

3. Diatomaceous earth

Diatomaceous earth is one of the more interesting methods of pest control, and is also completely natural and safe to use. It is made from a group of fascinating microscopic creatures known as diatoms. 

These tiny creatures are made from incredibly hard materials, which become razor sharp when crushed. Diatomaceous earth uses these sharp diatom bodies to lacerate and slice up the bodies of insect larvae. It is one of the most popular and effective treatments for any kind of larvae as well as slugs and snails. 

To use all you need to do is dust some of the diatomaceous earth around your plants roots or mix into the top layer of soil and repeat every 2-3 weeks.

4. Soil occlusion

This is an often overlooked method of control, but is highly effective. The life cycle of a fungus gnat requires newly developed adults to leave the soil to find a mate, and the recently mated adults to enter the soil once again to lay their eggs. By blocking off the soil, it effectively cuts off one of the life cycle stages of these insects. Many will die, and even more will not find a place to lay their eggs. This ends the life cycle and can easily result in the disappearance of fungus gnats completely.

To occlude the soil, you can find pre made covers, or simply make your own from some poly sheeting. Rockwool cubes that come covered in plastic can also be used by leaving as much of the plastic on as possible.

5. Environmental controls

Fungus gnats love high humidity, and higher heats. Stagnant water around the roots, poor airflow, and weak extraction are all highly beneficial for fungus gnat growth. By eliminating any or all of these factors, you can make it very hard for a fungus gnat infestation to thrive. 

Use good quality circulating fans to keep the air moving around the grow room and helping stagnant water to evaporate from the root zone, and ensure adequate ventilation is occurring to remove stale, humid air from the grow and draw in fresh clean air. Also ensure that you are not overwatering your plants, or that your watering cycles are not leaving too much water behind.

Although these pests are not a major threat to your crops, they can be a huge nuisance to have in the growroom or greenhouse. Identifying and addressing these problems early can make a huge difference in preventing a fungus gnat infestation.