Problem Identifier / How to Get Rid of and Kill Fungus Gnats

Where Do Fungus Gnats Come From?

The fungus gnat is a small fly. This pest infests plant containers, potting mix soil, and all types of organic decomposition. The main food source for fungus gnat larvae is organic matter found in the soil and fungi, but these insects are also known for chewing on roots. This is a major issue for indoor plants, potted plants, nurseries and greenhouses.

The fungus gnat is about the same size as a fruit fly, and generally found attacking plants indoors. You may also see fungus gnats on cannabis. Fungus gnats are attracted to the moistness of potting soil, with the adults laying as many as 200 eggs near the surface in organic matter. They then burrow deeper into the soil to feed on decaying material and fungi.

Fungus Gnat Larvae Damage
Fungus gnat larval feeding damage on poinsettia. PHOTO CREDIT Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University, Bugwood.org

In about two weeks, the adults come out of the soil, and the process is repeated. The life of a gnat is roughly seven days. Fungus gnats do not harm humans because they are unable to spread disease or bite. The main concern is damage from fungus gnats on cannabis and houseplants due to an exploding population. Once fungus gnats reproduce, they are difficult to eradicate.

Once fungus gnats start feeding on the roots, plants can die due to root rot. Seedlings are endangered due to the spreading of plant pathogens called Pythium. When there is an infestation, consistent prevention and management techniques are necessary to get rid of them.

Identifying Fungus Gnats

Adult fungus gnats are tiny, with the size usually between one-sixteenth and one-eighth of an inch. This is approximately the same size as fruit flies, which is the reason the pests are often mistaken for one another. The gnat larvae are a maximum of one-third inches long. Adult fungus gnats have a blackish-gray coloration, with either transparent or gray wings.

Despite the smaller size, the appearance is like a mosquito due to the long antennae and legs. The body of a fungus gnat is thinner than a fruit fly, with long antennae and legs. The body of the larvae is white or transparent, with a little black head. Both wings have veins in the shape of a Y. They spend most of their time on the surface of soil but may be seen flying around the plant’s drainage holes, or the pot’s outer edges.

Fungus gnats are not good fliers, so they can often be found walking on the soil. The flight is limited to short bursts. The flight of a fungus gnat is a lot slower than fruit flies, with an erratic nature like a mosquito. Despite being harmless to humans, gnats are extremely annoying because they tend to fly into drinks and people’s faces. Gnats can be killed easily with a single swat.

Fungus Gnat Life Cycle

The development of gnats consists of four different stages: the egg, larvae, pupa and male and female adults. The larval stage is often referred to as instars. Numerous generations of gnats are produced every year. Adult females lay yellowish-white eggs either separately or in clusters in cracks and crevices on surfaces, and in organic and moist debris.

Gnats prefer to lay their eggs on growing fungus. When the plants are located indoors, eggs are laid throughout the year. The larvae require approximately two weeks for feeding prior to pupating close to the surface of the soil in the thread chambers. The gnats remain in the pupal stage between three and seven days. Once adults, the lifespan of a gnat is roughly eight days.

The development from an egg to an adult takes three to four weeks. The specific lifespan depends on the temperature. As the temperature drops, the time necessary for development increases. An entire generation of gnats takes approximately 17 days to produce but is impacted by the temperature. Warmer weather enables faster development, meaning the number of generations produced in a year vastly increases.

The number of generations per year often overlap. Gnats are common in the spring and winter in areas where the temperatures are milder such as parts of California. During these seasons, gnats are also attracted to the additional water supply. When indoors or in coastal regions, gnats can be seen throughout the year regardless of the season.

What Does Fungus Gnat Damage Look Like?

The most common plant damage caused by gnats includes:

  • Root rot
  • Yellow or prematurely dropping leaves
  • Wilted leaves
  • Slow plant growth
Fungus Gnats Cannabis
Fungus gnat damage on cannabis or any plant can resemble everything from under or overwatering to nutrient deficiencies. Once you have an infestation, damage to the roots will cause nutrient issues, eventually leading to the death of the plant.

When the number of gnats is small, they are simply an annoyance, not causing any harm to plants. Once there is an infestation, the plant roots serve as a source of food for the larvae, leaving severe damage behind. The damage is much worse for seedlings due to the delicacy of the roots. A fungus gnat can also spread plant pathogens eventually causing the seedlings and plants to die.

The damage looks a lot like typical root issues including root rot. The lower leaves often turn yellow, before falling off prematurely. The growth of the plant will either slow or completely stop. If there is a major infestation, the entire plant will wilt, then start to die due to the excessive damage to the roots.

How to Get Rid of Fungus Gnats Naturally

You can get rid of fungus gnats naturally by using any of the options below:

  • Apply Trifecta Crop Control to the top layer of medium
  • Add gravel or pebbles in a thin layer on top of the plant
  • Use sticky traps
  • Clean out catch trays
  • Dry out the soil
  • Remove and discard the surface layer of the soil
  • Target pests in the immature stages as opposed to adults

As persistent as fungus gnats can be, we have had tremendous success in wiping them out completely. An application of Trifecta Crop Control at 2oz per gallon to the top 3/8” of soil is recommended and found to be effective. Apply by spraying, making sure the top 3/8″ of growing medium is saturated. Click here to view and/or download our full application guidelines.

Not only is a thin layer of gravel or pebbles on the surface of the potting soil pretty, but it prevents gnats from laying eggs. Gnats lay their eggs on the surface layer of soil, where they hatch. This is not possible in either gravel or pebbles. Thoroughly cleaning the catch trays will eliminate the gunk a fungus gnat uses to lay their eggs on.

Get Rid of Fungus Gnats
Sticky traps are a great way to help get rid of fungus gnats.

Indicator cards or sticky traps can be used for both cannabis and house plants. These papers are free of pesticides, inexpensive, and coated with a sticky material. Indicator cards are placed on the growing medium. The fungus gnat becomes stuck to the sticky surface and can then be disposed of. If there is any uncertainty as to whether the infestation is fruit flies or gnats, sticky papers will take care of both pests.

Gnat larvae require moist soil. This means allowing the soil to dry out is effective. The plants should not be watered until the top layer of soil is dried out. This will eliminate any gnats already in the soil, while discouraging the adults from laying more eggs. Most of the gnat’s life is spent in soil or organic matter as a larva or pupa. The best way to control fungus gnats is by targeting the immature life stages.

Controlling mobile adults during their short life cycle is much more difficult. The best way to eliminate issues with gnats is through the consistent decrease of organic debris and moisture.

To make certain all the larvae have been killed, the process should be repeated in two to three days. Removing the surface layer of soil will not prevent the eggs from hatching, but the soil can be stored in an airtight container to kill the pests, while keeping them away from the plants. Never move infected plants from one area to another.

How to Prevent Fungus Gnats Naturally

The best natural prevention methods for fungus gnats include:

  • Do not overwater plants
  • Water plants from the bottom
  • Keep extra soil by storing it in an airtight container with a tight lid
  • Regular inspection of plants

Issues with gnats generally occur late in the fall, or during the winter months for two reasons. First, houseplants kept outside when the temperature is warmer are often a breeding ground for gnats. Second, if these plants are brought back inside there is usually an increase in the population because the temperature is warmer. Fungus gnats on cannabis can generally be found through a visual inspection.

The adult fungus gnat can be seen resting on soil, plants, walls or windows. Sometimes, they can even be seen in flight. In addition to looking for the adults, look at any potted plants where there is organic debris or moist conditions to find the larvae. The adults can be trapped effectively with yellow sticky traps. You can find larvae by cutting a raw potato into chunks, then placing it in pots with the peel side up.

Do not overwater the plants because neither larvae nor adult gnats can survive when the soil is dry. Using moisture meters for correct moisture content of soil helps to dry the top layer of soil out while having no negative impact on the plants. Keeping the top layer of soil dry helps reduce colonization of fungus gnats. Do not move any infected plants to a different area. This will result in the infestation of even more plants. If the infestation is severe, consider throwing away the plant.

Your potting soil should be stored above the ground in a sealed container. This will prevent any infestation from occurring until the soil is used. Watering plants from the bottom whenever possible will keep the topsoil where gnats lay eggs dry. The plants will still have enough hydration to thrive.

Do not keep potting soil in the same bag it was packaged in because gnats can enter the bag easily and lay their eggs. Gnat eggs require air for survival. This means the containers will prevent both larvae and adults from entering. Containing any infestation before it increases is critical because a large infestation will kill cannabis plants.

Fungus Gnats in Greenhouses Using Biocontrol

You can biologically control fungus gnat larvae by using Hypoasps miles, a type of predatory soil mite or Steinernema feltiae, an insect-parasite nematode. You need to begin these biocontrols when your population of gnats is still at a lower level. Your options for parasitic nematodes include a watering can, a power sprayer – provided you have removed the screens, and an irrigation system.

Hypoaspis miles is a good choice for controlling fungus gnat larvae, and often they feed on other insects in the soil including shore fly larvae, thrips pupae and springtails. Hypoaspis miles and parasitic nematodes can effectively be used together. You need to use this method as a preventative measure. Correct cultural control treatments are essential when controlling fungus gnats in plants.

The curative rate is between 45 and 200 fungus gnats for each square yard. The preventative rate is between 35 and 70 fungus gnats for each square yard. You can have Steinernema shipped direct in shaker tubes. When using Steinernema, follow the tips below for the best results.

  • You should not store Steinernema feltiae for extended time periods.
  • Once you have inserted cuttings or sowed seeds, treat your crops as quickly as you can.
  • Apply Steinernema feltiae directly to your crops during the evening.
  • If your crops are slow growing, nematodes need to be reapplied at intervals of six weeks.
  • Wait seven days after applying nematicide prior to using Steinernema.
  • When applied, the temperature of your soil should be a minimum of 55 degrees Fahrenheit.
  • If the fungus gnats have already established a population, you need to allow between two to three weeks to see a decrease in their numbers.
  • During the application, your nozzle screens need to be a minimum of 50 mesh. You can also use coarser screens.
  • Water prior to and after completing your application because nematodes need moisture to be able to move.

When using Hypoaspis, follow the tips below for the best results.

  • Once your mites have been delivered, they need to be introduced to your crops as quickly as possible.
  • To achieve good activity, the minimum temperature of your soil is 60 degrees Fahrenheit.
  • The mites should be compatible with both beneficial nematodes and BT.
  • You can use mites as a preventative measure, mix them with your growing media prior to planting or sprinkle them on your surface soil.
  • Your soil should be moist but make certain it is not wet.
  • There are numerous pesticides negatively impacting Hypoaspis mites. Talk to your supplier or look at your compatibility table when controlling either pests or other diseases.
  • When using mites during planting, you should retain control for a period of six to eight weeks.

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