The best natural remedies to combat the fungus gnat

When we think about pests that affect our plants, we usually associate them with the warmer months when bugs tend to appear. However, we shouldn’t let our guard down during the colder months because some insects emerge when temperatures drop. One of these insects is the fungus gnat, which can silently ruin a crop.

What is the fungus gnat?

This dark-colored fly, belonging to the dipteran family, is often mistaken for a mosquito. They are small, measuring 1 to 4 mm, and live only a few days, less than a week, and love the moist soil in indoor plant pots, the perfect place to reproduce and die. They lay eggs in the substrate, up to 400 in each batch. Within a few days, larvae hatch from these eggs and begin feeding on the roots to grow. After three weeks, they become pupae and then undergo metamorphosis into adult flies.

As you can see, these insects reproduce rapidly and in large numbers, which can cause severe, sometimes irreversible, damage to plants.

How to detect it?

Sometimes, detecting a pest of fungus gnat isn’t easy until it’s too late. When a plant is infected, it may show several symptoms. Firstly, the plant stops growing, and its leaves begin to lose color and strength. Additionally, fungus gnats can carry various types of fungi such as Pythium, Phytophthora, Fusarium, Verticillium, or Botrytis, which could ultimately kill the plant.

The best way to identify fungus gnats is to inspect the substrate, where they live. When watering, you can gently tap the pot to disturb the soil, at which point you might see adult gnats scurrying across the soil or flying away. They don’t usually move around the leaves or the plant’s surroundings but instead stay on the soil almost all the time.

Why do fungus gnats appear?

This pest is very common in agriculture and gardens, especially in greenhouses or indoor grows. It tends to appear when the environment has humidity above 70% and a temperature over 23ºC, conditions that are easily met in indoor cannabis cultivation.

Additionally, these gnats love constant moisture and are particularly attracted to waterlogged soils.

How to get rid of fungus gnats

Once we identify the presence of this annoying pest, it’s time to act quickly. There are several possible solutions.

Drying the soil

As we’ve seen, fungus gnats love very moist soil, so it can be helpful to let the top layer of the pot dry out between waterings. The problem is that this measure might not be ideal for all types of plants.

Removing dead leaves

Another important step is considering that this insect feeds on decomposing organic matter. So, if dead leaves remain on the substrate, they provide food for the gnats and encourage their development.

Using vermiculite

Another helpful action is covering the substrate surface with a natural amendment such as vermiculite. This prevents gnats from laying new eggs and hatching existing ones in the soil. Simply spread a layer about 3 centimeters thick.

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Phototropic Traps

This type of trap consists of adhesive strips with a bright color, usually yellow, that attract insects, causing them to stick to the surface. They are not effective in completely eradicating the infestation since the problem is concentrated in the substrate. However, they help reduce the number of flies and allow for accurate identification of the insect responsible for the issue.

Potassium Soap

Potassium soap is one of the most commonly used products in cultivation for controlling certain pests such as aphids, whiteflies, and fungi like powdery mildew, sooty mold, or fungus gnats. Additionally, it provides an extra dose of potassium to the plant. In this case, the product is applied through irrigation water. If the leaves are also developing fungal infections, the use of a fungicide should be considered.

Neem oil

Neem oil is an eco-friendly insecticide widely used worldwide for its effectiveness. It is derived from the fruit and seeds of the neem tree, native to India. The active ingredient in this substance is a toxic compound called azadirachtin, which works both by contact and ingestion during the larval stage.

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Using nematodes

Nematodes are microscopic worms that are sold in a special powder form to be mixed with irrigation water. These nematodes attack larvae to parasitize and feed on them. As this is a biological solution, i.e. it uses living organisms, it is necessary to ensure that they have the optimum conditions for them to live. Thus, the soil must be somewhat moist at the time of pouring the nematodes. 

Diatomaceous earth 

Diatomaceous earth is a powerful natural remedy against fungus gnats. Made from fossilized microalgae, its sharp particles damage insects’ protective layers, causing dehydration and death. It only targets adult flies on the surface, not larvae underground. For best results, combine it with other methods like potassium soap.

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Replacing the substrate

This is the most drastic solution and should only be used if the situation is severe. It involves tackling the problem at its root—literally. The downside is that changing the substrate can stress the plant, and there’s no guarantee that eggs or larvae won’t remain, so it’s still necessary to combine this method with others for complete effectiveness.

These are some remedies to combat a fungus gnat infestation, but prevention is always best. This means maintaining proper humidity levels and avoiding overwatering, allowing the top layer of the substrate to dry completely, as mentioned at the beginning of the post. Vermiculite can also be used anytime to prevent these infestations.