Fungus Gnats On Tomatoes: What To Do?

Last updated on October 23rd, 2023 at 08:35 pm

Tomatoes are often attacked by various pests, such as fungus gnats. The small black insects are harmless to humans, but can cause a lot of trouble for tomatoes. We show you how to recognize, combat and prevent an infestation!

Fungus gnats (Sciaridae)

Fungus gnats belong to the family of species within the two-winged moths and to the suborder mosquitoes (Nematocera). The insects are distributed worldwide and prefer a humid climate. They are only a few millimeters in size and have a black, slender body structure, which is why they are often confused with fruit flies. The insects do not have a stinging proboscis and are therefore harmless to humans. However, they can pose a threat to tomatoes, with cuttings and potted plants being particularly frequently affected. However, it should be noted that it is not the fungus gnats themselves that damage the plants, but their larvae:

  • Larvae are about 5 – 10 mm in size
  • gray-white colored and black head
  • feed on dead plant tissue
  • but also from healthy roots
  • live in the soil
  • especially potting soil often affected
  • occur in summer as well as in winter

Fungus Gnats On Tomatoes: What To Do?

Note: fungus gnat larvae are often brought in with the purchase of potting soil.


Whether tomatoes are infested with fungus gnats can usually be recognized by the pests themselves. The black gnats are often noticeable when the tomato plant is gently wiggled back and forth. It is even possible that the white larvae can be seen in the substrate itself. Damage to the plants is most apparent during the larval stage, with symptoms including the following:

  • shrinking of seedlings and cuttings
  • stunted growth
  • hollow-eaten stems

Control fungus gnats

Fungus gnats can usually be controlled effectively and even biologically or with home remedies. To detect an infestation and control adults, it is worthwhile to apply yellow stickers. These are simply stuck into the soil and literally attract fungus gnats due to their yellow coloration. The pests stick to them and eventually die. It should be noted, however, that yellow stickers have no effect on the larvae, which are basically the actual pests.

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Biological control of fungus gnats

To effectively combat fungus gnats, the larvae must be tackled. For this purpose, the amateur gardener has a variety of different means at his disposal. Of course, there are also chemical preparations, but these should never be the first choice. It is better to first resort to biological control methods. For this purpose, the following methods have proven to be extremely effective:


Nematodes are among the nematodes that are tiny and often found in the soil. The little worms can be both beneficial and harmful. They have proven to be popular beneficial insects in the fight against fungus gnats, as they eradicate most of the larvae within 24 to 48 hours. Nematodes can be purchased at specialty stores and used to control fungus gnats on tomatoes as follows:

  • Add nematodes to the irrigation water
  • about 0.5 million per m²
  • get into the soil during watering
  • penetrate the larvae via body orifice
  • release symbiotic bacteria into their bloodstream

Note: When using nematodes, the soil should have a temperature between 15 and 25 degrees Celsius. Likewise, the substrate should be kept moist at all times, but excessive watering should be avoided.

Predatory mites

An alternative to nematodes are predatory mites, which are often contained in peat substrate. These beneficial insects can be bought with peat in specialized stores and can be used specifically against fungus gnats. Predatory mites are considered a natural predator of fungus gnats, as they eat their larvae. It is also practical that the beneficial insects can live for several weeks without food and are therefore also ideal for the prevention of fungus gnats. To get rid of the larvae with predatory mites, it is best to proceed as follows:

  • about 125 animals per m² for prevention
  • double the amount for control
  • repeat after 2 weeks in case of heavy infestation
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Note: Predatory mites can only be stored for a few days at a temperature between 15 and 20 degrees Celsius. Under no circumstances should the temperature fall below eight degrees Celsius, otherwise the animals could die. Therefore, storage in the refrigerator is not recommended!

Home remedies against fungus gnats

Many hobby gardeners report successful control attempts with various home remedies. However, it should be noted here that these have often proven effective only with a small infestation. If the fungus gnats have not yet spread or multiplied too much, an attempt to control them with the following home remedies might be worthwhile:


One of the most popular home remedies against fungus gnats are conventional matches. The red, flammable spot contains sulfur-containing substances that have a toxic effect on the larvae of the fungus gnat. Accordingly, they are very often used in combination with yellow stickers. The matches are used against fungus gnats as follows:

  • take several matches
  • stick them upside down into the ground


Garlic, like matches, contains sulfur-containing substances, namely allicin. Therefore, the bulb is also suitable as a home remedy against fungus gnats. The application is very simple and works as follows:

  • Cut the garlic open and stick it into the soil
  • or cut into small pieces
  • spread on substrate
  • Parsley and chives
Fungus Gnats On Tomatoes: What To Do?

fungus gnats do not like the smell of parsley and chives, so they avoid them. Hobby gardeners can take advantage of this aversion, and grow the herbs together with tomatoes. After all, parsley and chives can be cultivated together with tomato plants without any problems anyway. However, if you don’t have enough space, you can use the herbs to repel fungus gnats as follows:

  • Cut herbs into small pieces
  • sprinkle on potting soil

Prevent fungus gnats

Although fungus gnats can be controlled very well, their prevention is still the best choice. Since the insects are often dragged directly into the house with the potting soil, the substrate should be checked before planting the tomatoes. To do this, first take a sample of the soil and put it in a seed tray. A yellow sticker is now placed in the immediate vicinity and the soil is kept moist from then on. If there are fungus gnats on the yellow stickers after about three weeks, the soil is most likely infested. In this case, it is worth sterilizing the soil in the oven before using it:

  • Put potting soil in fireproof container.
  • Slightly moisten the substrate
  • preferably with a hand sprayer
  • Place the lid of the container loosely
  • sterilize substrate at 150-180 degrees Celsius
  • for about 30 minutes
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However, sterilizing the soil does not guarantee that tomatoes will not be attacked by fungus gnats. However, preventative measures can be taken throughout the tomato season to minimize the risk of infestation:

  • avoid permanently damp areas
  • always ventilate well, especially in the greenhouse
  • Allow the soil surface to dry out before watering the next time
  • avoid waterlogging
  • when cultivating in tubs – use pots with a drainage hole


  • James Jones

    Meet James Jones, a passionate gardening writer whose words bloom with the wisdom of an experienced horticulturist. With a deep-rooted love for all things green, James has dedicated his life to sharing the art and science of gardening with the world. James's words have found their way into countless publications, and his gardening insights have inspired a new generation of green thumbs. His commitment to sustainability and environmental stewardship shines through in every article he crafts.

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