Flies in potting soil – fighting fungus gnats

Sciarid gnats are between one and seven millimeters in size. The females lay their eggs in the earth, from which the larvae develop or hatch within a few days. These then develop into small black flies in a further three weeks, which then sit or buzz around the flower pot.

The fungus gnat is often mistaken for the fruit fly. In contrast to fungus gnats, fruit flies are reddish brown, have a thick abdomen and do not lay their eggs in the ground but in rotten fruit. While fruit flies fly in a straight line, fungus gnats fly clumsily and zigzag.

Damage

The fungus gnat causes the greatest damage in the larval stage. During this time, the larvae not only feed on dead, decomposing organic material in the soil, but also on the fine roots of the respective plant. The resulting damage to the roots can cause fungi and bacteria to penetrate the plant and damage it. If the substrate is then still permanently moist, this is usually fatal for the plant in question.

Cuttings and seedlings, but also older plants, are particularly at risk. The adult animals do no harm to the plant itself, they are just very unpleasant. Externally, many small black flies usually indicate an infestation. Damage to the above-ground parts of the plant can only be recognized by the generally unhealthy appearance of the plant in question.
You cannot attach an infestation to certain types of plants. If these parasites have not been introduced with the earth, the cause can also be due to unfavorable culture conditions or incorrect care.

Preventive measures

Hydroponics
In order to prevent these parasites, it is advisable to switch to hydroponics instead of cultivating in potting soil, provided the respective plant is suitable for this.
Advantages: In hydroponics, for example with seramis, perlite, gravel or other mineral substrates, fungus gnats cannot multiply. It is important here that residues of potting soil are removed from the plant or the roots without leaving any residue before it is converted into hydroponics. Another advantage of hydroponics is the good permeability, and watering errors are better tolerated by the plants.
Disadvantages: However, hydroponics are not suitable for all indoor plants. In addition, more regular fertilization and dead roots or dead organic material must be removed again and again.

Watering behavior
When watering , you should make sure that the substrate is always allowed to dry well before watering . The surface of the earth should never be wet. The drier the soil, the better, provided that the requirements of the plant allow it. You should pour more often but not much. In addition, it is advisable to always pour into the saucer, which is usually better for many plants anyway, with a few exceptions.

This does not protect against sciarid gnats, but makes it difficult for them to multiply. However, standing water in saucers should be avoided. Excess irrigation water, which is still in the coaster after about half an hour, should then be removed.Covering the soil with sand Another preventive measure is to cover the potting soil with a 0.5 to 1.0 cm thick layer of quartz sand with a grain size of 1-2 mm. The potting soil should be completely covered with sand. This quartz sand is supposed to prevent the females of the sciarid gnats from laying their eggs in the potting soil. Bird sand is unsuitable because, unlike quartz sand, it also contains lime in addition to numerous additives and, above all, many exotic and citrus plants cannot tolerate it.
Advantage: The advantage is that the plants in question do not have to be repotted in fresh soil.

Combat

Biological control
For biological control, the use of various useful insects, so-called beneficial organisms, is recommended, which can be obtained from specialist shops or the Internet (beneficial insects dispatch) without any problems. These beneficial insects only fight the larvae of the fungus gnat. After a few months, the beneficial insects themselves die off. If the infestation occurs again, the whole thing may have to be repeated. These beneficial insects pose no threat to humans or animals.

Nematodes (Steinernema feltiae)
The use of nematodes is said to be particularly effective. Nematodes are roundworms that penetrate the larvae and release their symbiotic bacteria there. The roundworms multiply and consume the larva completely. The larva then dies within 1-2 days and the nematodes attack the next larvae. The treatment may have to be repeated after 3-4 weeks. If the infestation is particularly severe, the treatment with nematodes should be repeated at shorter intervals.
Nematodes must be used within a few days of delivery according to the manufacturer’s instructions. The application should not take place in direct sunlight, because nematodes are sensitive to light, it is better to use the evening hours. During the treatment, the substrate must not dry out for a period of four weeks, but must also not be too wet.

Predatory mites (Hypoaspis miles)
Predatory mites are also useful insects that can be used both preventively and to combat sciarid fly larvae, among other things. They prefer to be in the upper layers of the soil. The larvae of the fungus gnat are on the menu of predatory mites and are destroyed by them. If there are no more larvae, predatory mites can survive in the soil for several weeks.

Bacterium Bacillus thuringiensis israelensis
In addition to nematodes, bacteria, especially the bacterium Bacillus thuringiensis israelensis, can be used to combat sciarid fly larvae. This bacterium forms permanent spores with protein crystals, which are poisonous for the larvae of the sciarid gnats. These bacteria, like nematodes, are added to the irrigation water and then poured evenly over the potting soil. Both nematodes and bacteria are available from specialist retailers.

Yellow boards or yellow stickers
Yellow boards are only suitable for combating adult fungus gnats, i.e. the little animals that buzz around the flower pots. They are ineffective against their larvae. These boards are provided with an odorless special adhesive and are attached to or near the flower pot.

The fungus gnats are attracted to these sticky stickers or the yellow color, stick to them and eventually die. These yellow or adhesive boards can also be attached as a preventive measure so that an infestation can be identified as early as possible.

Further control measures

Repotting
If only a single plant appears to be affected, it may be sufficient to repot the houseplant in fresh soil. The old soil should be completely removed from the plant and the roots should also be cleaned under running water if necessary.
If the same pot is used again, it must also be cleaned thoroughly. However, there is always the risk that these parasites are already contained in the new earth, so that repotting was ultimately in vain.

Earth sterilize
A more complex but effective method of control is the sterilization of potting soil. This can be done both with new soil and with soil that is obviously already filled with sciarid fly larvae. However, it is usually better to dispose of old, infected substrate.

First you take the plant and soil out of the pot. Spread the earth on a suitable surface and put the whole thing in the oven for about half an hour at 100 degrees. The larvae are killed in the process. It works just as well in the microwave. In the meantime, the roots of the plant are freed from all earth residues and rinsed thoroughly under running water. Then put the sterile soil in a clean flower pot and replant the plant.

Carnivorous Plants
Placing carnivorous plants in close proximity to infested plants can also be helpful, but only against the adult sciarid gnats floating around. The butterwort is particularly recommended here. As a rule, one carnivorous plant per window sill is sufficient. These attract the fungus gnats, which then stick to the sticky leaves of this plant and are decomposed. In this way, an infestation can already be decimated somewhat.

Fine knee highs or fleece
A normal fine knee high can be helpful but not particularly chic. This is pulled over the flower pot in question and carefully tied at the bottom of the trunk. This stocking then has to stay on the pot for 4-5 weeks. This prevents the female of the fungus gnat from laying their eggs in the potting soil. However, this does not protect against re-infestation if the stocking has been removed again. This is always possible.

Matches
If the infestation is still relatively low, you can also try to fight the sciarid fly larvae with conventional matches that you stick upside down in the ground. The sulfur components in the match head are supposed to kill the fungus mosquito larvae. If the infestation is already relatively strong, however, this method no longer works.

Chemical control

• Chemical control should only be used if nothing else has helped.
• In these cases, certain insecticides can be used.
• Thiacloprid or Imidacloprid are ideal here.
• These are added to the irrigation water and then administered.
• When using insecticides, you should always follow the manufacturer’s specifications.
• When pouring it is important to ensure that the entire substrate is moistened.
• The acquisition of such funds is actually only worthwhile for hobby gardeners, as they are relatively expensive.
• You can then use these against aphids, scale insects or mealybugs.
• However, there are a number of biological alternatives against sciarid gnats.
• As a result, purchasing such substances is only recommended in an emergency.

Conclusion
Indoor plants can be attacked by the sciarid gnat or its larvae at any time. The earlier you start fighting, the better the chances of success are of course. However, before resorting to chemical means of control, one should try one of the numerous biological helpers or aids, which is usually better for the plants in question.