Fungus gnats take their name from the fact that they feed on fungi and decaying plant material. They can be a common pest where humidity and moisture levels are high.
SYMPTOMS & IDENTIFICATION
These little bugs are about the same size as a fruit-fly, although they are darker in appearance. They are attracted to the moisture of the soil, and so are mostly seen walking around on soil surface, although you might also be able to spot them flying around the pot from time to time. Other signs to look out for are:
Root damage - Fungus gnat larvae feed on the roots of plants, so seedlings and small plants may get damaged
Yellowing leaves - Most plants don't get damaged from the fungus gnat, but yellowing of the lower leaves might occur (although it can actually be a symptom of too much water - which fungus gnats are attracted to).
Treatment: Fungus gnats actually don't do much harm to your plants, if any. But if you still decide that you want to try to make the flies go away, then you can try the following:
1. Make sure the soil dries out between waterings
These gnats are attracted to moisture in the soil, so ensuring that it gets the chance to dry out properly in between each watering will help reduce the risk of attracting them in the first place, and will deter them from staying
2. Use yellow sticky fly traps
These fly traps are readily available - you can pick them up from your local garden center or home improvement store
Position them near your plant (but clear of its canopy)
This will provide a way to see just how large the infestation is, as you will be able to physically see just how many flies they catch and how often you have to replace them
3. Build a cider-vinegar trap
Fill a shallow bowl with water, liquid soap and some cider-vinegar
Place it near your infested plants or near a window or source of light (adult Fungus gnats are attracted to light)
The gnats will be drawn to the smell of the vinegar and try to land, causing them to drown
This is a cheap and effective way of eliminating any Fungus gnats
You can even keep a trap near your plants as a preventative measure - the smell is non-invasive, and you could even use a decorative jar/container so that it’s less noticeable
4. You could also consider using Bacillus thuringiensis
This is a type of bacteria that works well as an insecticide
It’s best to spray either early in the morning or in the evening, as Bacillus thuringiensis doesn’t do well in sunlight
Note that this type of pest control also has a shorter shelf-life than typical chemical insecticides
Bacillus thuringiensis targets the larvae, so try to time the application to the larval stage if possible
CAN I AVOID THIS FROM HAPPENING AGAIN?
As mentioned above, you could consider leaving a cider-vinegar trap near your plants at all times to help prevent another infestation from taking hold - just remember to top it up on a semi-regular basis when the vinegar evaporates.
QUESTIONS ABOUT FUNGUS GNATS
WHAT DO THEY EAT? Their main food is fungi, leaf mold and organic material in the soil, so maintaining your plant’s general health will help to reduce the risk of drawing in Fungus gnats.
DO THEY BITE HUMANS? Luckily, Fungus gnats don’t really do any damage to plants or humans, so you don’t have to worry about them biting you.
HOW LONG DO THEY LIVE? A generation of Fungus gnats can be produced within around 17 days, although in warmer temperatures this can be accelerated. Adults only live for around 1 week, but can produce up to 300 eggs which have a relatively short gestation period (around 4 days), meaning that multiple generations can exist in the same pot. Therefore, if applying an insecticide, it may be necessary to use multiple applications to ensure that all eggs are eliminated.
CAN THEY FLY? Fungus gnats can certainly fly, however, they are quite weak fliers so are more likely to be found close to their host plant or near a window.
ARE MY PLANTS AT RISK? If your plants frequently stand in damp, poorly-drained soil then they may be more likely to attract Fungus gnats. Ensure that your pots have drainage holes and that you use a potting mix containing a higher proportion of perlite/ sand if you feel that it is not draining well. Also check your plants for organic debris. By making your plant’s conditions less hospitable for Fungus gnats, you should be able to deter them from taking hold in the first place.