I haven’t experienced this in years. Why? I’ve found the ultimate solution to fighting these annoying flies that are so keen to inhabit my plant pots. And it doesn’t involve chemicals, fly catchers or other expensive gadgets.
Sciaridae are a family of flies, commonly known as dark-winged fungus gnats. They look a bit like fruit flies. More than 600 species are known from Europe – over 1,800 from all over the world. They’re between one and seven millimetres long, have slender, darkly coloured bodies and dark wings. The sole purpose of these flies is to reproduce. They live for a few days only and feed exclusively on liquids. The larvae, on the other hand, feed on dead plant remains, fungi and the young roots of your plants.
Incidentally, larvae are one of the most important leaf-decomposing organisms in nature. Since only 10 percent of fungus gnats are male – and males have limited flying skills – their larvae are often found in a group, which makes it much easier to find a partner. Since the 17th century, there have been reports of the army worm: a crawling mass of thousands of fungus gnat larvae that can be as long as ten metres in total.
Fungus gnats either lay eggs into the pot soil from above or find their way into your pot from below by sneaking through the water drain holes. Female fungus gnats lay up to 200 transparent eggs, from which the larvae hatch after seven to eight days. This may well result in several thousand larvae per square metre.
Rule number one is simple: Don’t let your plant pots get waterlogged.
As fungus gnats like damp soil, drying out the soil is a good start. However, not all plants flourish in dry soil and on top of this, this measure will reduce, but probably not remove the fungus gnats completely. There’s always a chance that the population will recover. This method isn’t suitable for young plants either; when you dry out the soil, the flies will start eating the young, fragile roots.
The colour yellow attracts flies, so yellow sticky traps are a good way to fight fungus gnats. This method doesn’t work on all types of flies and won’t catch every single one, but it’s definitely worth a try.
Placing carnivorous plants next to infested pots is a good alternative to yellow sticky traps. Try butterworts or pitcher plants.
Fighting pests by using beneficial organisms is an approach that’s mainly applied in the commercial sector but works just as well on heavily infested flower beds or in winter gardens. Adding nematodes to the watering system is the most common method. Nematodes are tiny roundworms that feed on the larvae’s body fluids. By doing so, the nematodes pass on a bacterium that kills the larvae (Xenorhabdus) within about four days.
Heating soil in the microwave kills eggs and larvae within only three to five minutes. This can be a remedy for young plants that need little soil. Having said that, I might need to point out that you should only heat up the soil. Don’t place the entire plant in the microwave.
If everything else fails, chemical insecticides often serve as a last resort. Although most insecticides are highly efficient in fighting fungus gnats, they’re also very harmful to the environment and therefore not advisable. If you‘re planning to use insecticides on edible or smokable plants, keep in mind that plants take up and accumulate these chemicals. As with fertilisers, don’t use chemicals directly before harvesting your plants.
Switching from soil to a hydroponic medium, as for example clay pebbles, is sure to make all fungus gnats disappear. However, not every plant is suitable for hydroponics and the change-over is not unproblematic, especially for older or larger plants. Repotting plants can damage their root system and – in the worst case – cause plants to die. On top of this, cleaning the roots takes a lot of time and effort.
If all other remedies don’t work, repotting your plants might do the job. But only try this if there’s no other solution. Who knows, the new soil might also be infested or you might end up letting a female fungus gnat in through the window. If this happens, you’ll have to start from scratch.
You don’t want flies in your pots? Then make sure they can’t get in. Don’t worry, you don’t need to build a tiny door or buy an expensive cover, as there’s an easy solution: fine-grained sand.
But not every plant likes sand:
Citrus soil, for instance, has a pH value of around 6.5 – ideal to grow oranges or lemons. Bird sand, however, contains additives such as lime, which increases the pH value of the soil and impacts the success of your harvest.
Nevertheless, I always use bird sand – it’s available everywhere, it’s cheap and its increased pH value hasn’t harmed any of my plants. If you don’t want to risk it, go for quartz sand.
Cover the plant soil with a thin layer of sand (1-2 cm). This prevents flies from getting in and out.
If fungus gnats can’t get in through the surface of your soil, they’ll try to sneak in through the water drain holes. Make sure both access points are closed off. Here’s how:
This way, there’s not much soil for fungus gnats to enjoy (only the tiny bit above the water drainage holes) and they won’t manage to take over your plant pot.
Since I started using this method, I’ve seen hardly any fungus gnats. If your plants are in desperate need of help, try yellow sticky traps in addition to sand. This should get rid of unwelcome intruders in no time.
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