Fungus Gnats, which belong to the order of Diptera, look similar to mosquitoes, are a nuisance and can cause serious havoc on plants, but are harmless to people and animals. Around 2 to 3 mm (1/16 to 1/8 in) long, their delicate bodies are black and feature greyish-transparent wings, antennae and thin legs. In spite of their wings, they can hardly fly and typically crawl or or hover just above soil/ window sills.
The vast majority of fungus gnats are females. Each female is able to lay between 100 and 300 eggs into the little crevices within the soil. After hatching 4 to 6 days later, the larvae then feed on the plant's roots for a few weeks. They then go into a pupae-stage, only to re-emerge as adults and start over within under a week.
As fungus gnats are attracted to damp conditions and organic matter, like potting mix/ compost, for instance, they are often the result of high humidity, condensation in windows, poor ventilation and dampness around leaky pipes.
Allowing plants - or rather their soil - to dry a little between watering where possible; carefully checking and quarantining new plants or plants returning indoors for the winter; removing debris (dead plant material, etc) promptly and providing adequate ventilation all help to prevent infestations.
Signs of Infestation
Because fungus gnats tend to be drawn to sources of light, large groups can often be spotted near doors and windows. On the plant itself, they are usually only visible when disturbed by watering or blowing on the plant. The adults typically do not feed on the plant, but their larvae, which remain invisible in the soil, feed on organic matter - including the plant's feeder roots and delicate root hairs - within the soil. As a result, the plant loses its vigour, growth is poor and the plant wilts. Eventually, foliage turns yellow and drops off.
Dealing with Infestations
In greenhouses and gardens, natural predators - rove and/ or ground beetles - can be used to combat infestations. On house plants kept indoors, this is naturally not a suitable solution.
Non-toxic yellow sticky traps placed next to plants or hung nearby are, however, an excellent way of monitoring and catching adult fungus gnats, while larvae can be removed and destroyed after drawing them to the surface with a couple of chunks of raw potato placed onto the soil (a couple of hours should do the trick).
Weekly soil drenches - preferably done outside - will also help. Naturally, plants should be allowed to thoroughly dry and air well before bringing them back inside. Another effective measure is to install 18 Watt insectalite fly traps, which use ultraviolet light and pheromones to attract insects. Home-made sprays containing garlic and/ or hot pepper also help to deter these little monsters.
As a last resort, chemical pesticides containing pyrthrins or pyrethroids can be used to kill fungus gnats (adults; must be administered every two to at most three days for at least a fortnight), while Bacillus Thuringiensis Berliner var. israelensis allows effective control of larvae.Directions and safety precautions given by manufacturers of such pesticides should obviously be read and followed carefully.