Fungus gnats are an odd one in the gardening world. They have a habit of striking fear into the hearts of novice gardeners, they irritate horticulturalists and are overall unsightly but, in general, they aren’t actually a huge problem in and of themselves.
They are usually more of an indicator of overwatering or poor ventilation. That said, they can do considerable damage to seedlings and cuttings during their larval phase.
With so many tiny ‘flies’ that affect our plants, it’s easy to misidentify sciarids. At a glance, they are black, though closer inspection shows them to be greyish-brown.
The easiest way to tell sciarids apart from aphids is that they are very active and they don’t cluster tightly in one spot, as above.
Fungus gnats are twitchy and skittish, running about and flying at the slightest disturbance and rise in clouds where infestations are bad. The larvae are difficult to see on the soil surface, but are translucent yellow-brown, a black head, and leave a slime trail in damp conditions.
The speed at which sciarids reproduce is nightmare fuel. Females lay up to 200 eggs and they mature to adulthood within a month. Of this, it’s the two weeks as larvae that causes the damage to your plants.
They eat fungi and debris in the soil, but will also turn to root hairs, and the delicate new roots of cuttings and seedlings are especially vulnerable.
Before your plants are affected, you’ll generally see the adults flying about in clouds. The actual plants themselves will show symptoms synonymous with overwatering, so yellowing leaves and wilting stems.
On closer inspection, you’ll see the maggots in the soil. If not - and you haven't seen any adults flying about - it is just as likely that overwatering has caused the issue. If you see a grey powdery mould, then it's botrytis.
For indoor plants, as larvae are in the growing medium, it’s difficult to avoid bringing them into the home, unless you quarantine first, which is impractical in most cases.
Sticky yellow traps will prevent adult numbers growing along with good sanitation and ventilation.
Biological controls, such as nematodes Steinernema, bacteria Bacillus thuringiensis and the mite Hypoaspis are effective on smaller numbers, but repeated treatments are needed, therefore prevention is really better than cure with fungus gnats. Chemical controls, particularly pyrethrins, will work on adults but are ineffective on the larvae. Early identification and prevention are the best measures when it comes to sciarid control.