What are fungus gnats?

The autumn sciarid fly (fungus gnat) season is about to be upon us. Fungus gnats are at their most active in September and March, which means that larvae are growing and pupating in August and February.  This is the time to start thinking about gnat management and pro-actively controlling them.

Adult fungus gnat

Description

Sciarid flies and Glasshouse wing-spot flies are almost indistinguishable (unless examined under a microscope). They are slender flies, about 5 mm long, a dull grey colour with long antennae and large compound eyes.

The larva is a small maggot, about 7mm long, with a small black head, translucent body and no legs.

There are many species of fungus gnats, or sciarid flies, most of which are entirely harmless. The adults only live a few days – they mate, lay eggs and die. If the eggs hatch, they will be a problem next spring when they emerge as adults.

Generally. It is the adult flies that are the biggest nuisance due to their habit of flying in an erratic manner around people’s heads. They do not settle and when waved away return almost immediately. The flies are often confused with midges and mosquitoes and falsely accused by people in the vicinity of causing insect bites. In fact, their mouth parts are not able to penetrate human skin.

Fungus gnat larvae on potato slice.

The larvae feed on fungal growth and decaying plant material but some species can also damage the roots of seedlings or tunnel into the base of soft cuttings. When fully fed, the larvae pupate in the soil. During the summer the life cycle can be completed in about a month.

Control

Sciarid flies can only breed in damp soil, so if the top few centimetres of soil is dry, then they can’t successfully breed and the eggs won’t hatch. To keep the top layer of soil dry but still take care of your plants, use sub-irrigation watering methods.

Biological control: A nematode called Steinernema feltiae (synonyms. S bibionis, Neoplectana
carpocapsae) is commercially available. It works by entering the larva and releasing digestive bacteria that convert the larval tissue into nutrients for the nematode. The nematode then breeds and produces more nematodes to infect other larvae.