Fungus gnats are members of the Sciaridae family. They’re small (3-5 mm), dark, mosquito-like flies with long, slender antennae and long legs. It is particularly the Fungus gnat larvae that do the most damage.
Fungus gnats are especially fond of warm, moist environments in the proximity of plants. They can also survive the whole year round in greenhouses.
After mating the females lay 50-200 eggs that hatch within 2-3 days. The larvae over the course of a 2-3 week period go through four stages, at the end of which they are around 5 mm long, transparent white and have a noticeable black head.
The larvae pupate in the soil and appear three days later as adult gnats. With temperatures higher than 24° C, they reproduce continuously and with a 3-4 week life cycle, rapidly. The larvae feed not only on dead organic material such as moulds and algae, but also on living material like root- and stem tissue. They bore into the root and/or stem of cuttings, seedlings and young plants. Thanks to the feeding damage, a number of routes to secondary infection are opened up, giving plant diseases such as Pithium, Phytophtora, Botrytis, Fusarium and Verticillium an opportunity.
NB: Observe also the characteristic patterning in the wings
The larvae can cause direct damage by nibbling on roots and stems – they can even bore right the way through them. Indirect damage is also caused thanks to the larvae spreading nematodes, mites, mould spores and viruses.
Clear away dead organic material and use good quality compost.