Fungus gnats and their control

Fungus Gnats (aka Sciarid Fly)

Fungus Gnats are a huge problem in the nursery industry, especially in greenhouses and now, also, in indoor growing.

The adults are small black colored flies, about 3 millimetres long (1/8 “) that prefer to walk rather than fly, although they are quite capable of flight and that’s how they get from place to place. They are usually found on the surface of a damp medium and also in areas that are equally wet. They are most often found in cocopeat and soil media but also in NFT and other inert run to waste systems.

They lay their eggs in the soil. The larvae, which are clear colored with black heads, eat organic material and are particularly fond of plant roots. They are 1/24th” to 1/8th “ (one to three millimetres) and live in the medium amongst the plants roots. After the maggots have eaten their fill of roots, they then pupate for a few days before hatching into adults, mating and starting the cycle all over again by laying eggs. From egg to adult takes about three weeks at 72oF (22oC), more in cooler conditions and quicker in warmer conditions.

They cause damage to plant roots by feeding on them. Unfortunately, this damage exposes the plant roots to attack by fungal disease. The maggots also carry fungal diseases so they help spread root diseases as well as damaging roots. In one study, the maggots were shown to be carrying the spores of the diseases Pythium, Botrytis, Verticillium, Fusarium and Thielaviopsis. Even without the added pressure of diseases, roots damaged in this way can’t take up water and nutrients. If the plants are in flower, root growth has virtually stopped in favor of flowering so the plants can’t easily replace damaged roots.

Control is not easy but there is at least one good product available that is non toxic and will control these pests.

Ask your retailer for the best control.