Article 1-2 Fungus Gnats

This question deals with a successful grower who’s created a productive garden with good growing conditions, so many potential causes of the problem (overheating, crowding, mites) can be eliminated. He’s already had these problems and corrected them!

GnatEven experienced gardeners can be puzzled by fungus gnat symptoms. The larvae of these pests can destroy a garden, working out of sight as they chew the plant roots and drain the sap. Even the adults – tiny flies that hang around the bottom of the plant and run across the surface of the grow medium look harmless. Usually, growers only see a few tiny flies, and sometimes the flies lay their eggs near the plant’s roots and escape unnoticed by the gardener.

This hidden activity by fungus gnat larvae separates gnats from top growth – attacking insects like thrips or spider mites. Even careful examination of the root zone may miss these tiny larvae – the grower would see only damaged and discolored roots. Meanwhile, the baby bugs are:

  • Chewing and damaging root tissue, interfering with nutrient and water uptake.
  • Sucking sap from roots that was necessary for the needs of the plant. -Infecting the damaged roots with fungus disease.

This last activity is the reason these insects got their name – they carry disease spores on their bodies that can infect the damaged roots easily, creating more problems for the grower.

New fungus gnat problems in a garden usually occur in autumn (as cooler weather forces insects indoors) or spring (when over-wintering eggs outdoors hatch and the flies find their way into the grow room). Continuing fungus gnat problems can happen anytime of the year, indicating that an infected plant somewhere in the garden or nearby (house plants, or outdoors near the indoor garden) is serving as a continual source of these pests. Often the problem is traced to stock plants, which are usually neglected, old, and root bound

And good riddance! Because of the severe damage these pests can inflict on a garden, store staff and growers must be aware of how to identify and handle them. Bright yellow leaves – normally shaped, no wrinkles or spots – and very slow growth are strong clues to their presence in the garden. Have growers search for “tiny flies – like fruit flies” hovering near the base of the plant or on the grow medium. Once spotted, urge immediate treatment of all plants, not just the ones that look sick – the larvae can already be present and start to damage plants that still look healthy, and untreated larvae turn into more flies to re-infect the garden.

Plants recovering from fungus gnat problems still face the risk of disease problems – remember that these bugs can spread fungus spores to damaged roots. As a precaution, these plants should be given a treatment with a fungicide a day or two after pesticide application. A root drench is more effective than spraying the top growth. Follow a similar procedure to your use of pesticides, drenching the entire root zone with fungicide solution, with irrigation pumps off for at least several hours. Left over fungicide in the root zone will not interfere with nutrients, so it’s not necessary to drain and replace the fertilizer mix.

Yellow sticky cards are very useful as an early warning system for these flying pests, since the gnats are often attracted to the bright yellow surface. Soon, new gangs of these bugs will be pulling “home invasions” on our gardeners as the milder weather will allow gnat eggs to hatch out of doors.

Fortunately, treating this pest is very straight forward:

Wilson’s Potting Soil Insecticide or Wilson’s Fungus Gnat Powder will eliminate fungus gnats from the root zone, usually with a single application of pesticide. These products are very gentle on the plant, making them useful for all grow mediums. We do not recommend stronger, outdoor pesticides (liquid diazinon 5% strength) since these can cause major damage or death to an indoor garden! Stick with safe, effective products that allow plants to recover quickly.

Using these products with “Potting Soil” plants is very simple: just sprinkle the powder on to the soil and water it in.

When growers use the newer grow mediums, however, a different procedure is necessary.