Fungus Gnats

Aug 6, 2017
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Stroudsburg, PA
Hardiness Zone
United States
I recently wrote this article for my greenhouse forum, and I thought I would share it with you because fungus gnats are a common greenhouse pest. (I am sure some of you are very knowledgeable about greenhouse pests; this article is aimed at the beginner to intermediate hobby greenhouse grower.)

Fungus Gnats in the Greenhouse

You hear it all too often, “Something is killing my plants!” There are countless problems that cause diminished plant health in the greenhouse, and one of the most common of these is pests. Of the numerous pests that can be found in the greenhouse, fungus gnats come in near the top of the list.

The warm and humid environment of the greenhouse makes an excellent habitat for fungus gnats. Adult gnats are small, dark, mosquito-like flies with grayish wings, about one-eighth of an inch long and are usually first spotted flying around the surface of greenhouse potting soil rich in organic matter. These adult flies are rather innocuous, nothing more than an nuisance, but their presence indicates that their larvae are already at work undermining the health of your plants.

The adult female lays her eggs in soil that is warm, damp and full of decaying organic material. The eggs hatch into the larval stage and, depending on the species of fungus gnat, quickly set off feeding on the beneficial fungi and algae in the soil or on the roots of the plants themselves. Either way, this infestation leads to diminished health in the plants in the form of stunted development or disease resulting from the pathogens carried by the fungus gnat larvae.

If you have flying fungus gnats, you can bet on the fact that you have fungus gnat larvae lurking in your greenhouse soil. The larvae are dark in color, but very difficult to see without the aid of a 10x-20x hand lens. However, the adults are visible to the naked eye. Once identified, you can mount your response to control or eliminate them.

The most effective way to thwart fungus gnats is prevention, and the easiest prevention method is to water your plants properly, especially during the winter. Potting soil that remains moist for extended periods of time attracts fungus gnats because the warm moist soil provides a steady supply of food for their larvae. Also, keep the surface of your pots clean of any dying, dead or decaying plant material.

Insecticides are not warranted in controlling or eliminating fungus gnats as there are so many effective natural remedies. Here are but a few:

· Allow the top layer of soil to dry, and then water as normal with a 3% hydrogen peroxide to four parts water solution. The larvae will die on contact and the peroxide will quickly break down into harmless oxygen and water molecules. You can repeat this as needed. Don’t worry about the fizzing; that will subside within a few minutes and is harmless to the plants.

· Top dress your pots with one-quarter inch of sand. Sands drains very quickly and will discourage fungus gnats from laying their eggs.

· Fill small containers half way with apple cider vinegar or cheap beer and a few drops of dish soap. Replace the tops or use plastic wrap to cover the containers. Poke small holes through the tops and place near the infested plants. As the old saying goes, they check in, but they don’t check out.

· Place yellow sticky traps horizontal to the soil surface to capture large numbers of flying adults. They are attracted to yellow and can be easily disposed of before they lay more eggs.

· Products that contain Bacillus thuringiensis var. israelensis, (i.e. Microbe-Lift or Mosquito Bits) is a highly selective biological pesticide that kills the larvae of many greenhouse pests. It is safe and has no harmful residues.

· Beneficial Nematodes. Nematodes are microscopic round worms that penetrate fungus gnat larvae, as well as other soil-borne pests, then release a bacterium that consumes the larvae from the inside out. The long-lasting nematodes are safe for use.

These are the most popular natural weapons you have in your arsenal for defending your plants against the pesky and destructive fungus gnat, an unwelcomed but common guest in the greenhouse.

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