How to Get Rid of Gnats on Vegetable Seedlings

By Nathalie Alonso Updated July 13, 2021

The presence of flying insects called fungus gnats in a vegetable garden poses a threat to seedlings and other young plants that have vulnerable roots. Vegetables growing in pots are more likely to be affected by fungus gnats, which are most common indoors, in greenhouses and nurseries. Fortunately, there are non-toxic ways to prevent and eliminate fungus gnat infestations.

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Fungus Gnats on Vegetable Plants

Fungus gnats are small insects that look like tiny flies, at lengths of ⅛ inch or less. Female fungus gnats lay their eggs in soil or potting media, which is how vegetable seedlings may become infected with larvae. Fungus gnat larvae eat fungi and decomposed organic material in the soil. While adult fungus gnats pose little threat to vegetable seedlings, in large numbers, their larvae can destroy the roots of the plants. You may notice trails of slime in the soil if a plant is severely infected.

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Fungus gnats can be active at any time of the year. Adult fungus gnats that fly out of the soil are drawn to sources of light, which is when you may first notice them. Though they can be extremely annoying, adult fungus gnats do not bite humans. However, according to the University of Minnesota Extension, fungus gnats are capable of transporting the spores of a fungus that causes damping off – a disease that kills seedlings.

Biological Controls for Fungus Gnats

One way to control fungus gnat larvae is to drench the soil with commercial Bacillus thuringiensis subspecies israelensis, a type of bacteria that occurs naturally in soil and is therefore considered safe for humans. The israelensis subspecies is toxic to fungus gnat larvae specifically. The bacteria coat the roots of the vegetable plants, explains HGTV, killing any larvae that tries to feed on them. Plan on applying this bacteria every five days.

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You can also apply a beneficial nematode – a type of roundworm – as a soil drench. For fungus gnats, the recommended nematode species is Steinernema feltiae, which works best in temperatures between 60 and 90 degrees Fahrenheit, according to the University of California’s Division of Agriculture and Natural Resources. Alternatively, purchase mites of the species Stratiolaelaps, which prey not only on fungus gnat larvae, but also on the pupae, the insects in the stage between larva and adult.

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Other Fungus Gnat Tips

Trapping adult fungus gnats is as simple as using flypaper. You can also make a gnat trap by placing a dish filled with a solution of water and apple cider vinegar and a few drops of dishwashing liquid, says HGTV. Per the University of California’s Division of Agriculture and Natural Resources, you can place raw potatoes in the soil of your vegetable seedlings to trap fungus gnat larvae that may be lurking. The raw potato lures the larvae away from the roots, making it easy to dispose of them.

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Using only pasteurized potting mix for your vegetable seedlings is one way to avoid a fungus gnat infestation. Any organic matter you incorporate into the soil should be composted. Since fungus gnat larvae like moist conditions, avoid overwatering seedlings, and make sure the soil drains well. If the infestation is very severe, it may be necessary to discard the vegetable plants. [external_footer]