What are those gnats on my seedlings?

Mar 10, 2015

What are those gnats on my seedlings?

Mar 10, 2015


It's not quite spring yet, so many gardeners have started seeds indoors. We lovingly check them every morning to see how they are are and if anything new has germinated. We probably wave away swarming fungus gnats, too. These nuisance pests are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Fungus gnats are relatively weak fliers and usually don't move around much indoors. Fungus gnats often remain near potted plants and run across (or rest on) growing media.

Most of the fungus gnat's life is spent as a larva and pupa in organic matter or soil.  It's more effective to target these immature stages rather than attempting to directly control the mobile, short-lived adults. Reductions of excess moisture and organic debris are key to reducing fungus gnat problems.

We can't let our seedlings dry out, but because fungus gnats thrive in moist conditions, especially where there is an abundance of decaying vegetation and fungi, we can mop up excess water, avoid overwatering and provide good drainage.

Don't use incompletely-composted organic matter in potting media unless it is pasteurized first, because it will often be infested with fungus gnats. Improve the drainage of the potting mix (e.g., increase the proportion of perlite or sand in the mix). If you have infested plants, don't move them to new areas where flies can emerge to infest other pots. In some cases you may wish to toss out severely infested plants. Purchase and use only pasteurized container mix or potting mix. Commercial growers often treat potting soil with heat or steam before using it; this will kill flies and the algae and microorganisms they feed on.

Fungus gnats have many overlapping generations each year. They can occur during any time of the year indoors. Insecticides are rarely warranted to control these flies in and around homes. However, if you do apply an insecticide for fungus gnats, consider using the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti). Several Bti products (Mosquito Bits, Gnatrol) are readily available in retail nurseries and garden centers, so these products may be the most convenient for home gardeners to use. Bti does not reproduce or persist indoors, so infestations in potting media might require repeated applications at about five-day intervals to provide control.

Excerpts and photos from



UC Master Gardeners of Napa County provide free home gardening advice. Visit, call or complete the Plant Problem Diagnosis Sheet  for assistance.

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