Gnat-like insect from amaryllis or potting soil?

Asked March 16, 2017, 6:52 PM EDT

We recently purchased two amaryllis bulbs and a new bag of potting soil. Shortly after the bulbs were planted and the growing process started, we noticed tiny insects appearing. They resembled fruit flies or gnats but are really tiny. They seem to like crawling up the window glass on the front door (faces east). They don't bite and don't seem to have caused any damage to the plants but we don't particularly like having them as houseguests. Since we purchased the bulbs and the soil from different places and we aren't sure which item may be responsible, we'd appreciate any knowledge you might have about them.

Hennepin County Minnesota soil fungas gnats

1 Response

They are probably fungus gnats. They can be controlled chemically or biologically and I prefer the biological control. This product is available at garden centers and nurseries like Bachmanns.
Gnatrol*Bacillus thuringiensis var.israelensismicrobiallarvae

If they are very few in number, drying out the soil between waterings is in many cases enough to get rid of them.

They are a common problem for house plants and they and their control has been answered before in this forum so I have copied the answer for you.

Sounds like fungus gnats (adult and/or larvae). They are not a beneficial and will spread to your other house plants.

Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in potted plants and houseplants. Fungus gnats often remain near potted plants and run across (or rest on) growing media, foliage, compost, and wet mulch piles. Females lay tiny eggs in moist organic debris or potting soil. Larvae have a shiny black head and an elongated, whitish-to-clear, legless body. They eat organic mulch, leaf mold, grass clippings, compost, root hairs, and fungi. If conditions are especially moist and fungus gnats are abundant, larvae can leave slime trails on the surface of media that look like trails from small snails or slugs.

Adult fungus gnats don’t damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants.

Most of the fungus gnat’s life is spent as a larva and pupa in organic matter or soil, so the most effective control methods target these immature stages rather than attempting to directly control the mobile, short-lived adults. Physical and cultural management tactics—primarily the reductions of excess moisture and organic debris—are key to reducing fungus gnat problems.

Visual inspection for adults usually is adequate for determining whether a problem exists. You will see adults resting on plants, soil, windows, or walls, or you might see them in flight. Besides looking for adults, check plant pots for excessively moist conditions and organic debris where larvae feed. Yellow sticky traps can be used to trap adults. Chunks of raw potato placed in pots with the cut sides down (not the peels) are sometimes used to monitor for larvae.

Because fungus gnats thrive in moist conditions, especially where there is an abundance of decaying vegetation and fungi, avoid overwatering and provide good drainage. Allow the surface of container soil to dry between waterings. Clean up standing water, and eliminate any plumbing or irrigation system leaks. Moist and decomposing grass clippings, compost, organic fertilizers, and mulches are also favorite breeding spots. Avoid using 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). Minimize organic debris around buildings and crops. Avoid fertilizing with excessive amounts of manure, blood meal, or similar organic materials. Screen and caulk leaky windows and doors to help prevent pests from coming indoors.

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.

I hope that helps!
Amy Dismukes

UT/TSU Extension, Williamson County