I received a call the other week about an odd phenomenon on a client’s porch- hundreds of tiny crawling insects grouped together, forming a large, crawling mass. This type of behavior – traveling together en mass, is actually fairly common for several insect species including specific types of insect larvae and caterpillars. In this case, I suspect the culprit was a large number of fungus gnat larvae.
Fungus gnats (Bradysia spp.) are common pests in homes and offices, often found living near or in houseplants and sunny windows. Adults are small—less than 1/8 of an inch long—dark-colored, and fragile. They’re poor fliers and don’t tend to move around much indoors, instead sticking close to potted plants. Their long legs and antennae might lead you to mistake them for mosquitoes, but fungus gnats don’t bite. While adult fungus gnats don’t harm people or damage plants, they can become quite a nuisance.
Fungus gnats lay their eggs in moist organic matter such as soil, potting mix, and decomposing plant debris. After about three days, the eggs hatch into larvae with shiny black heads and transparent bodies. These larvae feed on organic matter like mulch, mold, grass clippings, compost, and fungi. When conditions are right—typically warm, moist, and rich in organic material—many eggs can hatch at once, resulting in an abundant population of larvae. These larvae may feed on plant roots, causing stunted growth and root damage, especially in seedlings and young plants. Roughly a week later, adult fungus gnats emerge and the cycle begins again.
Controlling this pest is relatively straightforward. Start by visually inspecting all houseplants to determine if fungus gnats are present. Adults often linger near soil surfaces, on the edges of pots, or in sunny windows. Yellow sticky traps are an easy way to catch adults, while raw potato chunks can be placed in pots to attract larvae. After a few days, remove the potatoes—larvae and all—and discard them. Then, replace with fresh chunks if needed.
Also check for overly moist conditions or organic debris like fallen leaves, which serve as food and breeding material for larvae. Avoid overwatering and ensure plants are in well-draining soil. Try not to use potting materials that haven’t been fully composted, as these can harbor fungus gnat eggs and larvae. If you’re unsure about the quality of your potting mix, consider solarizing it (placing it in a sealed, black plastic bag in the sun for several weeks) before bringing it indoors.
Biological and chemical control options are also available. Applying Bacillus thuringiensis (BT) at five-day intervals can help manage larval populations. Alternatively, if you’re dealing with a heavier infestation, a pyrethrin-based spray provides fast-acting control—though treatments may need to be repeated as new generations emerge. Be sure to follow all label instructions. These products are typically applied to the soil surface, plant foliage where adults rest, or used as a soil drench.
While fungus gnats are most often mentioned in relation to indoor plants, they’re also common outdoors—hence the large mass spotted by our client. Though unsightly, outdoor infestations are typically harmless. If the mass bothers you, you can disperse them with a strong spray of water or use the control methods listed above.
If you have questions about fungus gnats or other insect issues, please contact us at uge3181@uga.edu or 706-359-3233.
