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Fungus Gnats

Fungus Gnats

Bradysia species

Insectalso: Soil gnats

Tiny dark flies that hover around houseplants, seed trays, and damp potting soil. The adults are a harmless nuisance, but their root-feeding larvae can stunt and damage seedlings and cuttings, especially in the overwatered conditions of indoor seed starting. They are one of the most common problems of indoor and greenhouse growing.

🔎 How to spot it

Adults are delicate, grayish-black, mosquito-like flies about one eighth inch long, with long legs and a single pair of wings, often seen running across the soil or flying weakly when a plant is watered. The larvae are tiny, clear to white, legless maggots with a shiny black head, living in the top inch or two of moist soil. A yellow sticky card set near the pots quickly confirms them.

🥀 Damage it causes

The adults do not bite or harm plants and are mainly an annoyance. The larvae feed on roots, root hairs, fungi, and decaying matter in the soil, and when numerous they damage fine roots and stunt or kill seedlings and cuttings that have not yet rooted in well. The feeding wounds can also open the door to root-rot fungi.

🛡️ Prevent it

Let the top one to two inches of soil dry out between waterings, since the larvae cannot survive in dry soil; this single change breaks the cycle. Avoid overwatering and improve drainage, use fresh, well-drained potting mix rather than soggy or reused soil, and do not leave standing water in saucers. Bottom water when you can, and cover the soil surface to discourage egg laying.

🧯 If it is already here

Hang yellow sticky traps to catch the egg-laying adults and monitor numbers. Drench the soil with the bacterium Bacillus thuringiensis subspecies israelensis (Bti), sold as mosquito bits steeped in water, or apply beneficial nematodes (Steinernema feltiae) to kill the larvae. Letting the soil dry hard between waterings, combined with sticky traps, clears most infestations without other sprays.

💡 Good to know

Fungus gnats thrive on excess moisture and organic matter, so a persistent problem is almost always a sign that the soil is being kept too wet. They are easy to confuse with the equally harmless shore flies. Because the whole life cycle runs in three to four weeks, consistent drying and trapping over a few weeks is usually enough to end an outbreak.

🌱 Plants it attacks

714 plants in the library can be attacked by this pest

Acorn SquashAdirondack Blue PotatoAdzuki BeanAfrican Blue BasilAfrican MarigoldAgapanthusAgeratumAgrimonyAji Amarillo Pepper🍓Albion StrawberryAlice du Pont Mandevilla🥔All Blue PotatoAlmondAloe VeraAmbrosia CornAmerican BasswoodAmerican Beauty Dragon FruitAmerican BeechAmerican PersimmonAmethyst Falls WisteriaAmish Paste TomatoAnaheim PepperAnemoneAngelique TulipAngeloniaAniseAnise HyssopAnjou PearAnnabelle Smooth HydrangeaAnnual VincaApeldoorn TulipApple MintApril Tryst CamelliaArbequina OliveArizona Sun Blanket FlowerArkin CarambolaArmenian CucumberAroniaArp RosemaryArugulaAshwagandhaAsian PearAsian PersimmonAtemoyaAtlantic Giant Pumpkin🥕Atomic Red CarrotAucubaAugust Beauty GardeniaAunt Molly's Ground CherryAutumn Joy SedumAvocadoBachelor's ButtonBalsam FirBalsam PoplarBanana PepperBarbara Karst BougainvilleaBartlett PearBay LaurelBayberryBeach PlumBeauregard Sweet PotatoBecky Shasta DaisyBee Balm🍅Beefmaster TomatoBenarys Giant ZinniaBengal Tiger CannaBetter Boy Tomato🥬Bibb Lettuce🍅Big Beef TomatoBig Boy TomatoBilberryBing CherryBitter MelonBlack BeanBlack Beauty EggplantBlack Beauty ZucchiniBlack Beluga LentilBlack Cherry TomatoBlack CrowberryBlack Currant🍉Black Diamond WatermelonBlack Kabouli ChickpeaBlack Krim TomatoBlack RaspberryBlack Sapote🥬Black Seeded Simpson Lettuce🥕Black Spanish RadishBlack Tartarian CherryBlack WalnutBlack-Eyed PeaBlack-eyed Susan VineBlood OrangeBloomsdale SpinachBlue Bird DelphiniumBlue Bird Rose of SharonBlue FescueBlue Lake Green BeanBluecrop BlueberryBocking 14 ComfreyBok Choy

For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.