Small Flies in Houseplant Soil: Fungus Gnats

Causes and how to tell them apart

1. Small dark flies at the pot[1][2]

   How to tell: Adults are delicate, gray to black, about 1/8 inch long, and look like fruit flies. They run on the soil and up the sides of the pot.

2. Larvae in the soil[1][3]

   How to tell: The young live in the soil and are white, translucent larvae with a shiny black head. Seeing these confirms the soil is the breeding spot.

3. Constantly wet soil[4][5]

   How to tell: Gnats feed on fungi in soil and are more common where soil stays wet. If the flies keep coming back, check how often you water.

4. Root damage with yellowing or wilting[3][5]

   How to tell: Adults are mostly a nuisance. If leaves yellow or wilt too, some larvae may be feeding on roots, but overwatering and root rot can cause the same signs.

What to do, step by step

1. Look for small flies running on the soil while you water to find which plant is breeding them.[2]
2. Let the soil surface dry as much as possible between waterings, without letting the plant wilt.[6][1]
3. Do not let plants sit in saucers or outer pots filled with water.[7]
4. Use yellow sticky traps to catch some adults. They help you see how bad it is but do not stop new flies hatching.[6][7]
5. If it persists, repot in fresh soil in a clean container after washing the old soil off the roots.[6][2]
6. Treat with a product made from Bacillus thuringiensis subspecies israelensis in the soil, which works only against larvae. Iowa State says insecticides are not needed.[6][2]

How to prevent it

• Avoid overwatering, since fungus gnats prefer moist areas and soils high in organic material.[3]
• Water less in winter, when gnat problems are most common.[1]
• Keep saucers and outer pots from holding standing water.[7]