Safe Ways to Kill Gnats in Flower Pots

Reduce Moisture in the Soil

• Female fungus gnats are attracted to moist soil in the top three inches of houseplants and lay eggs there. Water plants less often so soil will become drier and less attractive to female gnats and less able to keep larvae alive. Make sure all flower pots provide adequate drainage for excess water. Use a soil moisture meter to determine how wet the soil is. If the soil keeps feeling wet for days after watering for shorter periods, the plant needs to be repotted with fresh soil, or it will remain so moist that larvae will thrive in it.

Bacillus Thuringiensis

• Bacillus thuringiensis or Bti, are microbes that feed on insect larvae. There are numerous varieties of Bti, but the type that kills fungus gnat larvae is called Bacillus thuringiensis subspecies israelensis. Bti is commercially available for treating houseplants. Wear gloves and shirts with long sleeves and long pants when handling Bti products to avoid skin irritation. If you are susceptible to asthma attacks or other breathing problems, wear a mask. Pour onto the soil as a drench. Wash hands afterwards.

Nematodes

• Nematodes are microscopic roundworms that live in soil and feed on any insects they encounter, such as fungus gnat larvae. Nematodes enter the larvae’s body openings and secrete a toxin which kills the larvae in four days. There are many species of nematodes commercially available, but the species that does best to kill fungus larvae is Steinernema feltiae. Nematode eggs are added to soil with water.

Expert Advice

• If one houseplant repeatedly suffers from fungus gnat larvae infestations, remove the plant and check the inside of the flower pot for mold growth. Change the plant to a new flower pot and trash the old flower pot. If this still fails to eliminate the fungus gnats and all other methods have failed, then throw the plant in the trash.