Adults

• Adult fungus gnats are tiny insects that look like mosquitoes. They do not bite but they can be irritating when they gather near windows or other light sources in your home, according to Colorado State University. An adult fungus gnat can lay 200 eggs in your houseplants before it dies.
  
  

Larvae

• Fungus gnat larvae develop in the soil of your houseplants. They feed on fungus and organic matter. They also can feed on the roots of your plants, leading to wilting and a failure to thrive, according to Oregon State University.
  
  

Solutions

• Adult fungus gnats can be killed with pyrethroid-based insecticidal sprays that are designed to kill gnats or flying insects. You can eliminate the larvae living in your houseplants with microbial insecticides in the form of soil drenches. Because you could have several generations of fungus gnats living in your houseplants, you could need to spray and drench more than once to eliminate them all.