Fungus Gnats

Fungus Gnat

June 17, 2010

The gardens and yards have been hit with fungus gnats.  Fungus gnats are comprised of a number of species in the insect order Diptera, family Sciaridae (dark winged fungus gnat species Bradysia coprophila, impatiens and paupera) that are about the same size as a mosquito.   These tiny little creatures cause minimal damage to herbs such as oregano where they were having a field day.  They can however, cause considerable damage to young seedlings as well as aiding the introduction and spread of disease in the garden and houseplants.  The biggest problem with gnats in general outdoors aside from the damage is their swarming tendency.  Walking through clouds of gnats is not the most pleasant thing.  Fungus gnats are more of a concern for houseplants and in greenhouse operations.  In these confined spaces every time you walk past infested plants a cloud of them will be set into flight creating an annoyance along with the damage they cause.  Fungus gnats are attracted to wet soil and since we've had a couple weeks of rather wet weather we have fungus gnats both indoors and outdoors.  Adult fungus gnats lay their eggs in the damp soil where the resulting larvae stay until developing into adult fungus gnats then the cycle repeats itself.  The key clue here is the fungus gnat larvae cannot survive in dry soil.  Fungus gnats control is dependent on where they are but most likely if you have them outdoors some will get indoors simply by coming in the door while you are going out.

Outdoor Fungus Gnat Control

• Allow soil to dry between waterings.  This will kill off fungus gnat larvae reducing the fungus gnat adult population in your gardens.
• Place small bowls of cider vinegar with a drop or two of dishsoap amongst infested plants.  They are attracted to the cider vinegar so will try to feed but the lack of surface tension caused by the dishsoap will serve as a trap.
•  Bacillus thuringiensis (Bt) is a bacteria that feeds off of the larvae of certain insects.  It can be applied to the soil and sprayed on the leaves of plants to kill off certain carterpillars and larvae.  Two brands are Bonide Organic Thuricide (concentrated liquid) and Safer's 5161 Caterpillar with BT Garden Dust.  Before you resort to using Bt, there is some concerns that it is not as benign as first appeared.  There is a good report here for things to consider before using Bt.
• Standard white incandescent and CFL outdoor lighting will attract fungus gnats to your gardens.  Change these bulbs out for the yellow insect repelling bulbs or better yet go solar or low voltage on a timer.  Put any security lights on a motion activated sensor so they are not constantly on to attract insects.
• Do not resort to using a bug zapper.  Electronic bug zappers are popular but very much eco-unfriendly and your neighbours will hate the constant noise.

Indoor Fungus Gnat Control

• Let the soil dry in potted plants before watering.
• A fly swater works well but can make a mess.
• Use lights to your advantage.  The gnats are attracted to lights including television.  If you let your soil dry between waterings the larvae will be killed off.  The remaining adult flies will die off in light fixtures they can get into leave these on for an hour or so.  Turn off anything that lights up including televisions and computers for the same period of time.
• Insecticidal soap (homemade or purchased) can be effectively used to control fungus gnats on houseplants.
• Set out a few bowls of cider vinegar with a drop or two of dish soap near where the fungus gnat infestations are to trap them.

Happy Gardening!

Garden Gnome
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