Fungus Gnat Problem
Answered by: Conrad Richter
Question from: Karen Corless
Posted on: June 18, 2000

I would appreciate any help that you could give me with this problem:

I have been growing herbs, oregano, basil, parsley, etc., in pots in my house and on the screened in porch. I am getting tiny little flying gnat-like bugs all around them. What can I do to prevent this?

The gnat-like insects are fungus gnats. They look like fruit flies that congregate on the soil surface. They are generally more of a nuisance than a problem for the herbs. The larvae feed off organic matter in the soil mostly, though they can attack the roots of plants too. The adults that you see flying around do not attack plants, but can be nuisance every time you water or move the plants.

Fungus gnats love wet soil with lots of organic matter. Most modern potting soils are high in peat and that makes them highly prone to fungus gant infestations. The best defense is to modify your watering. You should let the soil become dry to the touch but not dried out completely so the plants wilt. When dry then water thoroughly so the water comes out of the bottom of the pot. If no water comes out when you water, you either have not watered enough or your drainage is inadequate (which will compound the fungus gnat problem).

The best organic treatment is a biological control called Scanmask (available from Richters). This is a beneficial nematode that attacks the larvae. When used properly, Scanmask will gradually reduce the fungus gnat population. You can use a pyrethrum and soap spray combination also to help control the adults. Yellow sticky traps also help to catch some of the adults.

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