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Sandy's Garden

Fungus Gnats

Fungus gnats are annoying indoors

By: Sandy Feather 2012
Penn State Extension


Q. I have been fighting a losing battle with fungus gnats in my houseplants.  I have tried sticky cards, butter on a brightly colored surface, and finally insecticidal soap.  I have caught many on the sticky paper, and the insecticidal soap works for about a day, but they are back the next morning in full force.  Short of transplanting everything, do you have any solutions? 
  
  A. Fungus gnats are tiny, dark colored flies that commonly infest all kinds of houseplants. The adult flies do not damage plants, but can be a nuisance because they fly in your face, land in your drinks, fall in your food, and generally make pests of themselves. They often congregate near windows. Their larvae can damage plants because they feed on organic matter in the potting soil, including small feeder roots that plants depend on to absorb water and nutrients from the soil.
  

Ferns


Fungus Gnat Damage

Fungus gnats are most damaging to seedlings and freshly stuck cuttings; they are unlikely to harm established houseplants unless the population is extremely high. They are most problematic in our homes through the winter and early spring. These native insects are common outdoors, and we often bring them into our homes when we take plants outside for the summer where they become infested, and then bring them back indoors in fall.

amaryllis flowers


Best Strategy

One of the most important factors in getting fungus gnats under control is managing soil moisture. Fungus gnat larvae require a moist environment, and may die if you can allow the soil to dry out thoroughly. Avoid overwatering, and do not allow plants to sit in saucers filled with runoff water. If possible, allow the soil to dry out pretty thoroughly between waterings. Never take it to the point where the plant wilts, though. You can also repot plants into fresh potting soil, which will get rid of existing eggs and larvae. Covering the soil surface with a layer of sand discourages egg-laying by adults.

Controls

There is a Bacillus thuringiensis var. israelensis (Bti) formulation sold under the trade name Gnatrol that controls fungus gnat larvae. It is used as a soil drench, meaning that you water the plants with it. Ultra-fine horticultural oil, neem and insecticidal soap may be sprayed on the foliage to control the adults.


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