Fungus gnats are small, black flies which more closely resemble a mosquito than a fly. The flying adults are harmless to the plant but are a major nuisance to plant owners and greenhouse operators.
Where the fungus gnat crosses the line between nuisance and pest is at the larva stage. At this stage the larva consume organic material to survive and although that usually consists of potting mix material it can also include healthy plant root fibers.
Symptoms and Damage:
Fungus gnats are usually noticed before any real damage to the plant occurs. They can be seen scattering when the plant is watered and will often search out water sources leaving you to find them near kitchen and bathroom sinks. The damage is minimal and normally results in stunted growth and seedling failures.
Habitat and Life Cycle:
The fungus gnat is a short lived fly lasting approximately 10 days and consists of an egg stage, larval stage followed by a pupal phase and finally emerges as an adult fungus gnat to start the process all over again.
Although its life span is so short the female fungus gnat is capable of producing up to 200 eggs in that time and its the larva stages that often times consume young plants roots. The adults are harmless to plants but a major annoyance in green houses and especially in the home.
Fungus gnats prefer warm and damp locations with little wind. The young feed on fine root hairs and decaying organic matter in potting soils and mixes while the adults consume only liquids making the only irritating.
Prevention and Control of Fungus Gnats
There are many benefits to avoiding chemicals when dealing with insect invaders. The harsh chemicals in some insecticides may cause more harm to an already injured plant not to mention the possible harmful effects to you or your loved ones.
✔– First choices for attempting insect control.
✔– Used when green don't solve the issue.
✔– Only use when blue options have failed and the plant can not be discarded.
Prevention✔- Removing the damp part of the fungus gnats requirements is a great place to start. Let your house plants dry out between waterings, at least 2" of soil on the top should be dry before the next watering. And remember to empty house plant pot drip saucers of any water, fungus gnats love hanging around these areas. Keep windows screens in good shape, especially near house plants and remember to inspect the stems, leaves and soil of any potential new additions to your house plant collection.
Manual✔- If the soil is heavily contaminated it may be possible to vacuum up the top 2 or 3 inches of soil and replace it with sterile potting soil or mix. Vacuuming can also help to control flying adults. Yellow sticky traps are very effective in controlling the adult fungus gnats. Slices of potato place on the soil surface can be used to control and monitor fungus gnat larva who are immediately go to feed on them. Just collect the slices in 4 - 6 hours and repeat until no larva are seen on the potato slices.
Organic✔- Neem Oil Spray sprayed on the leaves and onto the surface of the potting soil works well to control populations. Pyrethrins such as Bonide Pyrethrinare a natural insecticide made from chrysanthemums and works well to kill delicate fungus gnats.
Biological✔- Bacillus Thuringiensis (BTi) is a biological insecticide which available commercial and attacks fungus gnat larva in the soil.
Chemical✔- Any insecticide for flying insects that can be used on house plants will work. Repeat applications may be needed.
More information on Fungus Gnats:
Fungus Gnats - http://www.ext.colostate.edu/pubs/insect/05584.html
Wikipedia - http://en.wikipedia.org/wiki/Sciaridae
Greenhouse Canada - http://www.greenhousecanada.com/content/view/1286/