Question
Hello, i just recently purchased a Fukien Tea bonsai tree
and after about 5 days I started noticing some fungus gnats
living hppily around and on my tree. I have spotted and
killed about six or seven each night for about 3 days now
and nothing seems to be diminishing their numbers. I was
wondering if it would be safe to use Malathion to get rid of
the flies or if there is a better option because i know the
Fukie Tea bonsai can be rather delicate. Any information
would be helpful. Thank You.

Answer
Hi Stephen,

Unless you have some decaying garbage lying about, the fungus gnats are coming from the soil of your plant. The gnat larvae or eggs feed on decaying organic matter, usually rotting roots when found in potted plants. As the larvae mature, they develop wings and fly off to deposit more eggs elsewhere. The winged adults only live for about a week before they die. Spraying the adults will shorten their lifespan, but new generations will keep emerging from the soil. So, you have to treat the larvae.

The Malathion label does not list fungus gnat larvae, which may mean it is not effective or it simply has not been tested on gnat larvae. In addition, the label does not indicate if it is safe to use on Tea plants. Again, this is probably because it has not been tested. Thus, there is some risk in using Malathion as a soil drench and it may or may not be effective in killing the gnats.

However, even if the Malathion is effective in killing the larvae, the problem of decaying roots remains and that is a far bigger threat to the health of your plant. You didn't indicate how you water your plant, but it is important that you allow the soil to dry out as much as possible between waterings. The drying out of the upper levels of soil will deprive the gnat larvae of the moisture they require to survive. The drying out will also stop the root rot that is likely occurring.

If there is any loose soil on the surface of the rootball, scrape it off and discard it. That may eliminate many of the larvae and will help the plant dry out sooner. In addition, be sure to provide lots of sunlight and warm temperatures to help the plant and to promote the drying process. If you have been misting, stop.

If you follow these instructions, I think you will see a gradual decline in the gnat population over the course of several weeks.

Please let me know if any of this is unclear or if you have any additional questions.

I have written an article on safe and effective treatment of plant pests that  will email to anyone who sends me a request at my address below.

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Will Creed, Interior Landscaper
Horticultural Help, NYC