Question
My ficus started droping its leaves when I noticed these gross little black gnat/fly things all over my stairwell (where the ficus is kept.) Then I found these little worms in the soil. The worms are sort of like earthworms but are smaller than an inch and seem to be the cause of my ficus trees ill health. Can you please suggest something. I've tried the soap and water thing and a 3-n-1 killer on the soil, yet they are both sill around.

Answer
Hi Cassie,

Fungus gnats are a pretty good indication that the soil of your Ficus tree is staying too wet and the roots may have already begun to rot. It is the wet soil or possibly inadequate light that is very likely causing the tree's "ill health."

Let's start with the light because that is of primary importance for a Ficus tree. You didn't indicate what the available light is, but most stairwells do not have a lot of natural light. Ficus trees must be directly in front of an uncovered window in order to thrive. Skylights and artificial lights don't help very much. If your Ficus does not have adequate light, then nothing else matters and you will slowly lose the tree. So let me know your lighting situation.

Adult fungus gnats fly around and are an annoyance, but they are not harmful to people. Each gnat lives for about 5 days. The trick is to get rid of the next generation - the gnat larvae that live in the top layer of the soil. The larvae feed on decaying organic matter. Decaying pine bark in potting mixes and decaying plants roots feed the larvae.

Try to keep the soil as dry as possible, allowing at least the top 1-2 inches of soil to dry out between waterings. Remove all loose soil from the surface and put a light layer of coarse coir (coconut husk) or sand or diatomaceous earth on the soil surface. These substances have sharp edges that carve up the larvae.

Another safe technique is to place ?inch slices of raw potato on the surface of the soil. After a day or so, discard the slices along with the larvae inside. Repeat this until there are no more larvae in the potato slices.

For more serious infestations try Knock-Out Gnats to treat fungus gnats available from Gardens Alive for about $20. See http://www.gardensalive.com/item_display.asp?ProductNumber=1962

Detection trick: Add a little water to the soil and then look very closely for tiny fungus gnat larvae swimming in the water as it pools on the surface. You need good light and good eyes to see them. If you don't, then your plant is probably gnat free.

Prevention is often the best remedy. Use sterile potting mixes that are free of bark chips. The potting mix should have ample drainage material, such as perlite so that it drains well and allows the soil to dry out frequently. Fungus gnats can nearly always be traced back to overwatering and/or poor soil quality.

The insecticidal soap and most other pesticides are not effective against gnat larvae and often aggravate the problem by keeping the soil moist.

To attract and capture the adult flying gnats, pour a little red wine into a small glass (cooking wine will work, if you don't want to waste the good stuff!) and leave it out for the day. Repeat the treatment as necessary.

The worm-like critters are not harmful to the tree. If they continue to be a problem after you have eradicated the gnats, then try plunging the pot up to its rim in a tub of water for a half hour. The flood will cause the worms to rise to the surface in search of air.

Please let me know if any of this is unclear or if you have any additional questions. If you would like to e-mail me some photos, I may be able to provide some additional insights.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

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