Question
I have two medium-large (3-4 feet tall and well-spread) multi-stem dieffenbachias in good condition. They appear to be affected by black flying aphids (? At first I thought those were fruit flies!)梖or the first time in many years that I have lived with these plants.  I am trying to remedy the situation by application of soapy solution (hand-dish soap) because the house conditions do not readily allow anything more radical (no shed, no yard, no spare bathroom), and the plants in their planters are also quite heavy.  I just started the treatments, and want to ask you for advice on the next steps in case this remedy is not enough. Many thanks. Leah

Answer
Hi Leah,

Those small black flying gnats are not aphids. They are fungus gnats. The gnat larvae live in the soil where they mature, develop wings and then fly out of the soil in search of other places to lay eggs. The winged adults only live for about a week before dying of "old age." The key to treating fungus gnats is to eradicate the larvae living in the soil.

The larvae require constantly damp organic material to feed on. This is potting soil that is kept too moist and rotting roots near the surface. If you recently added fresh soil, that soil was probably the initial source of the larvae. Keeping the soil too moist is what is keeping the gnats alive.

Remove and discard all loose soil not in direct contact with the roots from top surface. That will eliminate many of the gnat larvae because they mostly live in the top few inches of soil. Then, allow the soil to dry several inches deep into the soil before adding any water. Do not attempt to treat the larvae with any liquids, including soap. You can sprinkle a thin layer of sand on the surface. The sharp edges of the sand will carve up the larvae as they move to the surface.

The adults will die off in a week or so. If you follow these directions and keep the soil as dry as possible, you should see a gradual decline in the gnat population without using any sprays or pesticides.

I have written an article on treating indoor plant pests, including fungus gnats, that I will email for free to you (or anyone else) who emails a request to me at [email protected].

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

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

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