Question
Hi, the pots of my house plants seem to be infested with fungus gnats. The tiny bugs are always flying around everywhere and it's getting pretty annoying. Especially when I water the plants the bugs always come flying out and it seems impossible to stop them. I have looked up ways to kill these insects but nothing is really showing up. I have tried to make an apple cider vinegar trap but it doesn't appear to be working. I also have an insecticide based on pyrethrin, will this work? If not do you have any tips on how to effectively eliminate the fungus gnat problem with my plants? Thanks

Answer
Hi Hongzhi,

Fungus gnats are the symptoms of a larger problem. The gnat larvae live in the soil and feed on decaying organic matter in the upper couple of inches of soil. Unless you effectively treat these larvae, you will keep getting new gnats to replace the winged adults that die on their own after about a week.

Unnecessary repotting is the most common cause of fungus gnat problems. The added new soil is often contaminated with the larvae. In addition, repotting is usually done improperly with soil added to the top of the original rootball. The added soil retains moisture for too long and provides the wet environment that nurtures the larvae. When soil stays too moist for too long, the roots begin to rot. The larva feed on the decaying roots and rotting roots are a death sentence for potted plants.

So the first lesson here is to avoid repotting unless absolutely necessary, and it rarely is. There is not space here for me to explain how to determine when to repot, but I have written extensively on the topic of repotting.

Here is how to treat the gnat larvae. Remove all loose soil from the surface of the rootball. Leave only enough soil to just barely cover the roots. Then, allow the soil to dry as deep into the pot as possible - just short of the wilt point. If the top inch or soil gets good and dry, the larvae that live mostly in the uppermost layer, will die from lack of water. When you do water, add just enough so that the top layer of soil dries out again in a week or less.

The adult flying gnats will die on their own accord within a week or so. Don't expect instant results, but you should see a gradual decline in the gnat population if you are very careful with the watering.

I have written detailed articles on treating indoor plant pests and on repotting that I will email for free to you (or anyone else) who emails a request to me at [email protected]. I have also written an indoor plant care book in a PDF format that I can sell you if you contact me at my email address.

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

If this information has been helpful, please click the Rate Volunteer bar below and enter a rating and NOMINATION for me. I am a volunteer on this site so Ratings are the only compensation I receive for answering plant questions.

Need more information? Visit my website at:
A link to HorticulturalHelp.com

or email me at [email protected] or call me at 917-887-8601 (EST)
 
Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

Visit my website at: A link to HorticulturalHelp.com