Fungus gnats? Insecticides in heat?

(Deborah Ballanti) #1

Hi all.

I have recently noticed many small black flies/gnats congregating in the soil of a number of my bonsai…from maples to spruce.

I live in Sacramento, have been watering a lot due to high temperatures and recently started using organic fertilizers which I simply spread on top of the soil. After reading online, I am guessing these are fungus gnats attracted to the moisture and fertilizer. I could be mistaken and would be interested in other thoughts.

I will remove the fertilizer today and try watering less.

The questions: Can I use an insecticide? If so, recommendations on which one? Can it be applied in this heat? (90’s - low 100’s).

Thanks for any input.

(Frank Corrigan) #2

Removing the organic and allowing the surface to dry out between watering should do the trick. I would try that first before going for any insecticides.

(Deborah Ballanti) #3

Thanks, Frank. Will do. Appreciate the input.

(Mike ) #4

Might try Insecticidal soap. If have used it in the past. Mild on the tree in the head too.

As mentioned, etting the tree safely dry out is best preventative


Fungus gnats are a real nuisance during the “rainy season” here in coastal SoCal and love to get inside the house. I have taken to using mosquito dunks, which have the bacteria bacillus thuringiensis (Bt). I get the kind that are granular in a dissolvable packet and break them open to sprinkle in the garden, flower beds, etc. I also sprinkle them in my nursery cans that have a higher organic substrate portion. At the low rate that I sprinkled them out, it doesn’t seem like it would do much, but there has been a noticeable difference for several years. I also use a garden store boxed organic fertilizer which lists Bt in the ingredients.

(Darth Masiah) #6

you could use a top dressing of growstones gnat knix if you dont want to spray anything. the growstones have sharp edges that kill the gnats. probably the simplest approach ever.

(Deborah Ballanti) #7

Thanks to all for the suggestions. I got rid of the organic fertilizer for the time being and let the soil dry out a bit…seems like they are gone for now. I will definitely save your suggestions.