Biological Control of Fungus Gnats: Bt toxin or Parasitic Nematodes

I seem to be having a problem with fungus gnats with several of my plants and it appears to be spreading. I know the fungus gnats won't actually hurt the plants but for aesthetic reasons, I would like to get rid of them. Because this is indoors and also because of my ethics and concern for my safety, I would like to avoid using any chemical pesticides. I tried one application of a pyrethrins and sulfer spray produced by Bayer call Natria, however they came back and it also smelled horrible so I don't plan to use that again. I was thinking of using either a Bt toxin product formulated for fungus gnats(Gnatrol) or parasitic nematodes that attack the larvae(Nema-Globe is one brand). The Gnatrol has a longer shelf life (1-2 years) than the nematodes(1 month refrigerated) so if I need to use it again the Gnatrol would be a better bet, however the nematodes can establish a sustainable population in the media so it may offer longer lasting protection. On another note, the Gnatrol ($8.50) is significantly cheaper than the nematodes($19).

Does anyone have experience with one or both of these methods and what have your experiences been with them? Thanks everyone.

 

That is good to know that it can be a sign of degraded medium, however I don't think that is the case in my situation. Excluding one orchid, all of the orchids affected have been repotting into fresh media within the last few months and my growing space (my house) is not humid or hot enough to cause really rapid breakdown of media. I am just realizing that the gnats on some of these orchids may actually not be breeding in the mix, as it isn't especially far(10-12ft) from the fruit bowl and the bin we keep food waste in until we get around to taking it to the actual compost pile. They may just be migrating over there from the fruit bowl/food waste bin, where there are occasionally some fruit flies.

I am not sure when the one orchid excluded above was last repotted as I bought it in its current pot about 9 months ago, but the bark mix in it looks solid and fresh(checking by look and feel from both the top and through the drain hole). That plant is also growing like crazy and looks super healthy, which is another sign(in my mind) that the mix is not bad. Growing as well as it is, I really don't want to disturb it if I don't have to. This orchid ( a No ID grocery store phal), has had fungus gnats on and off for probably about a month or two and it is pretty far from the kitchen in a room that the door is usually shut on, so I know its not fruit flies. All the others, I just noticed had gnats in the last few days.

Another question, are fungus gnats a different species (or group of species) than ordinary fruit flies or do we just call them different names in different settings? They seem visually indistingushable, but I know that doesn't necessarily mean much.

 

I went with the parasitic nematodes and it seems to have worked with only one application. Its been about a week and no sign of gnats. I'll probably do another application in a little while to try to establish a sustained population.