This is no longer news as it was originally posted in 2011, but it’s still a good technique for controlling this pest.
Here’s the deal.
Raymond A. Cloyd (Kansas State University, Dept. of Entomology) and colleagues hung these dryer sheets around infected plants and found they did have repellent properties. Fungus gnats avoided the area. (I’m told there’s also anecdotal evidence these sheets repel mosquitoes as well) 🙂
The chemicals in the sheets were analyzed and found to contain Linalool — known to be toxic to mites and Citronella — lemon scent we already know has short-term repelling characteristics. There were other chemicals associated with insect repelling as well but you get the idea. These things tend to work.
They are not going to be considered strictly organic by any stretch of the imagination.
And just to make life a bit more interesting, other products in the class may contain other chemicals that don’t work as well so you’re going to have to do your own trials if you decide to go down this road.
My bottom line
Fungus gnats survive on soil algae for the most part and this is caused mostly by overwatering. So fix your watering first. Don’t rely on one more set of chemical cures to counteract poor garden management practices.
Having said that, if I were still starting hundreds of thousands of seeds every spring in the nursery, I’d have to think about hanging them around the propagation house like clouds. 🙂
We never really had a serious outbreak but here and there (mostly due to variations in the ventilation patterns or soil differences) I’d see them pop up.
A bit of insecticidal soap as a spray and drench also worked nicely for me.