The following was received via email: I have a Bay, 'Laurus Nobilis' in a pot which is outside for summer. When I bring it indoors for winter, is there anything i should do to prevent possible fungus gnats coming in with it & thus contaminating my indoor plants. I thought that perhaps removing the top few inches of potting soil would help. I live in Calgary, Alberta & the Bay tree i bought came from California, already heavily infested with fungus gnats. I purchased some nematodes & have used those yellow sticky sheets, both of which have helped. The tree is in good condition. Thank you,
I have heard that a quarter inch of sand on top confuses the knats and they don't lay eggs -- no egg no pupil/larve. However this does require you get rid of them to begin with though. They actually go for fungus but as you well know eat roots as well.
Did a search on sand and fungus gnats and lots of hits! Seems a lot of pot growers do that trick (except they mention 1/2 inch). Anyway this site had a good explaination. http://www.saferbrand.com/articles/fungusgnats.htm and recomends sand. A old book I read discussed bed for propagation and the last thing it mentioned was putting a quarter inch of sand on top. I didn't use the quarter inch of sand because I had on explaination for it at the time -- but I now know why it is there :)
Coarse sand works somewhat. Cinnamon(Cassia sp.) sprinkled on top of the soil surface works. But you should address the soil moisture levels first, if the top soil surface is dry it does hinder them. Aluminum foil over the surface, limits their access, but also limits oxygen exchange.