Q. Our house plants seem to breed a small black gnat. We use potting soil with our African violets and herbs. Anything we can do? It has been suggested soapy water spray? Fungus Gnat A. The gnat larvae or immature forms feed on organic matter in the soil and frequently on the roots of plants as well. There is an organic spray that can be used for fungus gnats which contains a bacterium called Bt. There are different strains of Bt so the one you use to control wormy tomatoes, corn earworm or grape leaf skeletonizer won’t work on fungus gnats. The strain has to be israelensis or sometimes designated as H-14 or strain AM 65-52. One product that contains this form of Bt is manufactured by Valent Biosciences and called Knock-Out®. It is available online from Gardens Alive. It is a liquid and applied to the soil. I have not seen it available in town. Follow the label directions. If you have a plant with fungus gnats in the container soil, do not bring it in the house. These little guys will spread to other soils in other containers if the soil has enough decaying organic matter in it.             Curiously enough, research indicates that some repellency of fungus gnats was provided by Bounce® fabric softener dryer sheets.