Q. Dear Neil: We bought a house recently and are stumped by a fungus gnat infestation. We have absolutely no houseplants, yet we have fungus gnats in the house and on the insides of the windows. We have had a pest control operator apply an organic remedy without results. Weâve had a plumber look for leaks and he found nothing. We donât know where to turn next. Do you have any suggestions? A. At the risk of telling you things you already know, fungus gnats live in the green âmossyâ algal growths that develop where there is constant moisture. Iâm sure thatâs why you had the plumber look for leaks. As a horticulturist, Iâm trained to look for poorly draining pots, wet greenhouse floors and other similar surfaces. But in a structural pest kind of situation youâd need to stick with the certified pest control operator or an entomologist with the Texas AgriLife Extension Service of Texas A&M. Itâs possible there could be seepage through the roof and into side walls of your house, or it could be from cracks in the foundation or brick or stone veneer. Iâm really out of my element. A structural engineer or a really good building contractor might be better able to advise you. For what itâs worth, a single spray usually wonât eliminate fungus gnats because they are present in several different life stages. You might be able to kill the flying adult gnats, but there would be eggs waiting to hatch. It usually takes time. And in the process, you must find the source of the moisture that is giving them a place to establish their home.