Fungus gnats, Bradysia spp., in greenhouses cause economic losses to horticultural producers by damaging young root systems during plant propagation, by spreading soilborne diseases, and by reducing the marketability of the crop. In a greenhouse cage study, our observations suggested that bagged soilless growing media or rooted plant plugs from wholesale distributors may be sources for the introduction of fungus gnats into commercial greenhouse facilities. To evaluate these possibilities, carefully collected samples of bagged soilless growing media stored in the greenhouse, as well as bagged soilless growing media and rooted plant plugs delivered from midwestern wholesale distributors, were incubated under controlled conditions in the laboratory. Fungus gnats emerged from soilless media stored in the greenhouse, soilless media delivered from wholesale distributors, and from rooted plant plugs delivered from wholesale distributors. These results demonstrate that pasteurization of even bagged soilless media may be essential to effectively managing greenhouse populations of fungus gnats. However, pasteurization is not an option for responding to contamination of rooted plant plugs. Preliminary evidence is provided that application of entomopathogenic nematodes may offer potential as a method for managing fungus gnats in plant plugs, so long as treatment is early. Other arthropods found contaminating soilless media and rooted plant plugs included the western flower thrips, Frankliniella occidentalis (Pergande), Collembola, Acari, Formicidae, Staphylinidae, Psychodidae, and other Diptera.