Severely injured plants make poor growth, go off color, and may drop their foliage. Potted plants may lose vigor and the leaves turn yellow, without any visible injury showing on the above-ground parts. Root hairs are eaten off, as are the small feeding roots. The chewed roots have small brown scars on the surface. In cases of severe feeding, all that is left is the center part of the root.
Maggots can also cause injury to underground stems.
Severe infestations have occurred in carnation benches. Fungus gnats can cause serious damage to such flower crops as African violets, carnations, cyclamen, geraniums, foliage plants, and poinsettias. Any crop that is grown in a high organic soil like peat moss, may harbor the pests. They are laid singly, or scattered in groups or strings of 3 to 10. Eggs hatch in 4 to 6 days.