Do you know that planters and potted plants are the number one source for winter fungus gnat?? Fungus gnats thrive in dampness and decaying vegetation. With adults looking similar to miniature mosquitoes and the larvae are worm like about 1/4" long.
Before treatment be sure to do a proper identification, in order to do a successful treatment.
Source: Techletter Vol 6 No 4
February 18, 2015