Drain flies, also called moth flies, are small flies that develop in decaying organic matter in sink or sewer drains. Their wings are covered with fine hairs, giving them a moth-like appearance. When many adults emerge at the same time, they can become a nuisance in homes or a health concern in commercial kitchens, processing plants or health care facilities. Regular inspection of drains and elimination of breeding sites are essential for keeping drain flies under control.
Read more about biology and management about these pests in a new UC IPM Pest Note: Moth and Drain Flies by David Theuret and Alec Gerry, Entomology, UC Riverside. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74167.html.