First use clear packing tape, with holes punched in the top and place over drain to see if there are drain flies coming out of the drain. Use clear tape so you can see thru it and punch holes in it for ventilation.
Use the Invade Bio Drain Gel to eliminate the source from the drains
Use the Stryker 54 Aerosol to treat drain flies as a space spray to kill on contact
(A) 1 x 1 qt of Invade Bio Drain Treatment Apply 4 oz. to each drain around the edges to coat the sides. Repeat daily for 5-7 days and then every 1-2 weeks for ongoing maintenance. For drains that have a very heavy layer of scum, clean the drain with a brush before beginning treatment for immediate results and then maintain with Invade Bio Drain to prevent further build up.