Sewer Flies in Shower Drain

This video was taken inside of a residential town home. Sewer flies and fly larvae can be seen inside of the P Trap underneath a standup shower. Incorrectly pitched sewer lines or a sewer backup are the only two reasons waste water is able to enter a P Trap under the floor which will cause fly larvae to grow and reproduce which will lead to sewer flies. We thoroughly flushed the drain to get rid of the sewage. If the lines are not pitched correctly, the sewage will come back which will lead to more sewer fly issues.