Fly management in commercial kitchens is an investigation, and you are the detective. With cockroaches as a close second, flies probably are the most significant insect pests associated with improper sanitation levels, and the foundation of fly management in commercial kitchens is built upon good sanitation practices. Understand what you are stepping into.
The worst fly pests, such as dark-eyed fruit flies (Drosophila repleta), often are associated with the presence of lesser pest species, such as drain flies (Psychodidae). Flies will use drains for breeding sites, so focus on drains for your baseline treatments. Find and remove, and then treat the breeding sites, and you likely will reduce pest presence by 100 percent.
Remember, only focusing on adult flies will not solve the issue. Consistent presence of adults indicates either continued introduction or an unresolved breeding site. Take your time, note what is needed, and price accordingly.