SEAWEED FLIES – House is located 20 meters from the sea. In late fall, during a spell of warm weather, we are infested by small (3/16 in.) slender flies that seem to breed in seaweed. They come out of seaweed in hoards when it is disturbed. Once inside they seek the windows. Difficult to kill, need to be squashed after the swat. Once the window is cleared, they reappear in no time. Never had them in the house in spring until this year. Result of egg laying? Petit-Rocher
These most likely are kelp flies (Diptera: Coelopidae). Their larvae feed on stranded and rotting seaweed, and they can become quite abundant. About all you can do is to keep all windows and doors tightly closed or screened.