Caulk or cover all openings into a home to prevent flies from entering. Efforts to exclude flies must be done prior to when they enter buildings. For example, cluster flies rarely are found indoors until late winter and spring but typically enter buildings during late August and September. Insecticides used for control should only be considered as a supplement to other controls. Serious problems exist with insecticide-resistant flies and many fly populations are now difficult to control with insecticides.
Spot treatments with insecticides applied to areas of high fly activity are most efficient.
For example, flies that tend to rest in dark corners can be controlled by applications to these areas. Cluster flies are controlled by treatments applied to upper stories of building exteriors immediately prior to periods when flies move indoors for overwintering. Where fungus gnats are a problem, insecticides can supplement the cultural control of reduced watering. Houseplant aerosols that contain pyrethrins or resmethrin, applied at two to three-day intervals for three to four weeks, should eliminate most of the adult fungus gnats. These products slowly release the insecticide as a vapor and provide long-term control.