Although the tubes in your fly zapper may appear to be working and giving out plenty of light after use of around 8000 hours run time the amount of useful insect attracting UV light coming from your old lamps drops to a level where it will no longer be attracting insects to your bug killer. These Replacement UV blacklight bulbs for bug killer machines fit most 11w zappers that use TGX11 bulbs such as Insect-O-Cutor F1, F2, HyGenie M11