Fruit flies, biting flies, and other flying insects are naturally drawn towards light, which is part of what makes the DynaTrap Insect Traps so effective. Keeping replacement bulbs on hand if your DynaTrap needs a replacement bulb prevents any interruptions in your defenses against these pest insects. Each package comes with one 9 watt replacement bulb that fits DynaTrap models DT3009, DT3019, and DT3039. DynaTrap Insect Traps need fresh and powerful bulbs to attract the maximum amount of insects away from you and towards the trap. For the best results, it is recommended to replace each bulb after 3,000 hours of use or approximately every 4 months. Once your trap is equipped with fresh bulbs, you can rest assured your trap will deliver the protection you have come to rely upon.