It may be that after adulthood, the mouth degenerates and cannot eat, or it may be that adult fireflies only have a lifespan of 5 days, and they are all used to mate and breed the next generation.
Adult fireflies eat only a little bit of dew and pollen until they starve to death. In fact, most of the fireflies die in the hands of humans. Now, due to technological development, the fireflies are disappearing little by little.