Maria strolled the halls of the generation ship, knocking on the vents. A maintenance robot was thirty meters behind her, spraying insecticide.
Humanity was traveling to a new home, but we didn't want to bring old pests with us. We had to get rid of the rats and roaches before we landed. We couldn't have the new civilization infested with vermin before it even had a chance to grow.
She turned a corner and knocked again, listening. A small voice called out "Thanks for the warning!" Dozens of tiny feet scurried in the walls, running out of range of the poisonous fumes. The creatures had been called brownies, fairies, leprechauns, and countless other names. They had been with humanity since the beginning, and now they were following us into space.
Greg looked up as Maria knocked on the wall. "What are you doing?"
She smiled. "Someone has to warn the duct sprites."
He laughed. "You and your silly superstitions."