That’s because certain species, including the most common one – Hippodamia convergens – prey on pests. There’s at least one Internet website that refers to ladybugs as the “Tyrannosaurus Rex” of the Insect World because of its predator tendencies.
Like many insects or animals, ladybugs, while useful, are misnamed. It isn’t a bug, but a beetle. Beetle lore has it that the ladybird beetle – as it’s known in Europe – was named after the Virgin Mary in the Middle Ages.