When you see a wasp, you probably fear a possible sting, but don't worry, they can't sting humans. Their stingers are modified for injecting their eggs into caterpillars that the hatched young then feed off of, killing the caterpillars. Because the caterpillars are crop pests to corn, the wasps do a service to farmers. Scientists have found that the wasps' secret in tracking down the caterpillars is their strong sense of smell. Plants that have been attacked by caterpillars emit an odor which the wasps pick up on.