The wasp symbolizes India itself. Mrs. Moore admires the wasp for its beauty, especially since it is such an Indian wasp and not at all British. However, she completely underestimates or ignores its power to sting.
In the same way, she admires India's beauty, forgetting that this is a country desperately trying to survive and keep its own identity. That beautiful "sting" will eventually be aimed at the British raj, as she finally understands at the end of the story, as she leaves to return to England.