gadfly

Short definition from Wiktionary

Noun

1. 1
   Any dipterous (“two-winged”) insect or fly of the family Oestridae (commonly known as a botfly) or Tabanidae (horsefly), noted for irritating animals by buzzing about them, and biting them to suck their blood; a gadbee.
2. 2
   A person or thing that irritates or instigates.
3. 3
   A person or thing that irritates or instigates: A person who upsets the status quo by posing novel or upsetting questions, or attempts to stimulate innovation by being an irritant.
4. 4
   Synonym of gadabout (“a person who restlessly moves from place to place, seeking amusement or the companionship of others”).
5. 5
   A person who takes without giving back; a bloodsucker.