gadfly
n
1 fly that stings horses and cattle 虻; 牛虻.
2 (derog 贬) annoying person, esp one who provokes others into action by criticism, etc 惹人讨厌的人(尤指用批评等手段怂恿别人采取行动者).
He says nothing about the poor selection of goods in the shops, or about those unfortunate people who have to travel from the country to the city every day to get to work.