Pieters is the event's defending champion, and he was set to make his first start since Darren Clarke added him to the European roster for Hazeltine. Instead, the 24-year-old Belgian posted a note to his social media accounts Thursday morning explaining his decision.
"The very unfortunate worst case scenario just happened," Pieters wrote. "One bee sting on Monday during the golf challenge against Joost Luiten, led to a major allergic reaction with flu symptoms and forced me to pull out of my favorite tournament of the year, the KLM Open."
Pieters is one of the hottest players in the world, having followed a fourth-place showing at the Olympics with a runner-up finish in the Czech Republic and a win in Denmark. That victory, his third European Tour title in a little over a year, came after Pieters played the first two rounds alongside Clarke and it helped him edge Scotland's Russell Knox for Clarke's third and final wild card pick.
Pieters' next event is now expected to be the Porsche European Open in Germany, held Sept. 22-25, the final European Tour event before the Ryder Cup.