The forward has captained the side on a number of occasions and was awarded the Player's Player-of-the-Year at the club's annual awards at the end of the 2005/06 season. Horstmann repeated that feat at the end of the 2006/07 season when he was named Player of the Year and secured a dream double with both the Player of the Year and Player's Player of the Year award at the 2008/09 awards dinner.
Head Coach Richard Hill recently announced Horstmann would not be offered a new deal and was free to look for a new club.
"I've had a great time at Worcester over the past seven years, but now was probably the right time to move on and experience something different in my career," said Horstmann. "I will take with me plenty of happy memories and a lot of good friends from Worcester."