Resistance woes compound when a weed biotype resists multiple herbicide modes of action. In 2005, weed scientists identify triple-stack resistance of glyphosate, PPO inhibitor, and ALS inhibitor resistance in waterhemp. By 2009, quad-stack resistance to glyphosate, ALS inhibitors, triazines, and PPO inhibitors to an Illinois waterhemp biotype is identified. By 2009, HPPD-inhibitor resistance surfaces in both Iowa and Illinois in triple stacks with other modes of action.