A critical density of four third-instar larvae per 900 cm² for European chafer, Rhizotrogus (Amphimallon) majalis (Razoumowsky), in winter wheat, Triticum aestivum L., was derived from smallplot greenhouse and field experiments conducted under favorable crop growing conditions at several Ontario and Michigan locations from 2001–2003. On average, plant weight was decreased by 14% and plant stand by 11% between zero and four larvae per 900 cm². In a commercial field under moisture stress, a yield loss of 35% occurred at a density of two third-instars per 900 cm². In short-term greenhouse experiments, density-dependent mortality was evident, whereas low larval recovery in field experiments indicates a high level of overwintering mortality, regardless of larval density. Winter wheat seed treatments of neonicotinoid insecticides, clothianidin, imidacloprid, and thiamethoxam provided protection from damage by larvae, but the level of protection was inconsistent between greenhouse and field small plots, and there was no apparent difference in protection amongst active ingredients or between application rates. There was little evidence of larval mortality owing to seed treatment, which supports the suggestion that neonicotinoid insecticides protect seedlings from loss by a nonlethal mechanism. Overall, we estimate that a low rate of neonicotinoid insecticide used at larval densities just less than the critical density will mitigate winter wheat losses by 85%.