Western cherry fruit fly, Rhagoletis indifferens Curran (Diptera: Tephritidae), is the major quarantine pest of sweet cherry, Prunus avium (L.) L. (Rosaceae), in the Pacific northwest of the USA and in British Columbia in Canada. Although spinosad bait (GF-120 NF Naturalyte® Fruit Fly Bait) is used for the control of R. indifferens in this region, the effects of alternate food sources on fly responses to this bait have not been studied. In this study, experiments were conducted to determine mortalities of flies exposed to hydrolyzed protein baits in the presence of sugar only and sugar + yeast extract food. All baits contained Entrust® (termed 'spinosad alone'). When flies were exposed to GF-120 with or without added ammonia compounds (uric acid, ammonium acetate, and ammonium carbonate) for 48 h, mortalities were higher in the presence of sugar only than in the presence of sugar + yeast extract, but when flies were exposed to spinosad alone, mortalities were similar in presence of either of the two foods. In another experiment comparing GF-120, Nu-Lure, Mazoferm, Baker's yeast extract, and spinosad alone, mortalities in the GF-120, Mazoferm, and Baker's yeast extract treatments were higher in the presence of sugar only than in the presence of sugar + yeast extract, but in the Nu-Lure and spinosad alone treatments, mortalities were similar in the presence of either of the two foods. Overall results suggest that the indirect effects of yeast extract food on mortality are dependent on bait type and that mortalities caused by spinosad alone and baits are similar. Nu-Lure and spinosad alone may have an advantage over other treatments for fly control, because their effects do not appear to be affected by the presence of nitrogenous food.