In principle, multiple weed biological control agents may provide effective weed suppression, even when individual species provide inadequate suppression on their own. This key idea in weed biological control is called the “cumulative stress hypothesis”. I experimentally evaluated this phenomenon for two biological control agents of Canada thistle: the stem mining weevil Ceutorhynchus litura and the gall fly Urophora cardui. Initial results for thistle biomass suggest weak effects of the gall fly and antagonism in the combined effects of the two agents. In particular, galling and its impact were reduced in plants attacked by the weevil. Results question whether cumulative stress is imposed on Canada thistle by these two insects.