Phytophagous insects from different feeding guilds may compete indirectly via altering the chemical defenses or nutritional quality of their shared host plants. Gall-formers are understudied in this context but may be susceptible to this mode of competition early in their life history, when they may be particularly sensitive to changes to the specific reactive tissue needed for gall development. Here, we conducted a natural experiment to investigate the effect of folivory of Canada goldenrod, Solidago altissima, by Trirhabda beetles on gall induction success for the goldenrod gall fly, Eurosta solidaginis. We monitored oviposition events and gall development on individual Solidago ramets at sites differing in their levels of Trirhabda folivory. We found a strong inverse relationship between Trirhabda leaf damage and successful gall induction rates. These results suggest Trirhabda beetles may negatively impact Eurosta demographics and highlight the need for further study on how factors affecting gall induction may influence the structure of insect communities.