Biological control agents used to manage alien vegetation are generally viewed as providing an ecosystem service, owing to reduced ecological and economic costs of invasion following their release. In particular, gall-formers are popular as biological control agents because they are host-specific and therefore considered low risk. However, galls can also be considered to be ecological engineers, because they provide nutritional resources for native invertebrates. We tested whether native invertebrates had formed associations with the gall-forming fungus Uromycladium tepperianum, introduced into South Africa to control the Australian invasive alien tree Acacia saligna, by collecting U. tepperianum galls and monitoring emergence. We found that a number of invertebrates had formed associations with the biological control agent, among which was the important citrus pest, Thaumatotibia leucotreta (false codling moth). We used pheromone-baited traps to ascertain if this supplementary source of T. leucotreta increased their abundance in orchards close to patches of gall host, but did not find this to be the case. We did find, however, that control measures used by farmers explained T. leucotreta abundances in traps, which may have obscured detection of any effects of a nearby host for the pest. Nevertheless, this study illustrates the first case of a host-specific classical biological control agent providing resources for an economically significant crop pest. We conclude that although biological control agents are strictly vetted to ensure host-specificity, introduced biological control agents that become abundant and can act as ecological engineers pose risks when native biota form associations with them, resulting in a number of possible cascading ecosystem effects. In addition, there could be economic consequences when these associated species include agricultural pests. We conclude that not just host specificity, but potential ecological effects of biological control agents, should be considered in their selection.