We built a DYMEX population dynamics model of the invasive land snail, Cochlicella acuta, and one of its dipteran parasitoids, Sarcophaga villeneuveana. The snail lifecycle consists of different size classes, based on shell height, as both parasitism and reproduction are influenced by snail height. The model reveals the likely role of small, cryptic snails in maintaining populations in the face of biological control and suggests that suppression of snail populations may only be possible with unrealistically high parasitism rates and with repeated annual spring releases of flies. The current model appears to describe the phenology of both the snail and fly adequately; however, construction of the model identified numerous knowledge gaps. Snail control would benefit from a thorough understanding of its basic biology and what drives its population dynamics – aspects which, surprisingly, have been largely ignored. Otherwise, any control measures will likely be unpredictable, with no real understanding of how or why management options may or may not work. We highlight the importance of modelling, preferably at an early stage, to synthesise available knowledge and data into a framework that helps target research at an early stage, and the benefits of experimentation via modelling.