Although the life history of the invasive terrestrial slug Arion vulgaris Moquin-Tandon, 1855 has been well studied, there are no long-term studies on the variation in its abundance. In 2003–2022, we used plasticine baits to monitor annual fluctuations in the abundance of a local Arion vulgaris population in Central Europe and studied the factors that influence these fluctuations. The consumption of plasticine, which is a reliable proxy of the abundance of slugs, varied between different years by a factor of 43. The maximum intensity of consumption occurred in July. The variability in slug abundance, as estimated from plasticine bait consumption, was irregular across years. Oviposition took place from September–November. The number of eggs laid did not limit the abundance of the filial generation. Slugs spent the winter period in the egg stage (∼25% of the population) or as juveniles, whose winter mortality varied from 18–100% among years. Winter temperatures were not a significant predictor of slug abundance in the following season, whereas the weather in spring and early summer determined the abundance. Amount of precipitation in April and May had a positive effect (P = 0.01). In contrast, the number of days from April–July in which minimum temperature at a depth of 2 cm below the surface exceeded 18°C (i.e., the optimum temperature for slug development) had a negative effect (P = 0.045). These two factors combined explained 55% of the interannual variability in slug abundance (r = 0.74). The robustness of these results was backed up by the data from a spatially isolated nearby population, showing similar patterns of annual changes in abundance.