ABSTRACTInvasive species pose a significant threat to global biodiversity and ecological services. The specific threshold of the initial population size necessary for the successful establishment of an invasive species remains uncertain. This research delves into the effects of different initial population sizes on the colonisation and population dynamics of the globally recognised invasive pest, Cydia pomonella, within a controlled laboratory setting. The results revealed that even a single pair of C. pomonella displayed notable eggs laid (fecundity) and hatched (fertility) by producing offspring upon mating. Larger initial population size exhibited increased mating rates, leading to enhanced fecundity and subsequently more offspring, indicating that the development of C. pomonella population is directly impacted by the mating rate of the initial individuals. Projections based on F1 data indicate that the C. pomonella population size could persistently increase over a span of 150 days, with varying growth rates influenced by the initial population size. These findings indicate the importance of heightened monitoring and management in non‐invaded regions to avert early colonisation of C. pomonella, thus mitigating the threat of invasion and subsequent population expansion.