A temperature- and photoperiod-driven model reveals complex temporal population dynamics of the invasive box tree moth in Europe

The box tree moth, Cydalima perspectalis, is an invasive insect that has rapidly colonised Europe, damaging to natural and ornamental box trees. In its native habitat in China, the number of generations per year is variable, but the number of generations observed in European climates remained unclear. This is a key issue for understanding the rapid increase in population size and range, and for optimising control. We developed a temperature- and photoperiod-driven model to simulate the life cycle of this insect and development rates for each life stage. The model was calibrated on published data and validated with observations obtained in France and Switzerland. Model stability analysis showed that minimal temperature for larval development to be the most important parameter to estimate. Diapause parameters had little effect. We then explored the effects of temperature increases of 1 and 2 °C. The number of generations ranged from two to four at the various study sites. Climate warming will accelerate the insect life cycle, making it possible for the occurrence of one more generation per year. The key finding of this study was the complexity of population dynamics for this species. Some generations overlapped, making it difficult to identify the adult flight period clearly for each generation. Furthermore, various stages were potentially able to overwinter, not just diapausing larvae. Climate warming may also enhance this phenomenon in the future. Further explorations of the complex dynamics of this species are required, notably it remains unclear how successfully the various life stages survive winter temperatures. Further model refinements are also required to obtain more accurate estimates of box tree moth phenology. However, this is the first phenology model for box tree moth to be published, and our findings provide useful information for improving control of this pest.