Climate Warming and the Dynamics of the Invasive Box‐Tree Moth <i>Cydalima perspectalis</i> in the Suburbs of Basel (Switzerland) and in the Nearby Natural Box‐Tree Forest: A 15‐Year Study

ABSTRACT The non‐native invasive box‐tree moth ( Cydalima perspectalis ) causes severe damage to ornamental box trees ( Buxus spp.) and natural boxwood stands ( Buxus sempervirens ). Using two light traps, we recorded the seasonal flight activity and abundance of C. perspectalis in the suburbs of Basel (Switzerland) over a 15‐year period (2009–2023) shortly after the moth's introduction to Europe in 2007. In each year, we also assessed the grazing damage and number of box‐tree larvae in a nearby natural box‐tree forest. We recorded two peaks in the number of C. perspectalis caught in the settlement area (2009–2010 and 2017–2018). Cross‐correlation analysis showed that waves of C. perspectalis moths invaded the settlement area in the year after the peak years of forest damage. Moderate numbers of C. perspectalis were caught each year during off‐peak periods. The outbreaks of C. perspectalis resulted in a complete defoliation of the box trees in the forest, after which the moth population collapsed. The slowly regenerating box‐tree forest was recolonised by C. perspectalis , most probably from the settlement area. After the second outbreak, the box‐tree forest did not appear to be able to fully recover, as moderate numbers of C. perspectalis larvae fed on the emerging new leaves. The annual mean temperature measured in Basel increased by 1.5°C between 2009 and 2023. During this period, the first C. perspectalis moths were caught earlier each year and the last moths were caught later each year. Thus, the duration of flight activity of C. perspectalis (all three generations of a year combined) increased from 13 weeks in 2010 to 20 weeks in 2023, indicating the impact of climate warming. Our study shows that the dynamics of C. perspectalis in the settlement area are strongly influenced by the presence of box trees in the adjacent forest.