none For more than half a century Bucculatrix thoracella showed an extensive range expansion in Northwest Europe. The species was recorded for the first time from The Netherlands in 1967. Next it extended its range quickly, and at present the species is found throughout the country except on the West Frisian Islands. In The Netherlands the species produces two generations per year and it hibernates as pupa in a cocoon usually spun on a trunk of Lime (Tilia sp.). Population countings were performed at Nunspeet (province of Gelderland) since 1982. After settlement of the population a peak in abundance occurred, succeeded by a sharp decrease in numbers. Subsequently the numbers tended to fluctuate round this lower level of population density. These events might be partly due to the action of parasitoids which resulted in high losses of the population. Bucculatrix thoracella is attacked by a number of pupal parasitoids, e.g. the braconid Apanteles arisba. It is argued that the colonization of Northwest Europe (inclusive of the regions in Great Britain where the species did not occur until recently) took place from Central Europe.