Reasons for and effects of changes in the parasitoid complex of the horse chestnut leafminer, Cameraria ohridella (Lepidoptera, Gracillariidae)

The parasitism of the horse chestnut leafminer remains at a low level in Europe since many years. As a consequence, horse chestnuts in regions with more than 20years of C. ohridella infestation still suffer from heavy leafminer attack.However, results of our investigations show changes in the parasitoid complex over the years, reflecting three different strategies exemplified by three parasitoidspecies. (1) The abundance of the most common parasitoid, Minotetrastichus frontalis, seems to remain at the same low level, indicating that the parasitoid does not actively search for special leafminer hosts but rather encounter them by chance. The relative impact of these species decreases as other parasitoids gain importance. (2) Other species, like Pediobius saulius, seem to undergo a slowadaptation process. Their relative importance in the parasitoid complex increases at many locations infested for a long time. In the long run, parasitism of these species may reach levels exhibiting a controlling effect on the leafminer's populations. (3) A third group of species seems to follow the spread of its newhost and invade new regions. Starting in the year 2000, the originally eastern European species Cirrospilus talitzkii was recorded from C. ohridella in countries where it has never been found before on any other leafminer. Meanwhile, the species can be found as a regular element of the parasitoid complex of the horse chestnut leafminer in many south and central European countries.To the present, these changes in the parasitoid complex have no significant impacton overall parasitism of C. ohridella. However, there are measurable effects, like an increasing rate of pupal parasitism due to the action of P. saulius. In the longrun, changes in the parasitoid spectrum of the horse chestnut leafminer may increase the impact of parasitism and contribute substantially to the control of the invasive species in future.