Poor synchronisation is considered to be one important reason for the ineffective control of the invasive horse chestnut leafminer by native parasitic Hymenoptera. Parasitoids hibernating in dry horse chestnut leaves break diapause early in spring and presumably leave the vicinity, since no hosts are available when they emerge. As a consequence, the percentage parasitism of the first generation of the leafminer in early summer is low. The experiments presented below were designed to test this hypothesis. Horse chestnut saplings were brought on in a greenhouse and infested artificially with C. ohridella prior to parasitoid emergence in the field. These saplings were then exposed to parasitoid attack under natural conditions to eliminate the synchronisation problem. In addition, the parasitoid complexes of other leafmining hosts, which appear early in the season, were analysed. The results confirm that the most important parasitoids of the horse chestnut leafminer are active early in the season, long before the larvae of the first generation of the host are present. Nevertheless, poor synchronisation with the invasive host did not significantly influence the abundance of the most important parasitoid in the complex, Minotetrastichus frontalis, and consequently had no impact on the parasitism of C. ohridella. Nevertheless, a detailed analysis of the parasitoid community shows that certain species are affected by poor synchronisation, while others are probably limited by ecological parameters, such as a specialisation to foraging in particular strata of vegetation. Several leafmining flies were found early in spring infesting other host plants commonly planted in urban green areas, including species with a parasitoid complex similar to that of C. ohridella. The role of these alternative hosts in the food web associated with the horse chestnut leafminer should be subject to further study.
The horse chestnut leaf miner, Cameraria ohridella Deschka & Dimić, is a species of unknown origin that recently invaded most of Europe, causing serious damage to horse chestnut trees, Aesculus hippocastanum. Parasitism was studied over a period of three years in the region of Plzeň in the Czech Republic. Additional collections were made in Slovakia and Slovenia. The parasitoid complex, dominated by polyphagous idiobiont parasitoids of the family Eulophidae, is similar to that found in other studies in Europe. Minotetrastichus frontalis (Nees) was the most abundant parasitoid found, except in Slovakia where Pediobius saulius (Walker) dominated. One parasitoid species, the eulophid Cirrospilus diallus (Walker) was recorded for the first time from C. ohridella. A new method is proposed to calculate stage-specific and total parasitism rates. Parasitism rates of spinning larvae and pupae were higher than of feeding larvae; however, total parasitism was low. We estimated that between 1% and 17% of moths died from parasitism during the larval and pupal stages whereas the rate of mortality caused by other factors varied from 7% to 62%, depending on the locality, year and generation. The proposed method for calculating total parasitism, based on the integration of stage-specific parasitism rates, which takes into account the mortality not directly attributed to parasitism, is discussed and compared with other methods commonly used in studies on C. ohridella.