Reasons for and effects of changes in the parasitoid complex of the horse chestnut leaf miner (Cameraria ohridella)

摘要

Parasitism of the horse chestnut leaf miner (HCLM) (Cameraria ohridella) has remained at a low level in Europe for many years. As a consequence, horse chestnut trees in regions with more than 20 years of HCLM infestation still suffer from heavy leaf miner attack. However, results of our investigations show changes in the parasitoid complex over the years, reflecting three different strategies exemplified by three parasitoid species. (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 leaf miner hosts but rather encounters them by chance; the relative impact of these species decreases as other parasitoids gain importance. (2) Other species, such as Pediobius saulius, seem to undergo a slow process of adaptation; 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 reachlevels exhibiting a controlling effect on HCLM populations. (3) A third group of species seems to follow the spread of its new host and invade new regions. Starting in the year 2000, the originally eastern-European species Cirrospilus talitzkii was recorded from HCLM in countries where it has never been found before on any other leaf miner. Meanwhile, the species can be found as a regular element of the parasitoid complex of HCLM in many south and central European countries.To date, these changes in the parasitoid complex have no significant impact on the overall parasitism of HCLM. However, there are measurable effects, such as an increasing rate of pupal parasitism, owing to the action of P. saulius. In the long run, changes in the parasitoid spectrum of HCLM may increase the impact of parasitism and contribute substantially in future to the control of this invasive species.