Indigenous parasitoids associated with Dryocosmus kuriphilus in a chestnut production area of Emilia Romagna (Italy)

The associations occurring between the Asian chestnut gall wasp (ACGW) Dryocosmus kuriphilus Yasumatsu (Hymenoptera Cynipidae) and native parasitoids were investigated over the 3-year period 2010-2012 in a chestnut plantation located in the municipality of Castel del Rio (high Santerno Valley, Bologna province, Emilia Romagna, Italy). The chestnut trees were mixed with other plant species, including oaks, and were heavily infested by D. kuriphilus (which was first recorded in Emilia Romagna in 2008). In the survey period, a total of 12,015 spring galls were collected and an overall number of 7,094 insects emerged (5,182 D. kuriphilus and 1,912 parasitoids). The parasitoids, belonging to five families (Eupelmidae, Eurytomidae, Ormyridae, Pteromalidae, Torymidae), were presumably recruited from oak gall wasps. They were identified using morphological characters and the most represented genus was Torymus. Within this genus, the morphospecies Torymus flavipes (Walker) was identified. Mesopolobus was the second most abundant genus in 2010 and 2011. The percentages of parasitism (calculated considering an average of 3.5 cells) were 3.06, 10.18 and 0.09 in 2010, 2011 and 2012, respectively. The dramatic reduction of parasitoids found in 2012 might have been a consequence of biocoenotic dynamics, in particular of the instability of the interactions occurring between the ACGW and the generalist native parasitoids.