SOME BIOTIC LIMITING FACTORS OF DRYOCOSMUS KURIPHILUS (HYMENOPTERA CYNIPIDAE): A SURVEY IN CHESTNUT ORCHARDS NEAR FLORENCE (ITALY)

The biological control of the Asian Chinese gall wasp (ACGW) Dryocosmus kuriphilus, is influenced by several factors. In 2013, a survey was carried out in three chestnut orchards near Florence (Italy) to investigate ACGW parasitization by native parasitoids and the introduced Torymus sinensis. The following two methods were adopted: the first based on the dissection of cynipid galls with the aim of inspecting cells within each gall and the second based on monitoring adult emergence from galls. Parasitization patterns due to gall position related to plant height were also evaluated. Native parasitoids were more abundant in the upper part of tree crowns in June, while in July and August their activity was more evident in the middle and lower parts of chestnut trees. Conversely, parasitization of T. sinensis, was detected at all plant heights. Plant position within stands also affected the performance of both native parasitoids and T. sinensis, with parasitization that progressively decreasead moving toward the interior of the orchards. Gall necrosis was equally evaluated as a limiting factor of ACGW survival. Among the isolated fungal species, Gnomoniopsis castaneae (Tamietti), an endophyte fungus harmful to fruits, colonized both healthy and necrotic galls together with ubiquitous components of fungal communities such as Alternaria sp., Aspergillus sp., Fusarium sp. and Colletotrichum fioriniae. Our results evidenced that T. sinensis was able control the ACGW, while native parasitoids played only a secondary, though non-negligible, role. Conversely, gall necroses had a minor impact on pest control, as their efficacy was limited to the early stages of gall development.