Host-parasitoid phenology at two different altitudinal gradients, the case of the chestnut galls wasp in a new expansion area

Parasitoids adaptation to a new host is related to their own phenology and it’s closely connected to the thermal trend of their diffusion area. We studied phenology and interactions of Asian chestnut gall wasp (ACGW) with native parasitoids in two altitudinal areas in southern Italy. We were able to develop phenology models to describe timing of cynipid stages (larvae, pupae, adults and adults emergence) across all sites and years, as a function of accumulated degree-days. Our results showed that these models were responding more accurately to the prediction where they were developed. Parasitoid species richness was different among the different areas and a minor number of species was recorded at lower altitude. The phenology estimator used (average of the occurrence days weighted by its abundance on each date) to compare the same parasitoid species at two different altitudes made evident that the reduction of the developmental time of some ACGW host stages (larvae and pupae-adults) makes parasitoids less flexible in their use as resource. The timing comparison between ACGW preimaginal and adult stages and the specific parasitoid complexes, stressed that the shortest period of occurrence of cynipids in higher temperature areas has a negative effect on the host-parasitoids relationship favoring a decoupling of phenology for some parasitoids. Species as Sycophila variegata, Megastigmus dorsalis and Torymus auratus show off a reduced parasitization to ACGW at higher thermal conditions. Overall, these results suggest that both host phenology and parasitoids species are affected in a similar way by temperature, but that ACGW is faster than parasitoids species in accelerating phenology in response to spring and summer warming.The findings underline how the parasitoids adaptation process to new host is limited by the short duration of host presence in the galls and could explains the considerable variability in the parasitization of the cynipid gall wasp at the different altitudes where chestnut is distributed.