Testing ecological interactions between Gnomoniopsis castaneae and Dryocosmus kuriphilus

An emerging nut rot of chestnut caused by the fungus Gnomoniopsis castaneae was reported soon after the invasion of the exotic gall wasp Dryocosmus kuriphilus in Italy. The goal of this work was to assess the association between the spread of the fungal pathogen and the infestation of the pest by testing if: I) viable inoculum of G. castaneae can be carried by adults of D. kuriphilus; II) the fungal colonization is related to the number of adults inhabiting the galls; III) the fungal colonization of chestnut buds and the oviposition are associated. Fungal isolations and PCR-based molecular assays were performed on 323 chestnut galls and on their emerging D. kuriphilus adults, whose number was compared between galls colonized and not colonized by G. castaneae. To test the association between fungal colonization and oviposition, Monte Carlo simulations assuming different scenarios of ecological interactions were carried out and validated through isolation trials performed on 597 and 688 chestnut buds collected before and after oviposition, respectively. Although DNA of G. castaneae was detected in a sample of 40% of the adults developed in colonized galls, the fungus could never be isolated from insects, suggesting that the pest is an unlikely vector of viable inoculum. On average, the emerging adults were significantly more abundant from galls colonized by G. castaneae than from not colonized ones (3.76 vs. 2.54, P<0.05), indicating a possible fungus/pest synergy. The simulations implying no interaction between G. castaneae and D. kuriphilus after fungal colonization were confirmed as the most likely. In fact, G. castaneae was present in 33.8% of the buds before oviposition, while no association was detected between fungal colonization and oviposition (odds ratio 0.98, 0.71-1.33 95% CI). This finding suggests that the fungus/pest synergy is asymmetrically favorable to the pest and occurs after oviposition.