Host suitability of a mutualistic bark beetle-fungal complex implies an ecological adaptation to fringe habitat

[Objective]The Asian larch bark beetle,Ips subelongatus(Coleoptera:Curculionidae:Scolytinae),can pose a serious threat to larches,and occasionally to other conifers,like pines and spruces,showing a strong host expansion potential in the coniferous forest.Endoconidiophora fujiensis,a strongly virulent fungus to larches,has formed a stable association with I.subelongatus in northeastern China.However,host suitability of I.subelongatus on novel hosts and to what extent the E.fujiensis is involved in host range expansion process of its beetle vector remain unknown.[Method]In this study,we selected Sandy spruce(Picea mongalica),Korean pine(Pinus jezoensis),and Khingan fir(Abies nephrolepis)as potential novel hosts to test the host suitability of the I.subelongatus-E.fujiensis complex,with historical host Dahurian larch(Larix gmelinii)being as a control.The development performance and pheromone production of I.subelongatus on these four tree species indoors as well as the infection ability of E.fujiensis on these four vigorous trees outdoors were measured firstly.We then analyzed the differences in defensive chemicals(e.g.,monoterpenes and phenolics)of these four tree species induced by E.fujiensis,and determined the nutrient contents(e.g.,nitrogen and ergosterol)accumulated on the phloem lesion after E.fujiensis infection over a period of time.Finally,beetle tunneling/fungi growth performances in media amended with monoterpenes and phenolics were tested.[Result]Based on the development indices and aggregation pheromone productions of I.subelongatus as well as infection ability of E.fujiensis,both I.subelongatus and E.fujiensis showed relatively well host suitability on P.mongalica when compared to the historical host of I.subelongatus,L.gmelinii,followed by novel host P.jezoensis,whereas exhibited weak host suitability on A.nephrolepis.Further,the nutrient accumulation(e.g.,nitrogen and ergosterol)and chemical induction defenses(e.g.,monoterpenes and phenolics)after E.fujiensis infection were clarified to assess the fungal assistance capability to beetle vectors on four trees.Generally,E.fujiensis supplied nutrient and exhausted defense the most on L.gmelinii,followed by P.mongalica,P.jezoensis and A.nephrolepis.Finally,the key monoterpenes and/or phenolics were amended in culture media,wherein phenolics generally showed stronger inhibiting effect on the tunnelling behaviors of I.subelongatus and fungal growth of E.fujiensis than those of monoterpenes.[Conclusion]I.subelongatus-E.fujiensis complex can establish populations on novel hosts P.mongalica and P.jezoensis,instead of A.nephrolepis.Although the assistance effects of E.fujiensis to I.subelongatus on host adaptability varied among tree species,the nutrient production and defense exhaust of E.fujiensis on L.gmelinii are stronger than those on novel hosts,suggesting a closer coevolutionary relationship between"I.subelongatus-E.fujiensis"complex and the historical host L.gmelinii.