Symbiotic association of three fungal species throughout the life cycle of the ambrosia beetle Euwallacea nr. fornicatus

The presence of three symbiotic fungi: Fusarium euwallaceae, Graphium euwallaceae and Acremonium pembeum was detected in the larvae and adult beetles of Euwallacea nr. fornicatus (Coleoptera: Scolytinae) and from the brood galleries of four tree species. Isolations from female beetle mandibular mycangia recovered mainly G. euwallaceae and A. pembeum during adult maturation, whereas isolations from mature adult beetles revealed F. euwallaceae, almost exclusively. Only G. euwallaceae (from adults) and mostly G. euwallaceae (from larvae) were isolated from the guts of the examined beetles. Within larvae, G. euwallaceae was quantified at much higher concentrations for all the tested tree hosts, compared to F. euwallaceae and A. pembeum. Fungal mandibular mycangial loads at each of the adult beetle stages among all tested plant hosts were uniform, while larvae-fungal combinations varied extensively between the sampled sites regardless of the host species; a similar trend of inconsistency for the three fungal combinations was also observed in the galleries. The percentage of feral, naturally occurring, larvae carrying the symbiotic fungi was significantly high, while most of the pupae sampled from these trees did not carry any of the fungi. On artificial rearing medium, larvae fed and completed their development solely on F. euwallaceae and G. euwallaceae but not on A. pembeum. Laboratory-reared larvae fed with a single fungus harbored the other fungi as well. The occurrences and roles of F. euwallaceae, G. euwallaceae and A. pembeum in the beetle developmental stages and in the host trees, along with the life cycle of the beetle, are discussed.