Interactions between three symbiotic fungi associated with an invasive ambrosia beetle and their host trees in Israel

The ambrosia beetle, Euwallacea nr. fornicatus (Coleoptera: Scolytinae), is a new invasive species to Israel, reported in 2009. Consequently, it was found that E. fornicatus is comprised of a complex of at least three cryptic species that harbor different symbiotic fungal species, all highly specific to their beetle hosts. Recently, a monophyletic group of Fusaria within the F. solani species complex was defined, each with exclusivity to their Euwallacea ambrosia species. Isolations from E. nr. fornicatus female beetle mandibular mycangia and brood galleries in the attacked host trees revealed the presence of three novel fungal symbiotic species: Fusarium euwallaceae, Graphium euwallaceae and Paracremonium pembeum. The spores act as feed once the mature adult female beetles colonize the host plant, where they tunnel the brood galleries and release the spores that germinate, generating mycelial layers that eventually line the galleries and serve as fungal gardens for the developing larvae. Upon beetle attack, the mandibular mycangia comprise 99% of the total spore count of F. euwallaceae. G. euwallaceae, the main food source for developing larvae, appears on mass approximately a week after establishment of the new gallery. The role of P. pembeum is not yet clear. F. euwallaceae acts as a pathogen and is responsible for host plant damage. Xylem inoculations with each of the three fungi alone indicated that only F. euwallaceae survives for long periods but spreads very slowly, while the other symbionts disappear rapidly from live issue and survive in dead xylem after beetle emergence. The beetle attacks more than 50 tree species in Israel but only ten (representing different botanical families) are suitable for reproduction, among them in descending order, Acer negundo, A. buergerianum, Quercus robur, Q. pedunculiflora, Platanus spp., Ricinus communis and Persea americana. The symbiotic fungal interactions and their roles in host plants will be discussed.