New insights into the Scolytus multistriatus, Geosmithia spp., and Ophiostoma novo-ulmi association

Dutch elm disease (DED) is a destructive tracheomycosis caused by Ophiostoma novo-ulmi, an ascomycete that is devastating natural elm populations throughout Europe, North America, and part of Asia. The fungus is mainly spread by elm bark beetles (EBBs) (Coleoptera: Curculionidae, Scolytinae) that complete their life cycle between healthy and diseased elms. It has recently been shown that certain fungi of the genus Geosmithia, vectored by bark beetles, are involved in the DED pathosystem. Not only it has been observed that the two fungi share the same habitat in the host plant and during each developmental stage of the insect, but also there appears to be a parasitic association between them. By analyzing the presence and quantity of the two fungi on the bodies of elm beetles by a qPCR duplex TaqMan assay, this work aimed to explore the dynamic of the relationship among the adults of Scolytus multistriatus, O. novo-ulmi, and Geosmithia spp. at sites characterized by different DED severity levels during the elm growing season. We observed that, regardless of the epidemiological conditions, both fungi are always present on adult flickering insects. The proportion of the two fungal populations varies among sampling sites, with Geosmithia spp. being predominant in the non-epidemic sites. The hyperparasitism of Geosmithia toward O. novo-ulmi within this tri-trophic system should be investigated further for potential use as biological control agent.