Interactions between ambrosia beetles, their associated microorganisms, and the causal agent of the Dutch elm disease

Dutch elm disease (DED) has caused the near-global disappearance of mature elm trees due to the rapid spread of the pathogen Ophiostoma novo-ulmi, which induces lethal vascular blockages. While bark beetles of the genus Scolytus are the primary vectors of O. novo-ulmi, ambrosia beetles may also transmit the pathogen. These beetles are commonly associated with trees producing and emitting host volatiles. These volatiles can be produced in response to stressors, including infection by O. novo-ulmi and other pathogens, and can potentially make a DED infected tree attractive for opportunistic ambrosia beetles searching for stressed trees to colonize. We conducted a field study using Ulmus minor logs from healthy and DED-symptomatic trees, part of them treated with ethanol. Logs were colonized by the ambrosia beetle Anisandrus dispar, Xyleborinus saxesenii, Xylosandrus crassiusculus, and Xylosandrus germanus. Attacks occurred on both untreated infected logs and infected and healthy logs treated with ethanol, but emergence occurred only from ethanol-treated ones. Beetle emerged from colonized logs were also tested using qPCR to assess the presence of O. novo-ulmi on the insect body. Individuals from both infected and healthy logs tested positive, with pathogen loads exceeding those previously reported in Scolytus spp. This suggests that naturally occurring populations of the beetles are exposed and can carry O. novo-ulmi. Interaction assays also revealed that some mutualist fungi of ambrosia beetles can outcompete O. novo-ulmi, and that the bacterial mutualist Erwinia sp. can inhibit its growth. These findings indicate ambrosia beetles can potentially act as mechanical vectors of DED.