Ambrosia beetles are a group of tree-dwelling beetles that cultivate symbiotic fungi for the nutrition of every life stage. Invasive ambrosia beetles are pests of woody plants, including ornamental and fruit tree crops, as well as important forest tree species. Ambrosia beetles are attracted to stressed trees that produce ethanol, and their symbiotic fungi rely on ethanol for growth. Due to their unique life history (i.e., remaining within woody tunnels except during adult dispersal), management and prevention has proven challenging. We are studying the potential of entomopathogenic fungi as a means of control for three ambrosia beetle species: Xlysandrus germanus, Xylosandrus crassiusculus, and Anisandrus maiche. Insect-killing fungi possess spores that adhere to insect cuticle, potentially allowing the pathogen to be picked up and carried by the adult beetles as they search for suitable digging sites. When beetles are directly exposed to entomopathogenic fungal spores, and contained within petri dishes, mortality reaches 100% within 4-5 days. However, when beetles are similarly exposed and allowed to dig tunnels in ethanol-soaked pieces of wood, mortality falls to under 25%. Preliminary experiments investigating the effect of ethanol concentration on biocontrol fungi pathogenicity indicate that ethanol and/or the development of symbiotic fungi provides long-lasting protection of the ambrosia beetles from fungal infection. These studies provide important context for understanding how the cryptic ambrosia beetle life cycle may naturally limit infection, and thus biocontrol opportunities, of insect-killing fungi.
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