Ethanol tolerance of fungal strains underlies biocontrol of invasive ambrosia beetles.

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Citation abstract

Entomopathogenic fungi are widely recognized as biological control agents showing potential against insect pests that are difficult to manage with chemical pesticides. Invasive ambrosia beetles threaten a wide variety of tree crops and prove challenging to manage with chemical pesticides due to their unique life history: entrenched in the xylem of tree hosts, feeding on symbiotic fungi. Many ambrosia beetle species attack trees that are flood-stressed and producing ethanol, a key component of growth for their fungal symbiont. Ethanol causes osmotic and chaotropic stress in microbes and could prevent successful biocontrol of ambrosia beetles using insect-killing microbes. To investigate the potential of entomopathogenic fungi to manage ambrosia beetles, we conducted a series of experiments using 3 species in eastern North America (Xylosandrus germanus, Xylosandrus crassiusculus, and Anisandrus maiche). We tested the efficacy of 3 microbial-based commercially available biopesticides (Botanigard 22 WP: Beauveria bassiana; Met52: Metarhizium brunneum; and PFR-97 20% WDG: Cordyceps javanica) in a series of environments with and without ethanol. We found that B. bassiana was the most effective against all 3 beetle species, but that its pathogenicity was heavily dependent on the ethanol-tolerance of the strain used. We isolated 2 strains of B. bassiana from separate batches of Botanigard that demonstrated significant differences in ethanol tolerance, as well as pathogenicity, radial colony growth, and spore production in the presence of ethanol. Our results emphasize the importance of pest natural history in control methods, while revealing that the performance of biocontrol products varies across microbial species and exhibits strain-dependency.