Plant secondary metabolites and low temperature are the major limiting factors for Beauveria bassiana (Bals.-Criv.) Vuill. (Ascomycota: Hypocreales) growth and virulence in a bark beetle system

Abstract

Beauveria bassiana is a ubiquitous entomopathogen and widely used as a biological control agent for a variety of arthropod pests, including bark beetles. The North American spruce beetle (Dendroctonus rufipennis) is a major pest in forest landscapes and recent studies show that B. bassiana is pathogenic to beetles in the lab but successful field applications have been limited by abiotic and biotic factors within the study system. To understand how habitat conditions impact fungal ecology, we evaluated variation in B. bassiana radial growth across conditions representative of the spruce beetle habitat, incorporating a range of temperatures, competition with spruce beetle symbiotic fungi, as well as exposure to Engelmann spruce tree secondary metabolites, nutrient limitations, osmotic potentials, and sunlight. Pathogenicity to spruce beetle was subsequently quantified in tests varying beetle origin, temperature, and bioassay arena substrate. Three major findings emerged: (1) growth of genetically similar B. bassiana isolates varied considerably in response to abiotic conditions, suggesting significant within-haplotype phenotypic diversity; (2) low ambient temperatures and exposure to Engelmann spruce tree secondary metabolites, two conditions which are prevalent in spruce beetle habitat, strongly inhibit B. bassiana growth; and (3) pathogenicity varied across environments: all isolates appeared pathogenic under room conditions, but when beetles inhabited in planta bioassays under in natura conditions most isolates were not pathogenic. This is the first study to evaluate the inhibitory effects that a series of tree secondary metabolites have on B. bassiana growth, virulence and pathogenicity. These collective findings have implications for field applications of B. bassiana but also the interpretation of entomopathogenicity, and could help to explain discrepancies between laboratory and field tests.