Selection of Beauveria bassiana (Hypocreales: Cordycipitaceae) strains to control Xyleborus affinis (Curculionidae: Scolytinae) females

Background: Xyleborus affinis Eichhoff (Coleoptera: Curculionidae) is an ambrosia beetle reported to affect avocado trees (Persea americana Mill.). The use of the entomopathogenic fungus (EPF) Beauveria bassiana (Bals.-Criv.) Vuill. for ambrosia beetle control represents an alternative to insecticides.

Methods: This study was designed in two stages to select B. bassiana strains with potential to control X. affinis females. In the first stage, 19 B. bassiana Mexican strains from EPF collection, isolated from Coleoptera (CHE-CNRCB, http://www.gob.mx/senasica/documentos/coleccion-de-hongos-entomopatogenos), were tested. Analyses included radial growth rate, conidial yield, spore germination, and germ tube length. Results were analysed by Principal Component Analysis (PCA) to identify clusters within favourable growth phenotypes. For the second stage, 10 selected strains were re-analysed for virulence-related metabolic characteristic, including cell wall-bound cuticle-degrading enzymes-Pr1-like proteases and β-N-acetyl glucosaminidases (NAGase) chitinases, conidial hydrophobicity and monopolar germination parameters. A second PCA analysis was run for those virulence parameters analysed, and upon results strains CHE-CNRCB 44, 171, 431 and 485 were selected and tested against X. affinis females. Females were treated with a 1 × 10⁸ conidia mL^-1 suspension (recommended rate), using a Potter Tower.

Results: All strains showed insecticidal activity, inducing up to 58% mortality; about 30% dead beetles developed aerial mycelia (CHE-CNRCB 485) and the fastest mortality rate was t₀ = 1.95 (CHE-CNRCB 44).

Conclusion: Since all selected strains showed virulence against X. affinis females, results indicated the possibility of selecting B. bassiana strains based on multiple metabolic attributes, as a preliminary test to perform bioassays against order-related target insects.

Keywords: Ambrosia beetles; Conidial germination; Cuticle degrading enzymes; Growth rate; Hydrophobicity; Principal components analysis.