Laboratory Assessment of Metarhizium spp. and Beauveria spp. Isolates to Control Brachystola magna1 in Northern México

The Plains lubber grasshopper, Brachystola magna Girard (Orthoptera: Romaleidae), is the main insect pest of bean, Phaseolus vulgaris L. (Fabales: Fabaceae), in Chihuahua, Durango, and Zacatecas states of México. Satisfactory control depends on diligent application of insecticide; however, given its evasive ethology and relentless arrival at bean fields, management is difficult. The use of entomopathogenic fungi has been proposed as an alternative, because they are effective and environmentally friendly. The aim of this study was to isolate and identify taxa of entomopathogenic fungi based on morphological and molecular characteristics and evaluate biological activity under laboratory conditions against B. magna adults. The Hypocreales genera of Metarhizium Sorokin and Beauveria (Balsamo) Vuillemin entomopathogenic fungi were isolated and identified based on morphological and molecular characteristics. Under laboratory conditions, the life cycles were completed in 5.6 to 8.0 days for Metarhizium spp. and 6.0 to 6.7 days for Beauveria spp. The average times to kill, were significantly different (F = 13.3, df = 20, P < 0.0001), ranging from 3.4 to 5.7 days post-inoculation for Metarhizium isolates and 4.0 or 5.0 days for Beauveria isolates. Forty-five percent (nine) of the 20 isolates showed mycoses in ≥95% of the specimens treated with 5.5 × 10⁷ spores ml⁻¹ and maintained under laboratory conditions. The number of spores of both genera produced per adult grasshopper evaluated under laboratory conditions were significantly different (F = 8.08, df = 20, P < 0.0001); Beauveria isolates produced 8 × 10⁵ to 8.5 × 10⁶ spores per specimen, while Metarhizium isolates produced 3.9 × 10⁶ to 5.2 × 10⁸ spores per specimen. Isolates of both entomopathogenic genera were applied against Plains lubber grasshopper adults in the field. Shortly after application, treated grasshoppers were collected and maintained under controlled laboratory conditions. Metarhizium isolates killed 50–100% at 9.0 days post-treatment, while Beauveria isolates killed 65–90% at a field dosage of 5.5 × 10¹⁰ spores ha⁻¹.