Occurrence of entomopathogenic fungi in coconut soils, identification, physiological characterisation and pathogenicity towards Rhynchophorus palmarum (Coleoptera: Dryophthoridae)

Abstract

ABSTRACT Rhynchophorus palmarum is a significant pest of Cocos nucifera on the American continent. Therefore, this work aimed to search and characterise entomopathogenic fungi isolates to control R. palmarum under laboratory conditions. Sixty-three isolates of Metarhizium spp. were obtained from soil samples from different coconut plantations in four regions in Tabasco, México. The sites were edaphically characterised, and the presence of Metarhizium was almost null in the sites with 90% sand. The ITS region of the DNA of eleven isolates was analysed, and M. robertsii and M. anisopliae s.l. were identified. The mycelial growth (MG) of 38 isolates was evaluated, which were incubated at 25°C, 30°C, and ·35°C on Potato Dextrose Agar with Yeast for 14 d. The optimal growth for some strains was at 25°C and 30°C; for others, at 35°C. The MG colonies were small (< 1 cm) and irregular. The pathogenicity of the strains was evaluated in Tenebrio molitor larvae for 14 d with a conidia suspension 108 (conidia/mL). All the strains were pathogenic to T. molitor. However, only two strains (PAL 231.1, PAL 733.1) identified as M. robertsii reached cumulative mortality of over 60%. Then, these two strains were tested in adults of R. palmarum for 21 d with three conidia concentrations (107, 108, 109 conidia/mL). The suspension 109 conidia/mL of PAL 733.1 was the most pathogenic, with cumulative mortality of 60% and LT50 at 17 d. Therefore, the strain PAL 733.1 is proposed as a potential biological control agent against R. palmarum.