Which came first: The disease or the pest? Is there a host mediated spread of Beauveria bassiana (Ascomycota: Hypocreales) by invasive palm pests?

The red palm weevil (RPW) Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) is threatening the palm family worldwide, causing important economic losses. Current tactics to manage the weevil are largely based on chemical control, although the use of pesticides is hampered by several environmental constraints. Since the first introduction of RPW in Spain in 1996 and during its progressive spread around the Mediterranean basin, the number of reports of natural infection of RPW populations by entomopathogenic fungi (EPF) has been rising for 15 years, and this rise could support a pest-mediated EPF spread. To challenge this hypothesis, we assessed the usefulness of the region of elongation factor 1-alpha (EF1-α), Bloc nuclear intergenic region (Bloc) and inter simple sequence repeat (ISSR) markers, alone or in combination, to infer the relationships among Mediterranean Beauveria and Metarhizium strains isolated from the RPW. Second, the effect of abiotic factors, such as temperature, humidity and UV-B radiation, on the germination and growth of these EPFs strains as a function of their genealogy and geographic origin were determined. Finally, the pathogenicity of strains from different genetic clades was evaluated against larvae and adults of R. ferrugineus. The phylogenetic analysis based on the EF-1α gene identified eight different sequences among 24 fungal isolates of four fungal species. Similar clades were clustered when Bloc and ISSR analyses were performed. The results showed that strains of different origins were clustered in the same clade, and this outcome could be explained by an RPW-mediated EPF spread that was also influenced by time, geographical and other RPW related factors. Neither the response to abiotic factors nor virulence to RPW larvae and adults were related to the sequence type, with all B. bassiana strains well adapted to Mediterraneam climatic conditions. Taken together, these findings may help to select the best strain for RPW management.

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