Could sterile males be used to vector a microbiological control agent? The case of Rhynchophorus ferrugineus and Beauveria bassiana.

Rhynchophorus ferrugineus (Coleoptera, Curculionidae) is the most threatening pest of palms worldwide. The potential of gamma-irradiated males to spread a pathogenic strain of the entomopathogenic fungus Beauveria bassiana (Ascomycota: Clavicipitaceae) to control this pest was studied. First, the effects of gamma irradiation (15 and 25 Gy) on the mating success and performance of adult males irradiated at age one day were studied in the laboratory. Although male longevity decreased after irradiation (118.6 vs. 244.7 days for irradiated and control males, respectively) and their testes suffered from the treatment, fecundity of mated females did not depend on the irradiation status of the male (86.8 ± 5.5 eggs in 15 days). However, egg hatching was significantly lower in couples with irradiated males (31.4% vs. 86.5% for irradiated and control couples, respectively), and this value decreased after a second mating (6.1% vs. 85.9%). Therefore, irradiation did not affect male sexual competiveness but sperm quality. Second, a semi-field assay was carried out to evaluate infestation in young Phoenix canariensis caused by different combinations of couples with irradiated and/or B. bassiana-challenged males. The number of immature stages found in infested palms was significantly higher when females mated with untreated males and lower when mated with irradiated males (either B. bassiana-infected or not). Some females from the fungus-challenged treatments showed post-mortem hyphal growth, and this horizontal transmission proves that irradiated males could act as a vector for B. bassiana and should be considered as a new method to improve the biological control of R. ferrugineus.