Pathogenicity bioassays of isolates of Beauveria bassiana on Rhynchophorus ferrugineus

Background: The control of Rhynchophorus ferrugineus (Olivier), the main palm pest in the Mediterranean Basin, is problematic because of its biology and the current restrictions in many European countries on the use of chemical insecticides in urban areas. Entomopathogenic fungi have been studied as potential biological control agents, but information on their natural incidence is limited. Strains of Beauveria bassiana (Balsamo) Vuillemin were isolated from symptomatic insects collected on dead palms, and their pathogenicity against different instars of R. ferrugineus was evaluated in the laboratory.

Results: The overall percentage of infected insects found in Canary palms was 7%. In laboratory bioassays, hatching of eggs treated with three different isolates of B. bassiana was 41.2, 26.8 and 29.9%, significantly lower than the control (62.4%). Larvae and adults were treated with a single isolate in two ways: spraying each insect with a conidial suspension or feeding them with fruit portions previously immersed in the same conidial suspension. At the end of the two trials, the mortality of treated larvae was 88 and 92%, and the mean survival time was 10.4 and 11.8 days, significantly different from the control, where no insect died during the trials. Mortality and survival time recorded in either trial on adults did not significantly differ between treatment and control.

Conclusion: This study shows that the pathogenicity of wild isolates of B. bassiana differs among the tested R. ferrugineus instars. The low mortality of treated adults supports their use as vectors of B. bassiana as a potential tool for reducing R. ferrugineus populations.