Persistency of Indigenous and Exotic Entomopathogenic Fungi Isolates under Ultraviolet B (UV-B) Irradiation to Enhance Field Application Efficacy and Obtain Sustainable Control of the Red Palm Weevil

SIMPLE SUMMARY: Entomopathogenic fungi have the potential to control insect pests However, field application has issues with germination and pathogenicity due to ultraviolet irradiation. The purpose of this study was to investigate the persistence of different local and exotic fungal isolates of Beauveria bassiana and Metarhizium anisopliae in the laboratory under various ultraviolet exposure times, to obtain a fungal isolate enabling long-term management of red palm weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae), a major pest of palm trees around the globe. After 300 min of ultraviolet radiation, the colony-forming unit of a certain fungal isolate has survived. The persistence of certain fungal isolates to ultraviolet irradiation has shown to be promising. Finding a persistent fungal isolate would be helpful in increasing fungal germination and increase its sustainability against harsh environmental conditions in the field. The overall aim of this research was to obtain sustainable control of the red palm weevil, which has become a major invasive pest in many areas outside its native range. ABSTRACT: The red palm weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae) has become a key invasive pest and major threat to the palm tree worldwide. Several entomopathogenic fungi are used in insect biological control programs. In the present study, persistency of different local and exotic fungal isolates of Beauveria bassiana and Metarhizium anisopliae was evaluated under UV-B irradiation with different exposure intervals. Several factors, including ultraviolet (UV) light, significantly decrease germination rate of fungi, as UV penetrates and damages their DNA. Several studies have investigated that UV-resistant conidia germinate better under harsh environmental conditions. Seven local and exotic fungi isolates (“BbSA-1”, “BbSA-2”, “BbSA-3”, “MaSA-1”, “BbIDN-1”, “MaIDN-1”, and “MaIDN-2”) were tested in the current study under UV-B irradiation having different UV exposure times (i.e., 15, 30, 60, 120, 180, 240, and 300 min). The colony-forming unit (CFU) in each isolate was used to calculate the survival rate. Results indicated that survival rate of all the isolates decreased under UV-B irradiation for all exposure times compared to no exposure to UV-B irradiation. The CFU number decreased as the exposure time increased. Fungi isolates “MaSA-1”, “BbSA-1”, “BbSA-2”, “MaIDN-1”, and “MaIDN-2” could persist after 300 min exposure to UV-B, while the remaining isolates, such as “BbIDN-1”, and “BbSA-3”, could not persist after 300 min exposure to UV-B. The ultimate objective of the present research was to explore an ultraviolet-tolerant fungal isolate that might be useful in the field application for the sustainable management of the red palm weevil, which has become a key invasive pest in many regions rather than its native range. Most of the fungus isolates studied in the present work were collected from Saudi Arabia’s Al-Qatif region, where the red palm weevil has infested more than ten thousand trees, worth millions of riyals.