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.
ABSTRACT
Entomopathogenic fungi are regarded as effective biocontrol agents in pest management. Different fungi isolates exhibit varying degree of pathogenicity against red palm weevil [Rhynchophorus ferrugineus (Olivier)]. The pathogenicity of four native isolate from Saudi Arabia (three Beauveria bassiana named as BbSA-1, BbSA-2, BbSA-3 and one Metarhizium anisopliae regarded as MaSA-1) and three exotic isolates from Indonesia (B. bassiana coded as BbIDN-1 and M. anisopliae named as MaIDN-1 and MaIDN-2) was evaluated against red palm weevil under laboratory conditions. The isolates were applied to eggs (1 day old), larvae (3 and 35 days old), pupae (5 days old) and adults (10 days old). The average mortality rate of eggs and hatched larvae was 100% in all of the isolates except BbSA-2 and BbIDN-1, where mortality was 93.3 and 90%, respectively. The lowest mortality rate (73.3%) was recorded for BbSA-3 against 3-days-old larvae; however, all other isolates caused >80% larval mortality. Meanwhile, 93.3% mortality of 35-day-old larvae was noted for MaSA-1 isolate. The highest pupa mortality (80%) was observed for MaSA-1, while remaining isolates caused >60% mortality. The isolates BbSA-1 and MaSA-1 caused 61 and 74.3% mortality in adults, respectively. The tested fungi isolates exhibited high virulence against all life stages of red palm weevil. Local isolates had higher pathogenicity than exotic isolates. The findings of the current study suggest that entomopathogenic fungi could be used as biological control agents for the management of red palm weevil. However, field studies are needed to reach the sound conclusions and practical applications.
