Virulence of Metarhizium isolated from infected Oryctes rhinoceros L. (Coleoptera: Scarabaeidae) larvae around coconut plantations in East Java, Indonesia

Abstract

Background

Metarhizium is one of the entomopathogenic fungi (EPF) that has been widely reported as a useful agent for controlling the coconut rhinoceros beetle (CRB), Oryctes rhinoceros L. (Coleoptera: Scarabaeidae). Application of this fungus as a biopesticide is influenced by regional environmental conditions that affect the pathogenic activity against the targeted pest. Several studies have proven that the native fungal isolates have strong pathogenic activity than EPF introduced from other regions. The identification of local EPF species is a strategic approach to develop potential biopesticides with standard properties including host specificity, climate suitability, and significantly suppressing the target pest population. This study aimed to identify Metarhizium species isolated from infected CRB larvae with high pathogenicity to host pests in East Java, Indonesia.

Results

Thirteen isolates were obtained which were divided into 4 clades based on phylogenetic analysis by the ITS rDNA region, namely M. anisopliae var. lepidiotae, M. anisopliae var. anisopliae, M. brunneum, and M. majus. Identified Metarhizium species exhibited varied sizes of conidia, but fell within the size ranges reported in previous studies. Interestingly, the isolate MaSi produced conidial lengths well above the range of conidial sizes recorded and placed these isolates (M. anisopliae) in the MGT clade, known as MALC (M. anisopliae s.l. with large conidia). The present investigation reported that isolates MaLe, MaMa, MaWa, and MaSi produced higher mortality values than other isolates, supporting that M. majus and MALC had higher pathogenicity against O. rhinoceros larvae than other Metarhizium species. Neighbor-joining analysis showed a close resemblance between the isolate MaMa and the strain ARSEF 1946, which was isolated from the CRB. In addition, the isolate MaMa had the highest virulence against O. rhinoceros larval cadaver with a faster lethal time (for 50% mortality). This result indicated a possible relationship between phylogenetic status or DNA sequence polymorphisms with Metarhizium pathogenicity and host specificity.

Conclusions

The local species of Metarhizium have been isolated from infected CRB larvae, and M. majus isolates exhibited high pathogenicity against O. rhinoceros larvae. The close similarity of M. majus isolates and CRB-isolated strains suggests a possible relationship between pathogenicity and host specificity with phylogenetic status or DNA sequence polymorphisms.