Insecticidal activity of new Metarhizium isolates against different populations of CRB larvae from Papua New Guinea (PNG) and Solomon Islands (SI)

The coconut rhinoceros beetle (CRB) (Oryctes rhinoceros) is one of the most destructive insect pests of coconut and oil palm in tropical Asia and the Pacific Islands. A new variant, known as CRB-G, has spread into the Pacific Islands causing significant levels of damage. Biopesticides containing Metarhizium spp. are the strongest candidates for fungal biological control against the emerging CRB threat and selection of the most virulent and robust isolate could be the determining factor for success. In this work, CRB specimens with natural fungal infection were collected in Papua New Guinea (PNG) and Solomon Islands (SI). Putative entomopathogenic fungi were isolated, identified and multiplied on rice. These new isolates and some previously obtained from other Pacific countries were tested for virulence against CRB larvae populations at PNG and SI in lab bioassays. Four isolates identified as Metarhizium majus (conidia length ⁓10- 15 µm) were obtained from SI and five isolates identified as Metarhizium pinghaense (conidia length ⁓4-6 µm) were obtained from PNG. Conidia yield on rice was 10 times higher for M. pinghaense (109 conidia/g) in comparison with M. majus, which could be related with the differences in conidia size. The most virulent isolate was M. majus SI4 that caused 100% mortality in 20-23 days against the old variant (CRB-S) in bioassays carried out in PNG, whereas isolates of M. pinghaense did not show pathogenicity against larvae. M. majus SI4 isolate was also the most virulent against the mixed population (CRB-S and CRB-G) at SI and was selected for further evaluation under field conditions.