Performance of Metarhizium majus applied in artificial breeding sites to control larvae of coconut rhinoceros beetle (CRB) in Solomon Islands

The fungi Metarhizium spp. are well known as pathogens of CRB (Oryctes rhinoceros) but natural infection in the field is relatively low. This has led several affected countries to adopt the incorporation of Metarhizium spores into artificial breeding sites (ABS) as a release strategy. ABS are piles of decomposing palm logs and associated plant waste created artificially in the field and treated with the fungus. Female CRB are attracted to the decomposing material to lay eggs with commercial aggregation pheromone. CRB larvae, pupae and emerging adults may get infected and die. Although this strategy has been used for many years, the real impact on CRB population reduction and fungus dispersal has not been determined. In this work, a strain of Metarhizium majus was isolated from one adult CRB cadaver found in an oil palm plantation at Tetere, Guadalcanal. This strain was tested by applying nil, low (100g/ABS), moderate (200g/ABS) and high (400g/ABS) doses of sporulated rice to four 2x2m ABS arranged in a grid. 50 CRB larvae were added to each ABS. This design was repeated at three oil palm blocks: Tetere, Mberande and Nekama. CRB numbers inside the ABS were assessed every 2 months for 6 months. Before applying the fungus, soil samples were taken at 10m, 50m and 100m from each ABS. The soil samples were ‘baited’ with CRB larvae to detect Metarhizium sp. Soil baiting was repeated at 2- monthly intervals. Between 5 and 20 CRB larvae/ABS were found during the 2-monthly evaluations. M. majus was detected in Tetere soil samples from all distances/treatments before and after the inoculation of the ABS, suggesting the fungus was already established in this block after a M. majus-based commercial product was applied in 2017. In contrast, Metarhizium sp. was not detected in soil samples collected from Mberande and Nekama before ABS inoculation, but the fungus was detected later in soil samples from all treatments, even at 100m away from the site. This suggests that M. majus can spread up to 100m from treated ABS and persist for at least 4 months. M. majus may contribute to CRB control if high levels of inoculum can persist in breeding sites under field conditions.