Assessment of coconut palm damage caused by coconut rhinoceros beetle, Oryctes rhinoceros (Coleoptera: Scarabaeidae)

Coconut production in the Pacific Islands is threatened by coconut rhinoceros beetle (CRB). Distinctive feeding damage caused by adults is often the first indicator of CRB activity in a new location. Multiple approaches to damage assessment have been used, reflecting observers’ experience. The lack of standardisation makes it difficult to compare CRB impacts on coconut palms over time. We examine damage assessment from three perspectives: participants, power and purpose. Participants applied binary, 3-point and 5-point damage scales consecutively to photographs of damaged palms. Participant responses were more accurate using the binary scale compared with the 5-point scale. Experience with CRB improved accuracy, but this did not wholly compensate for the effect of scale complexity. Calculations of statistical power determined minimum sample sizes to detect a 20% change in palm damage was 50 palms for the binary scale, 150 palms for the 3-point scale and > 200 palms for the 5-point scale, when the change in damage was spread across damage categories. More complex damage scales did not increase the likelihood of detecting significant changes in palm damage. With real-world examples, including detecting new outbreaks and managing established populations, recommendations were developed to standardise damage assessment for CRB on coconut palms.