Comparison of binary and multi-point scales for monitoring damage by coconut rhinoceros beetle (Oryctes rhinoceros)

Oryctes rhinoceros (L.) (Coleoptera: Scarabaeidae), most commonly known as the coconut rhinoceros beetle or CRB, is an invasive pest that affects palms, especially coconut (Cocos nucifera) and oil palm (Elaeis guineensis) and their effects can be detrimental. For the past 40 years, CRB populations have been controlled effectively by the Oryctes nudivirus (OrNV). A recent incursion causing adverse impacts in the Pacific has prompted researchers to re-evaluate the existing management approaches, so effective population control can be achieved once again. A vital component of the management process that is often overlooked is the need for a robust damage assessment and monitoring tool. Various damage assessment methods have been developed and the use of damage scales has been the most common among them; however, their accuracy and potential to monitor changes over time have not been explored. A study was carried out to 1) Compare and identify the capability of three damage scales: binary, 3-point and 5-point, in assessing damage severity levels and 2) To determine whether assessors affected the accuracy of the damage scales. To achieve these objectives, 20 individual standard palm pictures were selected that represented damage from none to palms killed by CRB activity and administered in an online survey across respondents with differing knowledge of CRB. The results indicated that damage intensity can be measured through the 5-point scale, but the accuracy within the scale is low compared with the binary and 3-point scales. The online survey revealed that experience increases the accuracy of damage scale use among assessors. Training remains a vital requirement in the use of the damage scales if accurate and reliable results are expected.