Burrowing activity of coconut rhinoceros beetle on Guam cycads

Abstract

Guam's established population of non-native coconut rhinoceros beetle (CRB, Oryctes rhinoceros L.) began creating burrows in stem apices of several cycad species in a managed garden. We conducted an island-wide survey to determine the spatial patterns of CRB burrowing of stems of in situ Cycas micronesica. We also measured starch of healthy and unhealthy coconut leaf tissue and compared this with starch of cycad stem tissue. The starch concentration of the central unexpanded leaf in healthy Philippine coconut trees was ≈90 mg·g^-1, and that of unhealthy Guam coconut trees was ≈40 mg·g^-1. The starch concentration of the tissue within the CRB burrow locations on C. micronesica trees was 145 mg·g^-1. Burrowing of C. micronesica was restricted to female CRB adults and was found throughout the full latitudinal gradient of Guam. Our findings indicate Guam's unhealthy coconut trees are no longer nutrient-dense, and the female CRB population may have exhibited a phylogenetically distant host shift to the abundant C. micronesica plants for a starch-rich diet within the concepts of the 'ecological fitting' hypothesis. We add proximity to coconut tree habitats as a new threat to Guam's endangered C. micronesica population.

Keywords: Adaptive radiation; CRB; Cycas micronesica; Oryctes rhinoceros; ecological fitting hypothesis; host shift; starch.