Selective herbivory by Christmas beetles in response to intraspecific variation in Eucalyptus terpenoids.

Between-tree variation in the terpenoid composition in the foliage of six species of Eucalyptus was investigated in relation to its effects on herbivory by Christmas beetles (Anoplognathus spp.). All six eucalypt species showed considerable intraspecific variation in terpenoid composition; the cineole content ranged from 13% to 78% of the total oil in different E. melliodora trees, from 0% to 67% in E. conica, 3% to 79% in E. sideroxylon, 1% to 76% in E. camaldulensis, 3% to 60% in E. rubida and 20% to 79% in E. blakelyi. Levels of defoliation by Christmas beetles of nine E. melliodora and eight E. conica trees were quantified from frass traps placed under each tree, and were used to confirm the reliability of visual ratings of defoliation. Defoliation was assessed visually for all six species and was found to be unrelated to the total amount of terpenoids in the foliage of each tree, but strongly associated with the percentage of cineole in the terpenoid mixture. Levels of most terpenoid components were significantly inter-correlated, so it was not possible to determine which components directly affected defoliation. The dominant Christmas beetle at all sites was A. montanus.