Effects of defoliation by Eucalyptus weevil, Gonipterus scutellatus, and chrysomelid beetles on growth of Eucalyptus globulus in southwestern Australia

Abstract

Plantations of Eucalyptus globulus in southwestern Australia are defoliated by Eucalyptus weevil, Gonipterus scutellatus, and a complex of chrysomelid and scarab beetles, yet there is no information on the impact of beetle defoliation to tree growth in southwestern Australia. To address this shortcoming, we used insect exclusion trials, to compare growth of insecticide treated (and thus relatively undamaged) trees with untreated (and thus defoliated) trees to determine whether defoliation by G. scutellatus and other beetles reduced the growth and harvest volume of E. globulus trees. Our results showed some evidence of beetle defoliation reducing growth of E. globulus. Mean defoliation levels of the growing tip of untreated trees ranged from 18% to 33% across the duration of the study and were significantly greater than mean defoliation levels of 5–16% on insecticide treated trees. Seasonal peaks in defoliation of 30–80% to the growing tip of untreated trees were recorded between late spring and early autumn. The greatest impact of defoliation on tree growth was evident during the 2.5 year period of insect exclusion, when higher relative growth rates were recorded for insecticide treated trees, which were significantly different from relative growth rates of untreated trees at two of the four plantations. However, our results showed only a limited impact of beetle defoliation on the total volume at harvest. Initially small trees tended to suffer more severe defoliation than initially large trees. Effects of insect exclusion treatment on harvest volume were modified by the initial tree size and the relationship between the initial tree size and levels of defoliation.