MODELING THE IMPACT OF DEFOLIATION BY THE LEAF BEETLE, CHRYSOPHTHARTA-BIMACULATA (COLEOPTERA, CHRYSOMELIDAE), ON HEIGHT GROWTH OF EUCALYPTUS-REGNANS

A model of the effect of defoliation by the Tasmanian eucalyptus leaf beetle, Chrysophtharta bimaculata Olivier, on the height growth of Eucalyptus regnans F. Muell. in young plantations is described. Two separate artificial defoliation trials were set up to provide data to calibrate and test the model. The treatments involved combinations of different defoliation levels and timing with and without a repeat of these treatments in the following year as well as a treatment involving regular removal of refoliating shoots (disbudding). The effect of disbudding was found to be greater on growth than was low levels of defoliation. The model gave an adequate fit to the data, but not all combinations of treatments were available to calibrate and test the model. A small simulation study employing the model to investigate the effect of defoliation and disbudding on plantation economics indicated a potentially high benefit to cost ratio for control of C bimaculata, especially where refoliating shoots were grazed.