Relationships between annual rainfall, damage symptoms and insect borer populations in mid-rotation Eucalyptus nitens and Eucalyptus globulus plantations in Tasmania: Can static traps be used as an early warning device?

Routine forest health surveillance of Eucalyptus nitens and E. globulus plantations in Tasmania has detected more frequent borer-related mortality in mid-rotation plantations in drier areas of the state. Damaging borer populations are not usually detected using traditional health surveillance techniques until mortality has occurred. Appropriately baited static traps are known to attract a wide range of stem-boring insects and might provide a means of monitoring populations in at-risk plantations. The objectives of this study were to explore the relationships between rainfall, tree stress symptoms and stem-borer-related damage symptoms in plantation eucalypts, and the nature of the associated borer populations, to determine whether static traps might provide an effective early warning system.Trees from a number of mid-rotation plantations covering a range of rainfall categories were visually assessed for a wide range of damage symptoms, and two intercept panel traps baited with -pinene and ethanol lures were deployed in each of 19 compartments between November 2006 and March 2007. All beetles caught in the traps were identified to species level and the numbers tallied. The incidence of damage symptoms tended to be negatively correlated with mean annual rainfall and was higher in plantations from lower-rainfall areas. There were no clear relationships between aggregated trap catches of borers and annual rainfall or the incidence of damage symptoms. However, a greater range of borer species was trapped in drier areas and the number of species trapped showed a strong negative correlation with annual rainfall. There were also strong positive correlations between damage levels and the range of borer species trapped. The size of the populations of some species, in particular some of the scolytids, increased substantially with decreasing rainfall and was strongly associated with the incidence of damaged trees. Cerambycids were the only group significantly correlated with borer-specific symptoms, but further work is required to determine their utility for monitoring purposes.In vulnerable plantations an increase in the range of borer species, scolytid populations or specific indicator species caught in static traps might provide useful early warning of unacceptable levels of mortality or stem damage. As such, static trapping has the potential to monitor populations and allow for timely silvicultural intervention to ameliorate stress.