Insect defoliation of mixed-species eucalypts in East Gippsland

Summary Ground surveys were conducted in the coastal and inland mixed—species eucalypt forests of East Gippsland from 2003 to 2006, using ten permanent forest health assessment sites across the region, to determine the cause, extent and severity of extensive defoliation first observed in late 2003. The primary cause of defoliation in the 2003 and 2005 outbreaks were Doratifera spp. (cup moth) and Uraba lugens (gumleaf skeletoniser) respectively. Eucalyptus globoidea (white stringybark) and E. elata (river peppermint) appeared most susceptible while E. cypellocarpa (mountain grey gum), E. obliqua (messmate) and E. sieberi (silvertop) were generally more resistant to defoliation by these insect species. The initial assessment in November 2003 indicated high levels of defoliation with a mean of 71% (range: 30–85%) across all ten sites. Re-assessment of the sites in March 2004 indicated active defoliation had ceased and that foliage cover in the trees had improved with mean defoliation of 34% (range: 15–60%), an improvement of 37%. Subsequent defoliation late in 2005 was not as severe as the 2003 outbreak, with mean defoliation in November 2005 being 23% (range: 10–40%) across all ten sites. Re-assessment in March 2006 indicated that active defoliation had ceased and that levels of foliage cover in the trees had increased over the 2005–2006 summer, with mean defoliation of 17% (range: 10–30%) recorded, an improvement of 6%. Rather than being considered as one continuous outbreak by a single pest species, the two defoliation episodes should be regarded as separate events caused by separate insect species, although their cause(s) may have foundation in similar conditions. The U. lugens outbreak can be reliably linked to low rainfall in the region. The cause of the outbreaks by Doratifera spp. is uncertain because of insufficient knowledge concerning their lifecycle. While climatic factors are likely triggers, other factors such as eucalypt host susceptibility, foliage nutritional status, altitude, forest type, soils and geology may also influence the occurrence of Doratifera spp. outbreaks. Further, it still has to be determined whether the factors that trigger outbreaks affect Doratifera spp. directly or reduce parasitoid/predator populations that usually keep Doratifera spp. populations in check. As little is known of the predisposing factors for outbreaks (especially for Doratifera spp.), the appropriate nature and timing of any potential control measures are difficult to identify. Continued monitoring of all potential damaging insect pest populations and associated canopy cover is recommended as part of a structured forest health surveillance system in order to predict outbreaks and allow appropriate contingency plans to be made if required. However, as most native insect outbreaks in native forests are short term and part of the normal forest ecosystem, the implementation of control measures is usually not required. The potential for long-term effects of drought to increase outbreak frequency and subsequently affect tree growth and health should not be discounted, thus emphasising the need for regular monitoring.