Insect defoliation of mixed species eucalypts in East Gippsland, 2003-2006.  N. Collett and P. C. Fagg.  February 2009.  29pp. (unpubl.)

SUMMARY

Ground surveys were conducted in the coastal and inland mixed species eucalypt forests of East Gippsland from 2003 to 2006 using 10 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) 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 level of 71% (range: 30-85%) across all 10 sites. Re-assessment of the sites in March 2004 indicated defoliation activity had ceased and that levels of foliage cover in the trees had improved with mean defoliation levels of 34% (range: 15-60%), an improvement of 37%.

A subsequent defoliation event in late 2005 showed it not to be as severe as the 2003 outbreak, with a mean defoliation level in November 2005 of 23% (range: 10-40%) across all 10 sites. Re-assessment in March 2006 indicated defoliation activity had ceased and that levels of foliage cover in the trees had increased over the 2005-06 summer period with mean defoliation levels of 17% (range: 10%-30%) recorded, an improvement of 6%.

The 2003 outbreak was more severe than the 2005 outbreak, and rather than being considered as one continuous outbreak by a single pest species, both defoliation episodes require treatment as separate events caused by separate insect species albeit their cause(s) may have foundation in similar conditions. While the U. lugens outbreak can be linked to low rainfall levels in the region, the situation for Doratifera spp. is less certain due to lack of detailed knowledge concerning their lifecycle and causes of outbreaks. While climatic factors are likely triggers for outbreaks, other factors such as eucalypt host susceptibility, foliage nutritional status, elevation, forest type, soils and geology may also influence the occurrence of Doratifera spp. outbreaks. Furthermore, it still has to be determined whether the factors that trigger outbreaks affect Doratifera spp. or reduce parasitoid/predator populations directly which 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 would be 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 for 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 impact on tree growth and health should not be discounted, thus emphasising the need for regular monitoring.