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Insect outbreak impacts

DATE 22/12/2010

THE effect of insect outbreaks in eucalypt forests was highlighted in early spring this year when large numbers of eucalypts appeared to be dying off in the Channel and Taroona areas of southern Tasmania.

Investigations by Forestry Tasmania biologist Dick Bashford revealed the damage was caused by larvae of a Doratifera cup moth species and that the trees would begin to recover almost immediately after the larvae began to pupate.

An outbreak of this species has not previously been recorded in Tasmania and the unusual occurrence has drawn attention to the fact that eucalypts are susceptible to damage from insects.

Tasmania native leaf beetles feed on eucalypts and research shows they can have a significant effect on tree growth rates, particularly in plantations. Forestry Tasmania does not undertake management of leaf beetle populations in regenerating native forests but steps are taken to protect plantations against economically-damaging defoliation of eucalypts in plantations.

Chrysomelid beetles are the most serious insect pests in Tasmanian eucalypt plantations. Not only do larvae damage tree growth but the adult beetles also feed through the summer and autumn to build up reserves to survive the winter.

Research shows that for E. regnans loss of more than half new season’s foliage in late summer and removal of buds can cause significant loss of growth for at least four years. In tests E. regnans protected from insects for eight years had more than three times the growth than trees that weren’t protected.

Factors that determine insect outbreaks can include reproduction, survival and dispersal rates which are determined by weather conditions, season lengths, availability of food, and most of all the presence of natural enemies. These natural pests and diseases generally control outbreaks but there is usually a time lag – so we would not expect another similar severe outbreak next year.

All young plantations are monitored throughout the summer and, if leaf beetle populations are over a threshold level, they are controlled so that they do not cause economic damage to plantations.

Dr Jane Elek
Leaf Beetle Research Officer