Impact of Insects on Eucalypt Plantations in the Murray Valley

A report for the RIRDC/L&WA/FWPRDC/MDBC Joint Venture Agroforestry Program

by Rob Floyd and Grant Farrell

June 2007

RIRDC Publication No 07/085 RIRDC Project No CSE-72A

Who is the report targeted at?
The report will assist plantation managers, farm foresters and researchers to understand, manage and minimise insect damage in eucalypt plantations. It is also for researchers who seek optimal management of other types of plantation research trials.

Background
Major tree establishment programs aimed at eucalypt plantations for landcare and commercial wood production are being pursued in the Murray Darling Basin and elsewhere. One of the regions being developed is the Murray Valley around Shepparton and Deniliquin. Some 100,000 ha of plantations are proposed for the area. Many young plantations in the Murray Valley have incurred heavy defoliation from a range of insects including leafblister sawfly (Phylacteophaga spp.), autumn gum moth (Mnesampela privata), and a number of psyllid species (Cardiaspina spp.). The two most commonly planted species of eucalypts, Eucalyptus grandis and E. globulus, are quite susceptible to insect feeding and have, in extreme outbreaks, incurred high levels of mortality. A number of the older plantations have also been damaged by borers (Phoracantha spp.).

Previously, the impact of insect damage on the rate of production of eucalypt plantations in the Murray Valley had not been quantified, even though severe defoliation by insects has occurred in a number of plantations (Neumann and Collett 1992). The role of resistance in plantation pest management has been reviewed by Floyd and Farrow (1994), and preliminary observations on relative resistance of provenances of E. camaldulensis and E. globulus in the Murray Valley have been reported (Farrow et al. 1994; Floyd et al. 1994).

Aims/Objectives
The project’s aims were to:

Assess provenances and families of several species of eucalypts for resistance to pests of young trees, in order to incorporate resistance into breeding programs in seed orchards.

Measure the impact of insect feeding on the growth and form of young eucalypts grown in existing and new plantations established under irrigation or high water table sites in the Murray Valley, and to incorporate this information into growth and damage models.

Develop a set of recommendations for the environmentally sensitive management of insect pests involving insect resistance in young trees for use in farm forestry.

Evaluate the economic cost of insect damage within plantations in the Murray Valley through modelling management scenarios.

Results/Key findings
Levels of insect damage varied between sites, years and tree species. There was 0 to 54% defoliation per year. Within tree species, there was up to a three-fold difference in cumulative damage between provenances. Some provenances were resistant to autumn gum moth, but susceptible to leaf blister sawfly, or vice versa. There was a significant variation in tree growth between sites, irrespective of insect damage, and insect exclusion trials showed that some provenances performed better when sprayed while others did not.

This study tested a range of provenances at 1-3 sites per species (sites given below). The results indicate that some provenances perform better in terms of growth and insect resistance. However we caution that provenances may perform differently at other sites outside the study area. The key findings for each of the major tree species examined are listed below, followed by the results of the economic analysis.

E. grandis
Insect damage to this species was assessed at three irrigated sites at two localities, Mildura and Shepparton.
Damage

Cumulative damage levels from trials at Mildura and Shepparton were moderately high.

Six year old plantations incurred an 11% loss in tree volume due to insect damage.

Mean volume loss between seed sources ranged from 7%–41%.

The major pest species were autumn gum moth (Mildura and Shepparton) and brown lace lerp (Cardiaspina fiscella) (Shepparton).

Cumulative damage levels varied by approximately 20% between seed sources.

The most resistant seed sources were from Boambee (NSW), Lake Cathie (NSW) and Loxton selects (plantation selects from South Australia), while the least resistant seed sources were from Orara State Forest (NSW), Bruxner Park (NSW) and Coffs Harbour Seed Orchard. However all seed sources incurred very heavy damage (90%–100% defoliation) during the largest outbreak of autumn gum moth.

Taking into account both resistance and growth potential (in terms of plant volume), the seed sources with the best overall potential are Coffs Harbour Seed Orchard (Shepparton), Loxton selects and Pretoria (Mildura). Other seed sources with good potential include Pine Creek and Bruxner Park.

Seed sources from Bulahdelah are predicted to perform well when the probability of insect damage occurring is low, while seed sources from Woondum and Boambee are predicted to perform well when the probability of insect damage occurring is high.

Selection of a resistant seed source alone is insufficient to avoid economic losses from insect feeding in the event of a pest outbreak. Monitoring of pest numbers along with additional control measures such as the use of biological and chemical insecticides may be needed to optimise financial returns.

Damage

High levels of defoliation (up to 84%) occurred in the final year of trials near Deniliquin.

Single seed sources of the closely related E. dorrigoensis and E. kartzoffiana were more susceptible to autumn gum moth attack than E. benthamii and subsequently sustained higher levels of damage.

Autumn gum moth damage caused the high damage levels observed in the final year of assessment but leafblister sawfly was responsible for defoliation levels of up to 45% in younger trees.

These data are inconclusive as to whether there is significant insect resistance between E. benthamii provenances. Thus, the selection of material for planting should be based on other desirable characteristics such as growth rate and wood quality.

E. kartzoffiana and to a lesser extent E. dorrigoensis should not be planted where the probability of autumn gum moth damage is high.

E. benthamii seed sources from Nepean River and Kedumba Valley have the best growth potential.

High defoliation levels in the final year of the trial suggest that production losses due to insect damage are likely. Hence, it would be prudent to monitor pest numbers for timely application of control measures such as biological and chemical insecticides.

Damage

Moderate to high levels of defoliation (up to 67%) occurred in the final year of trials near Deniliquin. Subsequent assessment of the impact of damage on growth was not possible due to the end of the project.

Autumn gum moth caused high damage levels in the final year while leafblister sawfly caused defoliation levels of up to 26% in younger trees.

These data are inconclusive as to whether there is insect resistance in E. dunnii. Thus, the selection of material for planting should be based on other desirable characteristics such as growth rate and wood quality.

Seed sources from Koreelah State Forest, Acacia Creek and Haystack East had the best growth.

High defoliation levels in the final year of the trial suggest that losses due to insect damage are likely. Monitoring of pest numbers so that the timely application of control measures such as biological and chemical insecticides can occur, may be prudent.

Overall damage levels in the seed orchards near Deniliquin were uniformly very low to nonexistent.

While exhibiting high levels of insect resistance, Corymbia spp. had slower growth rates than other species grown in the same locations (E. dunnii and E. benthamii) and it is therefore best suited to localities with a high risk of heavy insect attack.

Cumulative damage levels for the trial in the ACT ranged from low to moderately high.

Five year old plantations incurred an 18% loss of growth due to insect damage.

Mean volume loss between seed sources range from 0.5% to 56%.

The major pest species at the trial site was Christmas beetles (Anoplognathus spp.). Intense damage occurred over a short period (a few days to a few weeks) each summer.

Less intense damage, caused by a number of other insect species, occurred for much longer periods (three months or longer) over the remainder of the year.

Cumulative damage levels varied by approximately 60% between seed sources.

The most resistant seed sources were from Geeveston (Tasmania), Wee Jasper (NSW), Rylstone (NSW) and Beechworth (Victoria) while the least resistant seed sources were from St Mary’s (Tasmania), Lorne (Victoria), Jeeralang (Victoria) and Mt Dromedary (NSW).

Provenances from Tasmania and the Bass Strait islands, which have previously shown resistance to autumn gum moth and leafblister sawfly, also tended to show resistance to Christmas beetle (cross resistant).

Taking into account both resistance and growth potential (in terms of plant volume), Geeveston provenance had the best overall potential performance.

Provenances from Traralgon (Victoria), Flinders Island (Bass Strait), and St. Mary’s (Tasmania) are predicted to perform well when the probability of insect damage is low, while Black Range (NSW) is predicted to perform well when the probability of insect damage is high.

Selection of a resistant seed source alone is insufficient to avoid economic losses from insect feeding in the event of a pest outbreak. Monitoring of pest numbers along with additional control measures such as the use of biological and chemical insecticides may be needed to optimise financial returns.

The extent of the recovery was thought to be a function of number of factors such as plant species, rotation time and silviculture practice. Specifically:

E. grandis sawlog plantations on longer rotation times (~25 years) showed full recovery.

E. globulus pulp plantations on shorter rotation times (~10 years for) showed only partial recovery and depending on the damage regime, this potentially caused a loss.

For longer rotation times such as sawlog plantations of E. grandis there was little impact.

For shorter rotation times such as pulp plantation of E. globulus, impacts where larger Adding insect control to monitoring had little impact on plantation IRR regardless of the management regime.

Most experiments to measure the impact of defoliation on plantation species report results for one or two growing seasons after damage. This gives information on resistance, but not tolerance.

For fast-growing trees in resource-rich environments (good soil, suitable climatic range, adequate rainfall or irrigation) tolerance, rather than resistant to damage, may be a more useful approach in dealing with insect attack. This is an area of eucalypt biology that has been little studied and warrants further work for its potential as a management option.

Development of decision support systems that allow economic analysis of changes in plant growth due to inputs such as environmental change and insect damage, should be considered. These would be a useful addition to the plantation manager’s tool kit. Having the right information to make pest control decisions in the rapidly diversifying operating environment of plantation management is essential. The work done to develop the Sylva Ento Model used in this report could serve as a starting point for such a project.

Development of biological control methods that either encourage or introduce natural enemies into a plantation at appropriate times should be considered. The former approach is likely to be more practical in the short-term. Research on encouraging natural enemies into plantations could look at the planting of other plant species as food sources or enhancing the structural or floristic diversity of plantations. These options would then need to be integrated with other silvicultural practices to ensure that plantation management is both cost-effective and practical.

Silvicultural control is often overlooked when considered pest control options. It is possible that varying the spacing, pruning, fertiliser regime, thinning regime, weed control practice or layout of plantings could reduce the amount of insect damage in a plantation. The effects of these practices on key pest species are almost entirely unknown and warrant urgent attention.

Monitoring the plantation for insect levels as part of routine plantation monitoring should make it as cost-effective as possible, and increase the manager’s ability to decide timing and costeffectiveness of insect control measures.