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Review of methods and recommendation for assessing defoliation Eucalyptus nitens from Paropsis Charybdis for breeding purposes

By Toni Withers, Elise Peters, Mari Suontama, June 2017.

Download SWP-T030 (pdf)

Executive summary

Eucalyptus nitens has a long tradition in commercial forestry in New Zealand. It is currently the most important Eucalyptus species in the country. As a fast-growing species of good form it could offer even greater opportunities to tree growers for solid wood production, in addition to its current use for pulpwood. The Eucalyptus tortoise beetle, Paropsis charybdis, remains the most serious pest of E. nitens in New Zealand. In 2011, tree breeders agreed that the natural resistance of E. nitens to P. charybdis attack should be further explored. As a result all 180 families (excluding the Tasmanian material) from the New Zealand breeding population was collected and sown. The resultant seedlings were planted out in Howdens block (FR507) on the Southern Wood Export estate in Southland as 30 replicates of single tree plots (5400 trees). These trees are now approaching six years of age and breeders want guidance on a method to assess the growth, form, wood density, and resistance to Paropsis.

A literature review and an assessment of a subset of trees in a small plot (FR509) in Kaingaroa Forest were used to evaluate different methods of scoring resistance to P. charybdis. Methods tested were single branch observations, crown damage index, Paropsis charybdis chewing score and crown density rating to assess P. charybdis damage in the breeding population in Howdens block.

Based on field trials and literature review, we recommend the following:

Supplement natural populations of P. charybdis at Howdens block by releasing approximately 2000 live adults into the plantation, in spring 2017. Higher population levels are required to assess defoliation score resistance of individual trees (to be greater than an average of none or trace). After assessment, an aerial spray may be required in late summer to prevent long-term pest impacts.

In late summer, climb each tree 1m and using extended electric pole pruners obtain one random sample of an adult foliage-bearing branch to quantify the proportion of adult leaves damaged by chewing and/or missing from the terminal 100cm. Calculate a branch chewing score. Photograph any live insects still present on the branch and record photo number for later identification.

Undertake a qualitative late summer visual rating of upper crown density on a 1-5 scale (Paropsis chewing score). There is potential to obtain an aerial drone photograph of each tree to compare rating to a visual image.

Assessment of the E. nitens breeding population at Howdens block using these recommended procedures should be undertaken before the trees grow any taller as their crowns become inaccessible. Furthermore, earlier testing provides faster decisions for the next generation of breeding trials.

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