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Susceptibility of Eucalyptus bosistoana families to EVB defoliation across sites and families Feb/March 2019

By Tara Murray & Ruth McConnochie, June 2019.

Download SWP-T078 (pdf)

Executive summary

In the absence of a PhD student being secured to continue the durable eucalypt health assessment work in time for the 2018-2019 summer season, Ruth McConnochie was contracted to assess paropsine defoliation and EVB (Paropsisterna varicollis) incidence on E. bosistoana families in February and March 2019. Trees at two sites were assessed using two methods; 1) a visual estimate of % crown defoliation and 2) a quantitative ‘occupied leaves per shoot’ count of eggs and larvae. Few EVB and only minor paropsine chewing damage was observed at the first selected site, Ngaumu. One hundred and fifty trees were assessed, but the small pest population precluding assessment of family-level susceptibility. At McNeill’s, where EVB has been well established for several seasons, 3-17 trees from each of 80 families were able to be assessed, giving a total of 889 trees. Both EVB and Paropsis charybdis were present at the site, however the EVB population was significantly larger. Defoliation varied significantly within and between families, ranging from 5% to 90% of the upper third of tree crowns. Average defoliation per family ranged from 27.3% to 63.3%. Defoliation of families from the Won Wron provenance was significantly higher than three of the other provenances. No EVB or P. charybdis eggs or larvae were detected on the shoots sampled at either McNeill’s or Ngaumu, however small numbers of eggs were collected ad hoc throughout the trials. Enoggera nassaui was reared from several EVB egg batches from McNeil’s, while E. nassaui, N. insectifurax and B. albifunicle were reared from P. charybdis eggs. Large numbers of Cleobora mellyii were also observed at the McNeill’s site along with some harlequin ladybirds.

An additional ad hoc survey for EVB was conducted at Cravens Rd (Marlborough) while collecting scion material in February 2019. The beetle was detected at the site for the first time, with 2 adult beetles detected on Southern provenance E. bosistoana families.

The data presented here provides further evidence that there is sufficient variation in defoliation sustained by different E. bosistoana families to warrant continued investigation into potentially heritable differences in susceptibility to paropsine attack. A fully replicated assessment conducted across multiple sites, species and families is still required to definitively inform selection of species and families that are less susceptible, or more tolerant, to EVB attack. This work needs to be conducted to coincide with peak egg laying and larval abundance so that the agent(s) (EVB or P. charybdis) causing the damage can be confirmed, and to reduce error in assessing preferences. Such a trial is scheduled to be conducted by a newly appointed PhD student (commencing August 2019) at sites across Marlborough and the lower North Island in the 2019-2020 field season.

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