Eucalyptus resistance to paropsine beetles
By Leslie Mann and Stephen Pawson, December 2020.
Download SWP-T118 (pdf)
A wide range of insects can infest Eucalyptus trees in New Zealand, particularly pest insects that are native to Australia. Australian paropsine beetles are significant pests in New Zealand where climatic conditions are similar to their native range and they have few natural enemies. To date, five paropsine species have successfully established in New Zealand. Paropsis charybdis and Paropsisterna cloelia (EVB) are most frequently observed in Eucalyptus plantations and cause the most damage. In 2019, EVB was still restricted to the South-East of the North Island, however it spread to the upper South Island in the summer of 2020. Both species cause considerable damage within plantations. Eucalyptus tolerance to insect defoliation is poorly understood, but must be quantified across species and families to select appropriate breeding lines for the New Zealand Dryland Forest Initiative (NZDFI) to establish a healthy, productive and durable Eucalyptus timber industry in New Zealand.
We aim to determine if specific Eucalyptus species or families (genetic lines) are more resistant or tolerant to paropsine browse. To answer this we have made year 1 assessments of paropsine defoliation among seven different Eucalyptus species (E. quadrangulata, E. bosistoana, E tricarpa, E. globoidea, E. macrorhyncha, E. camaldulensis and E. cladocalyx), clones of E. bosistoana, and families of E. tricarpa and E. bosistoana.
- The Crown Damage Index (CDI) is the best technique (tradeoff between precision and efficiency) currently available to assess paropsine defoliation of Eucalyptus.
- CDI results show clear differences in the susceptibility of Eucalyptus species to defoliation at Dillon and Lissaman sites. E. tricarpa, E. quadrangulata, E. camaldulensis, and E. bosistoana were heavily defoliated, E. macrorhyncha, and E. cladocalyx had low levels of defoliation and E. globoidea was intermediate between these two groups.
- There was substantial range in the defoliation within a species and individual outliers with minimal defoliation are candidates for further investigation.
- Baseline DBH and height measurements were taken to inform future estimates of tolerance after repeated sampling.
- Within family variability to defoliation of E. bosistoana was high, in part due to the low number of replicates at the Dillon trial. The rank order of defoliation amongst families provides a guide for further investigation of optimal families for breeding decisions.
- Variability in defoliation amongst E. bosistoana clones was less than the intra-family variability. The rank order of defoliation amongst clones identifies those clones that are most resistant to paropsine browse.
- Assessment of E. tricarpa families showed consistently high levels of defoliation that was consistent with that observed in the multiple species trial.
Work to date has provided estimates of the resistance of individual species, families, provenances or clones to paropsine browse. We now have baseline measures of height and DBH that can be remeasured in the spring/summer of 2020 to 2021. This concurrent re- measurement of both defoliation and growth will allow us to further refine our understanding of resistance and present the first result of paropsine tolerance.