Farm Forestry New ZealandEucalyptus resistance to paropsine beetles
By Leslie Mann & Stephen Pawson, June 2022.
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Executive summary
A wide range of insect herbivores infest Eucalyptus trees in New Zealand, particularly insects native to Australia. The most damaging defoliators are the paropsine beetles. Paropsis charybdis and Paropsisterna cloelia (EVB). In 2019, EVB was present in the North Island, however in the summer of 2019/2020 EVB spread to the upper South Island and now causes considerable defoliation to Eucalyptus. Few studies have examined Eucalyptus tolerance and resistance to insect defoliation. Understanding paropsine feeding preferences and their impacts is important to inform the selection of New Zealand Dryland Forest Initiative (NZDFI) breeding lines to establish a healthy, productive and durable Eucalyptus timber industry in New Zealand.
This research aims to determine how paropsine insects interact with plantation eucalypts in New Zealand. This was achieved by quantifying the resistance and/or tolerance of Eucalyptus pecies/families/genotypes to paropsine attack. Paropsine defoliation across seven different Eucalyptus species (variation at species level), E. bosistoana and E. tricarpa families (variation at family level), and E. bosistoana clones of different genotypes (variation at genotype level) were quantified. For each tree sampled, the number and length of the new growth shoots, height increment, DBH increment, and defoliation using the CDI (Crown Damage Index) were assessed to quantify defoliation and resistance/tolerance on two or three occasions between 2019-2021. CDI was used to measure resistance, whereas growth measurements (DBH, height and new shoot growth) were used to assess tolerance to paropsine browse.
Key results are:
- At the species level: E. cladocalyx, E macrorhyncha and E. globoidea were the most resilient to defoliation, whereas E. quadrangulata and E. tricarpa were the least resilient. E. globoidea and E. cladocalyx were the species with the highest growth rate (height and DBH increment).
- Large variation in defoliation was recorded between and within E. bosistoana families. There was a weak negative correlation between CDI and height growth but no correlation between CDI and DBH growth for E. bosistoana families. E. bosistoana family 805 was constantly resistant to defoliation across all measurements periods and recorded a good growth rate (height and DBH increment). All E. tricarpa families were heavily defoliated with minimal variation in browse observed between families.
- Substantial variation in the CDI and growth was observed within the same E. bosistoana genotype (i.e., individuals that were genetically identical). This indicates that other microsite factors are important influencers of defoliation and growth.
- Multiple comparison tests were not possible for clonal and family trials due to the large number of potential comparisons. Thus, tables containing the 10 best and 10 worst families for CDI, height and DBH increment are provided for the E. bosistoana family and clonal trials and the E. tricarpa family trial.
Recommendations:
All E. tricarpa families were highly defoliated and only moderate height gain was observed. E. quadrangulata was extremely defoliated and attacked by other insect pests as well. From a pest management perspective, these two species have low resistance to paropsines and continued development of these species should be reconsidered.
E. globoidea and E. cladocalyx and E. macrorhyncha seem to be promising species that are resistant to paropsine browse, and maintain strong growth rates (DBH and height). Moreover, some individual E. globoidea trees seem to be more resistant and/or tolerant. E. bosistoana families expressed variable resistance to paropsine defoliation.
They had some growth rate differences (height and DBH increment) as well that were not necessarily negatively correlated with paropsine resistance, leading to some tolerance abilities. However, questions remain regarding the influence of microsite factors. We recommend planting such individuals in controlled trials using homogenous conditions (soil, moisture, slope etc.) to assess the heritability of the observed resistance/tolerance in the absence of potentially confounding microsite factors.
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