The susceptibility of hybrid eucalypts to pests
Creators: name: Dungey, HS
Publisher: name: University Of Tasmania
Year: 2023

description: Interspecific hybridisation is potentially a very useful tool for tree breeders, particularly as a source of variation for important genetic traits. However, hybrids in several tree species have been shown to be highly susceptible to damage by some pests. Therefore, the primary aim of this study was to determine if hybrids in Eucalyptus were more susceptible to pests than pure species. To achieve this, the responses of a number of vertebrate, invertebrate and fungal pests to hybrid eucalypts and their parent species, were examined in several experimental field trials and in a natural hybrid zone. Firstly, susceptibility of hybrids to the fungal leaf disease caused by Mycosphaerella spp. was examined in an experimental field trial, including first generation (F1) hybrids and controlled crosses of E. globulus and E. nitens. Both the host preference and genetic variation in resistance were investigated. Heritabilites were low to moderate and disease severity was greatest on the F1 hybrids. Hybrids were more susceptible to damage by Mycosphaerella than both E. globulus and E. nitens. Secondly, the host species preference and hybrid susceptibility was examined for chrysomelid leaf beetles (Chrysophtharta spp. and Paropsis spp.), the gum leaf skeletoniser (Uraba lugens) and for brush tail possums (Trichosurus vulpecula) in a number of experimental hybrid trials. Heritability estimates were very low for possum damage on E. globulus, and high for E. nitens. The proportion of dominance variation for possum damage was consistantly higher than heritability estimates in E. globulus, and at least as high as heritability estimates in E. nitens. E. morrisbyi and E. gunnii consistently had the most possum damage and E. globulus, E. nitens and E. johnstonii the least. Responses of the pest species to the different hosts and hybrids varied but hybrids were generally intermediate in their susceptibility to the different herbivores when compared with their parent species. Thirdly, this thesis examines the host species preference of a number of insect taxa and the host susceptibility of E. amygdalina, E. risdonii and E. amygdalina x E. risdonii hybrids. In a previous study by Whitham et al. (1995) in a natural hybrid zone, hybrids were found to be more susceptible to both insect and fungal taxa than either E. amygdalina or E. risdonii . The genetic basis of this observed hybrid susceptibility was examined by determining the distribution of a number of insect taxa in an experimental field trial, where the pedigree of the majority of hybrids and pure species was known. Species richness was found to be greater on hybrids than pure parent species. Furthermore, F1 eucalypt hybrids tended to be more susceptible to attack than advanced generation hybrids, arguing against hybrid breakdown being the cause of the greater susceptibility of the hybrids. Mechanisms contributing to the observed responses on the E. amygdalina x E. risdonii hybrids were also examined. Leaf toughness and the oil content and composition of the parent species and hybrids was determined and the effect of the different oil components on the distribution of the insect taxa was discussed. Finally, the cause of preferential weevil damage on hybrids between E. amygdalina and E. risdonii in a field trial was examined. While other factors such as larval survival and egg loss were examined, oviposition by the eucalypt weevil Gonipterus scutellatus was highly biased towards the hybrid phenotypes. This lead to much larger larval numbers on hybrids and was therefore the primary factor contributing to the higher damage observed on hybrids in the field trial. Results are discussed in terms of the susceptibility of hybrids and their usefulness in temperate hardwood forestry. The possible nature of hybrid susceptibility in Eucalyptus is also examined.