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Summary
Christmas beetles (Coleoptera: Scarabaeidae) (Anoplognathus spp.) and paropsine leaf beetles (Coleoptera: Chrysomelidae) are common defoliators of eucalypts, including several important plantation species, throughout south-eastern Australia. It has already been demonstrated that populations and individuals of several eucalypt species vary in susceptibility to defoliation by these leaf-chewing beetles. The objective of this study was to determine whether significant variation in resistance to insect herbivory was present in Eucalyptus grandis genotypes selected from the hardwood plantation program of State Forests of NSW.
Grafted clones from three young trees with significantly less crown damage than neighbouring trees in a plantation established near Byron Bay, NSW, were tested for attractiveness to Anoplognathus chloropyrus adults in a binary choice experiment in the laboratory. Shoots from the resistant grafted ramets were paired with shoots from susceptible seedlings of similar age, and caged with feeding A. chloropyrus adults for 24 h. The Christmas beetles consumed significantly (P ≤ 0.05) less leaf area from the resistant shoots, compared to that consumed from the shoots from susceptible seedlings, when given access to both. The binary choice test method was also used to test feeding preferences of four paropsine chrysomelid species: Paropsis atomaria (adults and larvae), P. variolosa (larvae only), Paropsisterna beata (adults), and a Chrysophtharta sp. (adults), using the same E. grandis genotypes. No significant differences (P > 0.05) in feeding preferences were detected for P. variolosa larvae, P. atomaria adults, P. beata adults or the Chrysophtharta sp. adults. In contrast, P. atomaria larvae displayed a significant preference for foliage from the susceptible seedlings. In a larval development study using P. atomaria larvae, those reared on the susceptible foliage developed approximately five days faster and had significantly higher survival than those reared on resistant foliage.
Several leaf traits were also compared between the two types of foliage. Leaves of similar age from the resistant clones did not differ significantly from the susceptible seedlings in terms of carbon to nitrogen content or concentrations of the monoterpene, 1,8-cineole. They did, however, have significantly higher specific leaf weight (SLW). SLW may have the potential for use in rapid screening of E. grandis genotypes for susceptibilty to defoliation by leaf-chewing beetles that prefer young soft foliage.