Feeding Preferences of the Christmas Beetle Anoplognathus Chloropyrus
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184 Resistance to insect browsing in Eucalyptus grandis clones Feeding preferences of the Christmas beetle Anoplognathus chloropyrus (Coleoptera: Scarabaeidae) and four paropsine species (Coleoptera: Chrysomelidae) on selected Eucalyptus grandis clonal foliage Caitlin V. Johns1,3, Christine Stone2 and Lesley Hughes1,4 1Dept of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia 2Research and Development Division, State Forests of NSW, PO Box 100, Beecroft, NSW 2119, Australia 3 Current address: Plant Health Australia, 5/4 Phipps Close, Deakin, ACT 2605, Australia 4Email: [email protected] Revised manuscript received 23 December 2003 Summary screening of E. grandis genotypes for susceptibilty to defoliation by leaf-chewing beetles that prefer young soft foliage. Christmas beetles (Coleoptera: Scarabaeidae) (Anoplognathus spp.) and paropsine leaf beetles (Coleoptera: Chrysomelidae) are Keywords: insect resistance; defoliation; Christmas beetles; chrysomelids; common defoliators of eucalypts, including several important Eucalyptus grandis plantation species, throughout south-eastern Australia. It has already been demonstrated that populations and individuals of Introduction several eucalypt species vary in susceptibility to defoliation by these leaf-chewing beetles. The objective of this study was to Eucalyptus grandis is viewed in many overseas countries as a determine whether significant variation in resistance to insect desirable plantation species (e.g. Brazil, China and South Africa). herbivory was present in Eucalyptus grandis genotypes selected In Australia, however, the areas planted with E. grandis are in from the hardwood plantation program of State Forests of NSW. decline. A major disincentive to planting this potentially fast- growing species is its susceptibility to defoliating and stem-boring Grafted clones from three young trees with significantly less crown insects. Eucalyptus grandis is host to a wide range of herbivorous damage than neighbouring trees in a plantation established near insects including Christmas beetles (Anoplognathus spp.) and Byron Bay, NSW, were tested for attractiveness to Anoplognathus paropsine chrysomelids (Coleoptera: Chrysomelidae) (Carne et chloropyrus adults in a binary choice experiment in the laboratory. al. 1974; Stone et al. 1997). Both these groups of insects regularly Shoots from the resistant grafted ramets were paired with shoots cause extensive damage to E. grandis plantations in north-eastern from susceptible seedlings of similar age, and caged with feeding NSW (A. Carnegie, State Forests of New South Wales (SFNSW), A. chloropyrus adults for 24 h. The Christmas beetles consumed pers. comm.). The only management options for minimising the significantly (P < 0.05) less leaf area from the resistant shoots, impact of these defoliating insects available at present is the timely compared to that consumed from the shoots from susceptible application of insecticides and the use of optimal silvicultural seedlings, when given access to both. The binary choice test practices to maximise growth. The impetus for this study was method was also used to test feeding preferences of four paropsine the observation that during heavy Christmas beetle infestations chrysomelid species: Paropsis atomaria (adults and larvae), in an E. grandis plantation in north-eastern NSW, occasional P. variolosa (larvae only), Paropsisterna beata (adults), and a trees sustained little damage compared with surrounding trees Chrysophtharta sp. (adults), using the same E. grandis genotypes. (A. Carnegie, SFNSW, pers. obs.). If this apparent resistance to No significant differences (P > 0.05) in feeding preferences were Christmas beetle attack is heritable, incorporation of this material detected for P. variolosa larvae, P. atomaria adults, P. beata adults into a breeding program could help decrease the susceptibility or the Chrysophtharta sp. adults. In contrast, P. atomaria larvae of E. grandis plantations to insect herbivory. This could be even displayed a significant preference for foliage from the susceptible more beneficial if the genotypes proved resistant to feeding by seedlings. In a larval development study using P. atomaria larvae, other insect herbivores, such as chrysomelid beetles (Floyd and those reared on the susceptible foliage developed approximately Farrow 1994). five days faster and had significantly higher survival than those reared on resistant foliage. Several chemical and physical leaf traits are known to affect the extent of eucalypt defoliation by insects (e.g. Edwards et al. 1993; Several leaf traits were also compared between the two types of Steinbauer 2001), but the underlying causal mechanisms are still foliage. Leaves of similar age from the resistant clones did not being elucidated (e.g. Lawler et al. 1998). Many of these leaf differ significantly from the susceptible seedlings in terms of traits are intercorrelated and are often associated with leaf carbon to nitrogen content or concentrations of the monoterpene, phenology (e.g. Ohmart et al. 1987; Steinbauer et al. 1998), 1,8-cineole. They did, however, have significantly higher specific confounding simple interpretation. Host quality, in terms of factors leaf weight (SLW). SLW may have the potential for use in rapid such as leaf toughness and nitrogen content, changes as leaves Australian Forestry 2004 Vol. 67, No. 3 pp. 184–190 185 mature (e.g. Steinbauer et al. 1998). Older eucalypt leaves tend cleaned every 1–2 days. Food material consisted of young shoots to be tougher, less nutritious and better defended (e.g. Landsberg from E. grandis from a source separate to that used in feeding 1990; Ohmart 1991). We chose to measure three leaf traits preference trials. When larvae reached the pre-pupal stage, a layer previously shown to be related to insect feeding behaviour on of vermiculite (1–2 cm thick) was added to the jar below the filter eucalypts: foliar concentration of the two terpenes 1,8-cineole paper, thus providing a substrate into which they could pupate. and α-pinene (Edwards et al. 1993; Li 1993; Stone and Bacon When adults emerged they were added to stock colonies. 1994); foliar carbon:nitrogen ratio (C:N) and specific leaf weight (g cm–2) (SLW). In general, fast-growing plants tend to have lower Eucalyptus grandis test material C:N ratios than slower but better defended plants (Coley et al. 1985), while SLW is a leaf parameter indicative of leaf toughness In April 1997, shoots from three 2-y-old plantation trees exhibiting (Steinbauer 2001) and leaf sclerophylly (Landsberg and Gillieson noticeably less damage from Christmas beetle feeding in the field 1995) in eucalypts. While these three traits may not directly govern were grafted onto potted juvenile seedling rootstocks, grown from susceptibility of E. grandis to defoliation from Christmas beetles seed originating in the same seed orchard as the original field and chrysomelid leaf beetles, they are relatively easy and cheap material. We refer to these grafted ramets as the Byron Bay to measure. resistant clones. The total amount of foliage available from each of the Byron Bay clones was limited, so the clonal material was Shepherd et al. (2000) assessed variation to defoliation by Christmas pooled for testing and we did not make any comparisons between beetles across a series of eucalypt clones including E. grandis and the individual clones. The susceptible seedlings were grown as E. grandis x E. urophylla hybrids. He reported that the interspecific potted stock plants from seedlots of families from the same seed hybrids were more susceptible than the E. grandis individuals orchard. All foliage used in the feeding preference trials was from tested. Our study extends Shepherd’s findings by comparing plants of similar age. routine E. grandis seedlings with clonal material originating from three field trees exhibiting partial resistance to Christmas beetle To confirm that the original trees exhibiting resistance were defoliation. E. grandis, fresh leaf samples from both the resistant clones and the susceptible test seedlings were sent to Dr Michael Gillings In particular, we addressed the following questions: (Key Centre for Biodiversity and Bioresources, Macquarie 1. Is the original resistance to Christmas beetle defoliation University, Sydney). DNA fingerprinting analysis using the LP- maintained in grafted clones? RAPD method was carried out on the leaf samples (Gillings and 2. Is foliage that is unattractive to Christmas beetles similarly Holley 1997). The resistant clonal genotypes were confirmed to unattractive to paropsine leaf beetles? be genuine members of E. grandis (M. Gillings, unpublished data). 3. If resistance to paropsine feeding is shown, is it consistent between species and between life stages within species? Feeding choice tests using Christmas beetles 4. Does foliage from resistant clones and susceptible seedling Preliminary feeding tests using Christmas beetles indicated that stock differ significantly in nutritional, physical and/or 24 h was sufficient for the beetles to forage over all the plant chemical traits such as C:N ratio, SLW and monoterpene material present and to consume up to, but not exceeding, two- concentration? thirds of the foliage. Shoots of similar age with fully expanded leaves were randomly selected from the resistant Byron Bay clones Materials and methods and paired with shoots of similar age selected randomly from the susceptible seedlings. Shoots within pairs were matched such that Collection