Martin J. Steinbauer

Native and Exotic Pests of Eucalyptus: A Worldwide Perspective

Eucalyptus species, native to Australia, Indonesia, the Philippines, and New Guinea, are the most widely planted hardwood timber species in the world. The trees, moved around the globe as seeds, escaped the diverse community of herbivores found in their native range. However, a number of herbivore species from the native range of eucalypts have invaded many Eucalyptus-growing regions in North America, Europe, Africa, Asia, and South America in the last 30 years. In addition, there have been shifts of native species, particularly in Africa, Asia, and South America, onto Eucalyptus. There are risks that these species as well as generalist herbivores from other parts of the world will invade Australia and threaten the trees in their native range. The risk to Eucalyptus plantations in Australia is further compounded by planting commercially important species outside their endemic range and shifting of local herbivore populations onto new host trees. Understanding the mechanisms underlying host specificity of Australian insects can provide insight into patterns of host range expansion of both native and exotic insects.

Oviposition preference of a Eucalyptus herbivore and the importance of leaf age on interspecific host choice

1. Paropsine chrysomelids are eucalypt folivores for which it has been hypothesized that the availability of suitable age class foliage (i.e. young foliage) is a more important determinant of host use than the physical and/or chemical characteristics of foliage from different hosts. This prediction was tested by assessing the oviposition preference of Chrysophtharta bimaculata (Olivier) on excised branches of Eucalyptus regnans F. Mueller and E. nitens (Deane & Maiden) Maiden.

Monoterpenes and epicuticular waxes help female autumn gum moth differentiate between waxy and glossy Eucalyptus and leaves of different ages.

The autumn gum moth, Mnesampela privata, is a native Australian species whose preferred host, Eucalyptus globulus (Myrtaceae), is an aromatic evergreen tree that has long-lived waxy leaves during the juvenile phase of growth. We compared the behavioral and antennal responses of female moths to whole leaves (new and old) and samples of foliar chemicals (from new and old leaves) from a typical E. globulus subsp. pseudoglobulus with responses to a glossy, half-sibling E. g. subsp. pseudoglobulus putative hybrid (the result of natural cross-pollination). We also studied larval survival and development on leaves from the same trees. In laboratory binary-choice assays, female M. privata laid more eggs on waxy leaves than on glossy leaves thereby confirming the nonpreference for the glossy tree that was observed in the field. Analyses of the monoterpenes and waxes of both trees revealed that they had comparable suites of monoterpenes and total oil contents but different suites of epicuticular waxes. Headspace extracts differed in the intensity of component monoterpenes. Gas chromatographic analyses with electroantennographic detection showed different patterns of monoterpene detection. Leaves of the glossy tree had a less diverse array of epicuticular waxes than those of the waxy tree. Electroantennographic screening of responses to wax extracts from leaves (new and old) from either tree revealed positive dose-dependent responses of female antennae to waxes from new leaves only. Binary-choice assays also revealed a strong preference by ovipositing females for new, compared to old, leaves whether they were from the waxy or the glossy tree. However, new leaves from either tree could be manipulated (by physical abrasion of epicuticular waxes) so that females would lay almost no eggs on them. Larval survival did not differ between groups reared on leaves from both trees (new and old). Over 70% of all larvae survived to pupation. However, larvae reared on leaves from the glossy tree took longer to pupate than those reared on leaves from the waxy tree. Also, larvae reared on new leaves from either tree did not perform as well as those reared on old leaves. Monoterpene and wax cues are suggested as helping female M. privata locate preferred hosts in native forests.

Specific leaf weight as an indicator of juvenile leaf toughness in Tasmanian bluegum (Eucalyptus globulus ssp. globulus): implications for insect defoliation

Summary The influence of watering regime, provenance and leaf-age were studied in relation to specific leaf weight (abbreviated SLW; inversely correlated with percentage leaf water) of juvenile leaves of Eucalyptus globulus subspecies globulus. SLW provides a physiological estimate of leaf “toughness” relevant to studies of insect herbivory. Leaves from saplings given continuous access to water had lower SLWs and higher water contents than leaves from saplings that were drip irrigated. Saplings of the King Island provenance produced leaves of consistently lower SLW compared with leaves from saplings of the Geeveston provenance irrespective of watering regime. Furthermore, leaves from the King Island provenance saplings exhibited less variation with respect to water content than leaves from the Geeveston saplings irrespective of watering regime. Generally, new leaves exhibited lower SLWs and higher water contents than old leaves irrespective of watering regime or provenance. The importance of leaf “toughness” (as indicated by SLW) to insect herbivory, and therefore its relevance to plantation managers, is discussed.

Oviposition preference and neonate performance of Mnesampela privata in relation to heterophylly in Eucalyptus dunnii and E. globulus

Abstract 1  The autumn gum moth, Mnesampela privata, is an endemic Australian geometrid that utilizes a number of species within the genus Eucalyptus as hosts. Based on field observations, the moth is thought to be leaf‐type specific for juvenile as opposed to adult eucalypt foliage. 2  Laboratory binary choice assays of the oviposition preference of host novice M. privata confirmed that eggs were more likely to be laid upon juvenile rather than adult foliage of Eucalyptus dunnii and two subspecies of E. globulus. This oviposition preference was not influenced by differences in leaf size or adherence to leaves by ovipositing moths. The high specific leaf weights common to adult leaves were associated with reduced oviposition. 3  Although neonates fed on both juvenile and adult leaves of most of the trees studied, performance was greater on juvenile as opposed to adult foliage. Juvenile leaves typically had lower specific leaf weights and were nutritionally superior to their adult counterparts. Specific leaf weights above 0.236 mg/mm2 (associated with low water, i.e. = 56.5%, and nitrogen, i.e. = 1.23%, contents) were associated with reduced larval performance. Younger adult leaves, those with lower specific leaf weights, allowed slightly greater larval consumption. 4  When ovipositing, this eucalypt‐specific moth discriminates between leaf types of its heterophyllous hosts in favour of types with the lowest specific leaf weight. Less tough leaf types, which are also higher in nitrogen, enable neonates to attain larger body weights.

Suitability of Eucalyptus and Corymbia for Mnesampela privata (Guenée) (Lepidoptera: Geometridae) larvae

Abstract  1 Mnesampela privata (Guenée) has a host list of 40 Eucalyptus and at least one Corymbia species. Larval survival and performance was studied on 19 species to investigate how certain leaf traits influence the suitability of different species.

Nutritional enhancement of leaves by a psyllid through senescence-like processes : insect manipulation or plant defence?

Some herbivores can modify the physiology of plant modules to meet their nutritional requirements. Induction of premature leaf senescence could benefit herbivores since it is associated with the mobilisation of nutrients. We compared the effects of nymphal feeding by Cardiaspina near densitexta on Eucalyptus moluccana with endogenous processes associated with senescence to assess the relative merits of an insect manipulation or plant defence interpretation of responses. Evidence supporting insect manipulation included increased size of fourth and fifth instar nymphs (in the latter the effect was restricted to forewing pad length of females) on leaves supporting high numbers of conspecifics and feeding preventing leaf necrosis. Intra-specific competition negated greater performance at very high densities. High and very high abundances of nymphs were associated with increased concentrations of amino acid N but only very high abundances of nymphs tended to be associated with increased concentrations of six essential amino acids. Contrary to the insect manipulation interpretation, feeding by very high abundances of nymphs was associated with significant reductions in chlorophyll, carotenoids and anthocyanins. Evidence supporting plant defence included the severity of chlorosis increasing with the abundance of nymphs. Leaf reddening did not develop because ambient conditions associated with photoinhibition (high irradiance and low temperature) were not experienced by leaves with chlorotic lesions. Leaf reddening (from anthocyanins) alone is not expected to adversely affect nymphal survival; only leaf necrosis would kill nymphs. For senescence-inducing psyllids, nutritional enhancement does not fit neatly into either an insect manipulation or plant defence interpretation.

Challenges to assessing connectivity between massive populations of the Australian plague locust

Linking demographic and genetic dispersal measures is of fundamental importance for movement ecology and evolution. However, such integration can be difficult, particularly for highly fecund species that are often the target of management decisions guided by an understanding of population movement. Here, we present an example of how the influence of large population sizes can preclude genetic approaches from assessing demographic population structuring, even at a continental scale. The Australian plague locust, Chortoicetes terminifera, is a significant pest, with populations on the eastern and western sides of Australia having been monitored and managed independently to date. We used microsatellites to assess genetic variation in 12 C. terminifera population samples separated by up to 3000 km. Traditional summary statistics indicated high levels of genetic diversity and a surprising lack of population structure across the entire range. An approximate Bayesian computation treatment indicated that levels of genetic diversity in C. terminifera corresponded to effective population sizes conservatively composed of tens of thousands to several million individuals. We used these estimates and computer simulations to estimate the minimum rate of dispersal, m, that could account for the observed range-wide genetic homogeneity. The rate of dispersal between both sides of the Australian continent could be several orders of magnitude lower than that typically considered as required for the demographic connectivity of populations.

Causes and Consequences of Host Expansion by Mnesampela privata

The autumn gum moth, Mnesampela privata, utilizes several species of Eucalyptus planted outside regions of endemism within Australia. We investigated whether foliar monoterpene composition influenced oviposition in the field on the natural primary host (E. globulus) and a novel host (E. rubida), both characterized by nonstructural epicuticular waxes. In the laboratory, oviposition preferences of females for species and families of known host, novel hosts, and non-hosts that were characterized by both nonstructural and structural waxes but also varied in foliar concentrations of the purportedly toxic plant secondary metabolite (sideroxylonal) were studied. Although M. privata laid as many eggs on trees of two families of E. rubida as they did on trees of two families of E. globulus, there were significant differences in the numbers of clutches of eggs laid. When combined with data for oviposition on another five families of E. globulus, we found a negative relationship between mean numbers of eggs and foliar concentration of α-pinene but a positive relationship between egg numbers and the concentration of α-terpineol. The field data suggest that female M. privata are just as willing to lay eggs on novel hosts with comparable foliar monoterpene compositions to those of the primary host, especially if they produce nonstructural epicuticular waxes. Oviposition assays in the laboratory endorse this mechanism of host plant hierarchy and support the long-held assumption of the host primacy of E. globulus. In laboratory assays, some larvae pupated on all hosts (except Corymbia eximia) but the number completing larval development was greater on hosts with softer leaves. Larval survival was also reduced on hosts with high concentrations of sideroxylonal but only if those hosts also had modest to high concentrations of monoterpenes. Larval survival was high on a host (E. macarthurii) with a high concentration of sideroxylonal but with virtually zero monoterpene content. This suggests that the monoterpene content of a host could antagonize the effect on M. privata larvae of its sideroxylonal content. The larval food plant most affected the fitness of female rather than male pupae. Of the known host expansion events, all have occurred in mixed species plantations. The co-occurrence in these plantations of either the primary host or other highly ranked species probably explains the eventual expansion onto the neighboring species of Eucalyptus and Corymbia.

Eight polymorphic microsatellite loci for the Australian plague locust, Chortoicetes terminifera

Few population genetics studies have been carried out on major locust species. In particular, an understanding of the population genetic structure of the Australian plague locust, Chortoicetes terminifera, is lacking. We isolated and characterized eight polymorphic microsatellite loci in C. terminifera, and described experimental conditions for polymerase chain reaction multiplexing and genotyping these loci. The number of alleles per locus ranged from 11 to 29 and the expected heterozygosity ranged from 0.797 to 0.977. One locus was found to be X‐linked. Results of cross‐taxon amplification tests are reported in four species of the Oedipodinae subfamily.