Helen F. Nahrung

Corymbia phloem phenolics, tannins and terpenoids: interactions with a cerambycid borer

Plant secondary chemistry mediates the ability of herbivores to locate, accept and survive on potential host plants. We examined the relationship between attack by the cerambycid beetle Phoracantha solida and the chemistry of the secondary phloem (inner bark) of two differentially attacked plantation forestry taxa, Corymbiavariegata and its hybrid with C. torelliana. We hypothesised that this differential rate of attack may have to do with differences in secondary chemistry between the taxa. We found differences in the bark chemistry of the taxa, both with respect to phenolic compounds and terpenoids. We could detect no difference between bored and non-bored C. variegata trees (the less preferred, but co-evolved host). Hybrid trees were not different in levels of total polyphenols, flavanols or terpenes according to attack status, but acetone extracts were significantly different between bored and non-bored trees. We propose that variations in the bark chemistry explain the differential attack rate between C. variegata and the hybrid hosts.

Host tree influences on longicorn beetle (Coleoptera: Cerambycidae) attack in subtropical Corymbia (Myrtales: Myrtaceae)

ABSTRACT Phoracantha longicorn beetles are endemic to Australia, and some species have become significant pests of eucalypts worldwide, yet little is known about their host plant interactions and factors influencing tree susceptibility in Australia. Here, we investigate the host relationships of Phoracantha solida (Blackburn, 1894) on four eucalypt taxa (one pure species and three hybrid families), examining feeding site physical characteristics including phloem thickness, density, and moisture content, and host tree factors such as diameter, height, growth, taper, and survival. We also determine the cardinal and vertical (within-tree) and horizontal (between-tree) spatial distribution of borers. Fewer than 10% of P. solida attacks were recorded from the pure species (Corymbia citriodora subsp. variegate (Hook)), and this taxon also showed the highest survival, phloem thickness, relative growth rate, and bark:wood area. For the two most susceptible taxa, borer severity was negatively correlated with moisture content, and positively related to phloem density. Borers were nonrandomly and nonuniformly distributed within trees, and were statistically aggregated in 32% of plots. More attacks were situated on the northern side of the tree than the other aspects, and most larvae fed within the lower 50 cm of the bole, with attack height positively correlated with severity. Trees with borers had more dead neighbors, and more bored neighbors, than trees without borers, while within plots, borer incidence and severity were positively correlated. Because the more susceptible taxa overlapped with less susceptible taxa for several physical tree factors, the role of primary and secondary chemistries in determining host suitability needs to be investigated. Nevertheless, taxon, moisture content, phloem density, tree size, and mortality of neighboring trees appeared the most important physical characteristics influencing host suitability for P. solida at this site.

Sirex woodwasp Sirex noctilio (Hymenoptera: Siricidae): revisiting some past perceptions

The Sirex Woodwasp, Sirex noctilio Fabricius, is the focus of extensive global research because of its pest status, damage potential and ongoing spread within commercial softwood plantations and native forest ecosystems worldwide. In this overview, some small but persistent errors in the reported biological attributes of S. noctilio and its biological control nematode are highlighted. First, an error is noted in the equations reported for estimating size/fecundity relationships, whereby the female S. noctilio prothorax should be measured in cm, not mm. Second, the temperature requirements for egg‐adult development time are recalculated to provide more precise estimates of lower thermal threshold (5.1 °C) and cumulative day‐degree requirements (1973) using modern software techniques. Third, the origin of the Australian laboratory culture of Amylostereum areolatum used in rearing Deladenus siricidicola is clarified: it was collected from S. noctilio in Mt Gambier, South Australia, around 1995. It is envisaged that these corrections will result in improved research efforts.

Population genetic analyses of complex global insect invasions in managed landscapes: a Leptocybe invasa (Hymenoptera) case study

AbstractIncreased rates of movement and the accumulation of insects establishing outside their native range is leading to the ‘global homogenization’ of agricultural and forestry pests. We use an invasive wasp, Leptocybe invasa (Hymenoptera: Eulophidae), as a case study to highlight the rapid and complex nature of these global invasions and how they can complicate management options. To trace the invasion history of L. invasa globally, we characterised the genetic diversity within and between populations from its origin and invaded regions using mitochondrial and nuclear markers. Three mitochondrial Haplogroups were identified, of which two are likely different species that appear to have been independently introduced into different parts of the world. One type (Mitochondrial Haplogroup 1) occurs globally, and is the exclusive type found in Europe, the Middle East, South America and most of Africa. The second type (Mitochondrial Haplogroup 2) co-occurs with the first-type in Laos, South Africa, Thailand and Vietnam, while a third type (Mitochondrial Haplogroup 3) occurs exclusively in Australia, its native range. The distinction of the two invasive Haplogroups was supported by analysis of newly developed simple sequence repeat (microsatellite) markers in populations from 13 countries. Further analyses using clustering methods and approximate Bayesian computation suggested the occurrence of hybridisation in the Laos population and revealed that an unsampled population was the origin of Mitochondrial Haplogroup 1. The analyses also showed little genetic differentiation within the invasive populations, suggesting a limited original introduction from a very small population followed by rapid, global range expansion in a stepwise fashion. Results of this study should provide some guidelines for characterizing invasion pathways of new invasive insect pests.

Multiple introductions of Sirex noctilio (Hymenoptera: Siricidae) in northeastern North America based on microsatellite genotypes, and implications for biological control

The invasive woodwasp Sirex noctilio (Hymenoptera: Siricidae) has been moved from Eurasia into regions in the Southern Hemisphere, where extensive tree mortality has occurred in pines (Pinus spp.) introduced for forestry. More recently this woodwasp was found in northeastern North America, where pines are native, and it is a species of concern due to the economic importance of pines. Understanding the genetic diversity of North American S. noctilio points to new areas of inquiry, particularly regarding the ability of parasitic nematodes to sterilize woodwasps, which could provide control methods in the US and/or Canada. We investigated the genetic diversity of 924 S. noctilio from nine populations from New York and Pennsylvania (US), Ontario (CA), and Queensland (AU) using nine microsatellite loci. To avoid inflating the number of populations estimated by Bayesian inference, we measured the full-sibling relationships of woodwasps within 13 trees and removed all but one member of each full-sib family from the genetic analysis, resulting in a final sample size of 741 S. noctilio. Within a tree, on average 39% of woodwasps did not have a full sibling, and there were 5.6 families with at least two full-sibling members per tree. The mean family size across trees was 1.9 when single offspring (i.e., no full siblings) were included. Given the short time span since invasion, variation within North American S. noctilio is likely due to differences among founding genotypes. Genetic analyses support the hypothesis that at least two separate introductions occurred. Within North America, genetic distance measures were greatest between a site in southwestern Ontario and all other sites, suggesting that this population could represent a separate introduction event. Two methods of Bayesian clustering also support this idea; they detected 4 or 5 distinct genetic clusters with little admixture between the southwestern Ontario site and other North American populations. The wasps from Australia, where biological control with nematodes has been successful, showed low genetic diversity and clustered with the southwestern Ontario population in one out of two Bayesian analyses. Within the Ontario subset of samples, high woodwasp activity level (i.e., attack and mortality of trees) was associated with one genetic cluster more strongly than another. Population variation should be taken into account in studies of S. noctilio spread and management within North America.

Invasive Insects in Mediterranean Forest Systems: Australia

The Mediterranean forest regions of Australia predominantly comprise native mallee scrub, eucalypt woodlands, exotic Pinus plantations, and commercial eucalypt plantations. Native forests have, so far, remained largely free of invasive exotic insects. The exotic pines, however, have five well-established and significant invasive pest insects: the bark beetles Ips grandicollis, Hylastes ater, and Hylurgus ligniperda (Coleoptera: Curculiondae), Monterey pine aphid, Essigella californica (Hemiptera: Aphididae), and Sirex woodwasp, Sirex noctilio (Hymenoptera: Siricidae), with the latter not yet present in Western Australia (WA). The exotic giant pine scale, Marchalina hellenica (Hemiptera: Margarodidae), was recently detected on pines in Adelaide and Melbourne and is under an eradication program. Many of the established pest species have had classical biological control programs implemented. European house borer, Hylotrupes bajulus (Coleoptera: Cerambycidae), a pest of untreated softwood, is established in areas around Perth, WA, and has been found in dead and live trees, as well as untreated timber. African black beetle, Heteronychus arator (Coleoptera: Scarabaeidae) is a major exotic agricultural and forestry pest in wetter parts of the Mediterranean forest regions in WA, where it was first recorded in 1938. Several other exotic polyphagous horticultural pests are occasionally associated with eucalypts. Australia is the origin of major insect pests on Eucalyptus species grown in Mediterranean regions across the globe. However, populations of these insects are generally effectively controlled by native species of natural enemies in Australia. At least five species endemic to eastern Australia, Gonipterus platensis and G. sp. nov. 2, (Coleoptera: Curculionidae), Paropsisterna m-fuscum (Coleoptera: Chrysomelidae), Phylacteophaga froggatti (Hymenoptera: Pergidae) and Cardiapsina fiscella (Hempitera: Psyllidae), have been introduced to Mediterranean regions of WA, where they initially caused extensive and severe damage to plantations of introduced eucalypt species (predominantly E. globulus) in the region. However, the level and extent of damage gradually decreased, and it has been hypothesised that improved control by endemic natural enemies has occurred.

Population genetics of the Australian eucalypt pest Thaumastocoris peregrinus : evidence for a recent invasion of Sydney

Biological invasions represent a major threat to agriculture and forestry across the globe. Thaumastocoris peregrinus is a small sap-sucking heteropteran bug that has recently invaded a number of eucalypt plantations worldwide from its native range in Australia. To date, no studies have examined the range of this insect within Australia, and its population genetics remain poorly understood. We sampled T. peregrinus from 16 populations from South East Queensland, across New South Wales (NSW) and Victoria to south-eastern South Australia, and generated microsatellite and mtDNA data for ~ 200 individuals. Population genetic analyses consistently revealed moderate levels of genetic isolation by distance among populations across the range. Nonetheless, T. peregrinus has undergone dispersal across large distances, as revealed by the presence of identical mitochondrial haplotypes in both South East Queensland and South Australia. Two populations within the Sydney area (NSW) were divergent from other populations based on STRUCTURE and factorial correspondence analysis. They also had relatively low allelic richness and haplotype diversity indices. These results suggest they are the result of a relatively recent invasion event, consistent with their discovery in 2001. Pairwise genetic distance analyses suggest that the source of the invasion may have been central NSW. Our study provides an important framework for understanding the biology of this pest in its native environment, and may have implications for determining how it has invaded multiple areas worldwide.

Native Defoliators of Australian Mediterranean Forest Trees

Eucalypts (Eucalyptus, Corymbia and Angophora), as native stands and as plantations, dominate the two Mediterranean forest regions of Australia. These two geographically separated regions – south-western Australia, and areas of South Australia, including the Green Triangle region around the southern South Australian/Victorian border – have likewise largely separate native insect species, in spite of a common, albeit non-endemic, resource (E. globulus plantations). Almost half a million hectares of E. globulus and around 300,000 ha of softwoods is planted in Mediterranean Australia. Native defoliators have been largely unproblematic in pine plantations, but many have achieved significant pest status in hardwood plantations. Defoliators in native forest are less well-known, but there are several examples of species that have been studied in detail. Eucalypts typically receive (and tolerate) relatively high levels of defoliation, and have a diverse array of phytophagous insect fauna from several Orders (Lepidoptera, Coleoptera, Hemiptera, Hymenoptera and Orthoptera) feeding on their foliage, despite it being sclerophyllous and high in plant secondary compounds. Here, we provide an overview of the endemic folivores of Mediterranean forests in Australia.

In short- or long-term relationships, size does matter: body size patterns in the Mesostigmata (Acari: Parasitiformes)

ABSTRACT We examined a dataset of body sizes comprising 3209 species of Mesostigmata to test two hypotheses regarding body size patterns: that species living in symbiotic relationships differ in size compared with those that are free-living, and that mite species show geographic body size gradients. We found that Mesostigmata in symbiotic relationships are considerably larger than those that are free-living, but the type of symbiosis (commensal versus parasitic) had little influence on body size. We also found that free-living mite species fit a Bergmann size cline (larger species in cool climates), but mite species in commensal relationships show a converse Bergmann size cline (larger species in warm climates) while no pattern was found for parasitic species. These results fit a resource-based explanation for both patterns. We hypothesize that animals that use more easily obtainable resources grow larger, through resource pulses in temperate zones for free-living mites and use of concentrated resources for commensal and parasitic species. Available microhabitats could also promote smaller sizes in free-living Mesostigmata: i.e. soil and plants offer more and smaller microhabitats to exploit and are hence occupied by smaller species.

Mechanical properties of silk of the Australian golden orb weavers Nephila pilipes and Nephila plumipes

ABSTRACT Silks from orb-weaving spiders are exceptionally tough, producing a model polymer for biomimetic fibre development. The mechanical properties of naturally spun silk threads from two species of Australian orb-weavers, Nephila pilipes and Nephila plumipes, were examined here in relation to overall thread diameter, the size and number of fibres within threads, and spider size. N. pilipes, the larger of the two species, had significantly tougher silk with higher strain capacity than its smaller congener, producing threads with average toughness of 150 MJ m−3, despite thread diameter, mean fibre diameter and number of fibres per thread not differing significantly between the two species. Within N. pilipes, smaller silk fibres were produced by larger spiders, yielding tougher threads. In contrast, while spider size was correlated with thread diameter in N. plumipes, there were no clear patterns relating to silk toughness, which suggests that the differences in properties between the silk of the two species arise through differing molecular structure. Our results support previous studies that found that the mechanical properties of silk differ between distantly related spider species, and extends on that work to show that the mechanical and physical properties of silk from more closely related species can also differ remarkably. Summary: Spider silk is a remarkably tough and extensible biomaterial. We identified macrostuctural and mechanical differences in outer web frame silk of two Australian orb-weaving spiders.