Characterisation of entomopathogenic nematodes and their symbiotic bacteria across eastern Australia and their potential in controlling Queensland fruit fly

Tephritid fruit flies (Tephritidae) include several destructive pest species that attack fruit and fruiting vegetables across many climatic regions of the world. The Queensland fruit fly, Bactrocera tryoni, is Australia’s most significant horticultural pest. It lays eggs in the fruit of over 40 plant families, including many important fruit and vegetable crops, making it one of the most polyphagous and most economically important species among all tephritids. So far, one of the most effective control measures is the use of synthetic insecticides. However, many insecticides have been abolished because of their environmental and human health risks and their impact on non-target organisms. Entomopathogenic nematodes (EPNs) of the families Heterorhabditidae and Steinernematidae and their associated bacteria Photorhabdus and Xenorhabdus, found in soils throughout the world, are important biocontrol agents of insect pests with soil-inhabiting stages. They have been commercially used as a safe alternative to chemical pesticides. However, the potential of EPNs to control B. tryoni is largely unknown. This PhD thesis comprises five chapters. Chapter 1 reviews the biology and ecology of the Queensland fruit fly, EPNs and their associated symbiotic bacteria. Chapter 2 comprises a survey and the identification of EPNs and their associated bacteria isolated from soils across eastern Australia. Chapter 3 provides an overview of the virulence of the 32 newly isolated and 4 commercial EPN strains on larval and pupal stages of B. tryoni. Chapter 4 provides the result of the effects of temperature on survival and persistence of 17 EPN isolates against larvae and pupae of B. tryoni under laboratory conditions. Overall, my research expands substantially the previously available information about EPN diversity in Australia and provides the first comprehensive genetic characterization of this Australian diversity. Furthermore, the EPN isolates such as Hi.HRN2, Hz.NAR1, Hi.ECCH, Hi.LMI2, Hi.QF6, Hb.HIE and Sf.ECCS showed traits, such as high virulence and long persistence, that make them potential candidates for the biological control of fruit flies. The survival and virulence of these isolates, even at higher temperature suggest that they could be a good biological control agent of fruit flies in Australian conditions. However, it is necessary to further check their efficacy in the field before development of applications at a commercial scale

Elastic and electronic properties of Ti2Al(CxN1−x) solid solutions

The elastic coefficients and mechanical properties (bulk modulus, shear modulus, Young\u27s modulus and Poisson\u27s ratio) of Ti2Al(CxN1−x) continuous solid solutions for x from 0 to 1 are calculated using ab initio DFT methods on 4×4×1 supercell models. It is shown that the properties of these solid solutions do not vary linearly with x. Although the lattice constant c is almost constant for x≤0.5, a increases linearly. For x\u3e0.5, c starts to increase with x while the rate of increase in a slows down. For x between 0.5 and 0.85, the elastic coefficients and the mechanical parameters show interesting dependence on x and crossovers, signifying the complex interplay in the structure and properties in Ti2Al(CxN1−x) solid solutions. The nonlinear variations in mechanical properties are explained in terms of subtle difference in the electronic structure and bonding between nitrides and carbides in complex MAX phase compounds

Theoretical study of the elasticity, mechanical behavior, electronic structure, interatomic bonding, and dielectric function of an intergranular glassy film model in prismatic β-Si3N4

This is the published version. Copyright © 2010 The American Physical SocietyMicrostructures such as intergranular glassy films (IGFs) are ubiquitous in many structural ceramics. They control many of the important physical properties of polycrystalline ceramics and can be influenced during processing to modify the performance of devices that contain them. In recent years, there has been intense research, both experimentally and computationally, on the structure and properties of IGFs. Unlike grain boundaries or dislocations with well-defined crystalline planes, the atomic scale structure of IGFs, their fundamental electronic interactions, and their bonding characteristics are far more complicated and not well known. In this paper, we present the results of theoretical simulations using ab initio methods on an IGF model in β-Si3N4 with prismatic crystalline planes. The 907-atom model has a dimension of 14.533 Å×15.225 Å×47.420 Å. The IGF layer is perpendicular to the z axis, 16.4 Å wide, and contains 72 Si, 32 N, and 124 O atoms. Based on this model, the mechanical and elastic properties, the electronic structure, the interatomic bonding, the localization of defective states, the distribution of electrostatic potential, and the optical dielectric function are evaluated and compared with crystalline β-Si3N4. We have also performed a theoretical tensile experiment on this model by incrementally extending the structure in the direction perpendicular to the IGF plane until the model fully separated. It is shown that fracture occurs at a strain of 9.42% with a maximum stress of 13.9 GPa. The fractured segments show plastic behavior and the formation of surfacial films on the β-Si3N4. These results are very different from those of a previously studied basal plane model [J. Chen et al., Phys. Rev. Lett. 95, 256103 (2005)] and add insights to the structure and behavior of IGFs in polycrystalline ceramics. The implications of these results and the need for further investigations are discussed

Survey and identification of entomopathogenic nematodes in the province of Cotabato, Philippines, for biocontrol potential against the tobacco cutworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)

Background: The tobacco cutworm, Spodoptera litura [Fab.] (Lepidoptera: Noctuidae), is a devastating insect pest of several crops. Entomopathogenic nematodes (EPNs) of the families Heterorhabditidae and Steinernematidae are used as an alternative control measure in lieu of the hazardous synthetic chemical applications. Results: A survey of naturally occurring EPNs was conducted across the province of Cotabato, Philippines, covering a total of 5 municipalities with 25 villages. Galleria-baiting technique was employed to recover nematodes from peanut and grassland soils. Out of 50 soil samples collected, only 5 samples harbored nematodes, indicating a recovery of 10%. Preliminary morphological data identified only one EPN under the genera Heterorhabditis (1 isolate), whereas 4 were facultative necromenic nematodes from the genera Metarhabditis (2 isolates) and Oscheius (2 isolates). Analysis of D2D3 segments of the 28S rDNA confirmed high sequence similarity to Heterorhabditis indica, Metarhabditis rainai, Oscheius insectivora, and Oscheius sp. This is the first record of H. indica and M. rainai in the entire region, whereas the first record for Oscheius spp. in the Philippines. Furthermore, the biocontrol potential of the local H. indica infective juvenile (IJ) populations (PIGCD1) isolated from peanut was assessed against the tobacco cutworm, S. litura, under laboratory conditions. The mean percentage mortality caused by H. indica on S. litura at 7 different concentrations ranged from 0-100% at 24 h post inoculation. The lethal concentration (LC50) required to kill 50% of the S. litura larvae population with H. indica was 7.13±1 (IJs/larva). Conclusions: The use of Galleria-baiting method is a convenient approach to detect EPNs including other facultative necromenic nematodes from the soils. Obtained data indicated that the local H. indica isolate can be a promising alternative measure to suppress the economically important insect pest, S. litura, and this may provide significant outlook to establish the biocontrol program in the country

Native Philippine Heterorhabditis indica isolates from banana and rice fields and preliminary results of their virulence against the larvae of super worm (Zophobas morio Fabricius Coleoptera: Tenebrionidae)

Background: Entomopathogenic nematodes (EPNs) of the families Heterorhabditidae and Steinernematidae are used as biological control agents of several insect pests in lieu of synthetic pesticides. Results: Using an insect-baiting technique, naturally occurring EPN isolates were collected from banana and rice fields in the municipality of Hagonoy, the Philippines. A total of 4 isolates were recovered from all the sites namely, HBP1, HBP2, HR3 and HR4. By combining morphometrical and ITS rDNA data, all the isolates were identified as Heterorhabditis indica. To initially assess their infectivity, virulence tests were conducted on the last instar larvae of the super worm, Zophobas morio Fabricius (Coleoptera: Tenebrionidae), the model insect. HR3 isolate had the highest percentage mortality (56%), followed by HBP1 and HBP2 (33%), which were comparable to the available nematode in the market, Heterorhabditis bacteriophora (44%), whereas the lowest was obtained in HR4 isolate (11%). Conclusions: This study accounted 4 more H. indica isolates in addition to pre-existing isolates in the country, expanding its habitat range and geographic distribution. This also provides baseline information on the potential biocontrol utilization of native EPNs against soil-dwelling insect pests of agricultural crops. Further investigations are required to assess their efficacy against several other insect pests of economic importance for the integrated pest management (IPM) programmes in the country

Elastic and electronic properties of Ti2Al(CxN1-x) solid solutions

The elastic coefficients and mechanical properties (bulk modulus, shear modulus, Young\u27s modulus and Poisson\u27s ratio) of Ti2Al(CxN1-x) continuous solid solutions for x from 0 to 1 are calculated using ab initio DFT methods on 4×4×1 supercell models. It is shown that the properties of these solid solutions do not vary linearly with x. Although the lattice constant c is almost constant for x≤0.5, a increases linearly. For x\u3e0.5, c starts to increase with x while the rate of increase in a slows down. For x between 0.5 and 0.85, the elastic coefficients and the mechanical parameters show interesting dependence on x and crossovers, signifying the complex interplay in the structure and properties in Ti2Al(CxN1-x) solid solutions. The nonlinear variations in mechanical properties are explained in terms of subtle difference in the electronic structure and bonding between nitrides and carbides in complex MAX phase compounds

Structure and properties of the low-density phase ι-Al2O3 from first principles

Of the various phases of transition alumina, iota-alumina (ι-Al2O3) is the least well known. It is considered to be the end member of the mullite series in the limit of zero Si content. The structural details of ι-Al2O3 are not available and its physical properties are totally unknown. Based on an appropriately modified structure of a high alumina content mullite phase close to the 9-1 mullite, we have successfully constructed a structural model for ι-Al2O3. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern obtained from samples that claim to belong to ι-Al2O3. ι-Al2O3 is a highly disordered ultralow-density phase of alumina with a theoretical density of 2854 kg/m3. The calculated total energy per Al2O3 is much higher than α-Al2O3 and the other well-known disordered phase, γ-Al2O3. Using this theoretically constructed model, we have calculated the elastic, thermodynamic, electronic, and spectroscopic properties of ι-Al2O3 and compared them with those of α-Al2O3 and γ-Al2O3. It is shown that the mechanical strength of ι-Al2O3 is much weaker with bulk and shear moduli that are only 57 and 42% of α-Al2O3. Phonon dispersion results and the subsequently calculated thermodynamic properties point to the fact that ι-Al2O3 is an alumina phase preceding γ-Al2O3 in the processing of alumina before reaching α-Al2O3. Electronic structure calculations show it to be an insulator with a direct band gap of only 3.0 eV at the Γ point and a higher ionic bonding character than α-Al2O3 and γ-Al2O3. It has a calculated static dielectric constant of 2.73 and a corresponding refractive index of 1.65 that are in agreement with reported data. Also calculated are the Al-K, Al-L3, and O-K edges of the x-ray absorption near edge structure in ι-Al2O3, showing sensitive dependence on its local bonding environment. However, only the weighted average of these spectra can be directly compared with the measured ones which are currently unavailable

Approximate lattice thermal conductivity of MAX phases at high temperature

The temperature dependent lattice thermal conductivity (κph) of MAX phases, Mn+1AXn are calculated using the Debye theory as outlined by Slack. At high temperature the formula derived by Slack is a reasonable approximation to estimate the lattice thermal conductivity. The calculation used the large data base of elastic coefficients of stable MAX phases established recently. It is found that MAX phases with A = Al have higher κph at 1300 K, and the majority of MAX carbides have higher κph than MAX nitrides. We have also calculated the minimum thermal conductivities of these MAX phases using the empirical formula suggested by Clarke. It is shown that the minimal lattice thermal conductivities of MAX carbides and nitrides are closer to each other in the 211 phases than in higher n phases. The calculated κph for 8 MAX phases at 1300 K are in reasonable agreement with experimental data, especially in Ti2AlC, Nb4AlC3, Ta4AlC3, Nb2AlC and Nb2SnC phases

A genomic approach to the stability, elastic, and electronic properties of the MAX phases

In this study, we report a comprehensive assessment on the elastic and electronic properties of 792 possible MAX (Mn+1AXn) phases with n = 1-4 using ab initio methods. These crystals are then screened based on their elastic and thermodynamic stability resulting in a large database of 665 viable crystals. All the experimentally verified MAX phases passed the screening. Various correlations among and between them are fully explored. In particular, the key elements in the interdependence between the elastic properties together with mechanical parameter derived from them and the electronic structure are identified. Detailed analysis of various correlation plots shows that there is a clear correspondence between bulk modulus K and total bond order density (TBOD). Calculations show a marked difference between the carbides and nitrides. This database is also used to test the efficacy of data mining algorithms for materials genome. We further identified several thermodynamically stable new MAX phases with unusual mechanical parameters that have never been synthesized in the laboratory or theoretically investigated. The complete database on the elastic and electronic structure together with the mechanical parameters for these 665 MAX phases compounds are included in the Supplementary Materials and fully accessible