Idris Barchia

Soil carbon dynamics under different cropping and pasture management in temperate Australia: Results of three long-term experiments

In addition to its important influence on soil quality and therefore crop productivity, soil organic carbon (SOC) has also been identified as a possible C sink for sequestering atmospheric carbon dioxide. Limited data are available on the impact of management practices on the rate of SOC change in agricultural soils in Australia. In this paper, results of three long-term trials (13–25 years) located near Wagga Wagga in temperate Australia were used to assess C dynamics under different tillage and stubble management practices, and under cropping intensities in pasture/crop rotations. Experimental results confirm the importance of management practices and pasture in determining first the steady-state SOC concentrations that are characteristic of given rotations and crop management systems, and second the rates of change of SOC concentrations as they approach steady-state concentrations in agricultural soils of this agro-ecological zone. A long-term crop/pasture experiment at a site with initial high SOC showed that the rate of SOC change in different treatments ranged from –278 to +257 kg C/ha.year over 0–0.3 m soil depth. Under continuous cropping, even under conservation agriculture practices of no-tillage, stubble retention, and crop rotation, the high initial SOC stock (0–0.3 m) present after a long-term pasture phase was, at best, maintained but tended to decrease with increased tillage or stubble burning practices. The effect of tillage was greater than that of stubble management. Increases in SOC were observed only in rotations incorporating a pasture phase. Our results suggest that improved soil nutrient and grazing management of permanent pasture can lead to an increase of 500–700 kg C/ha.year where the initial SOC concentrations are well below steady-state concentrations that could be expected after long periods of improved management. No difference was found between perennial pasture and annual pasture to the depth measured (0–0.3 m). Our results suggest that pasture holds the key to maintaining, and even increasing, SOC under crop/pasture in this environment.

Soil carbon stocks under different pastures and pasture management in the higher rainfall areas of south-eastern Australia

In Australia, pastures form the basis of the extensive livestock industries and are important components of crop rotation systems. Despite recent interest in the soil carbon sequestration value of pastures in the mitigation of climate change, little information is available on the soil carbon sequestration potential of pastures in New South Wales farming systems. To quantify the soil carbon stocks under different pastures and a range of pasture management practices, a field survey of soil carbon stocks was undertaken in 2007 in central and southern NSW as well as north-eastern Victoria, using a paired-site approach. Five comparisons were included: native v. introduced perennial, perennial v. annual, continuous v. rotational grazing, pasture cropping v. control, and improved v. unimproved pastures. Results indicated a wide range of soil organic carbon (SOC) stocks over 0–0.30 m (22.4–66.3 t C/ha), with little difference when calculated based on either constant soil depth or constant soil mass. Significantly higher SOC stocks were found only as a result of pasture improvement using P application compared with unimproved pastures. In this case, rates of sequestration were estimated to range between 0.26 and 0.72 t C/ha.year, with a mean rate of 0.41 t C/ha.year. Lack of significant differences in SOC stocks for the other pastures and pasture management practice comparisons could be due to inherent problems associated with the paired-site survey approach, i.e. large variability, difficulties in obtaining accurate site history, and the occasional absence of a valid control as well as the likely lower rates of SOC sequestration for these other comparisons. There is a need for scientific long-term trials to quantify the SOC sequestration potential of these other pastures and pasture management practices.

Evidence of superclones in Australian cotton aphid Aphis gossypii Glover (Aphididae: Hemiptera)

BACKGROUND Aphis gossypii is an important pest of cotton that has developed resistance to many chemicals used for its control. Any lack of understanding of its genetic structure, resistance status and host plant specialisation hampers effective management. RSULTS Eight microsatellite markers were genotyped for a collection of Australian A. gossypii field isolates from 55 plant species from major Australian cotton-producing regions. The aphids pirimicarb resistance status linked to the ACE1 (acetylcholinesterase) S431F mutation was determined by PCR-RFLP. Overall, the genetic diversity was low and there were only 13 multilocus genotype (MLG) groups found in a total of 936 aphids, suggesting asexual reproduction. Three MLGs (Aust-01, Aust-02 and Aust-04) represented 78% of all aphids tested. MLGs Aust-01 (41%) and Aust-02 (18%) were linked to the ACE1 S431F mutation and found on cotton and a range of hosts. Aust-04 (19%) hosted mainly on cotton (but also Asteraceae and Malvaceae) was predominantly susceptible to pirimicarb. Given their abundance and widespread occurrence, these three clones were considered to be superclones. CONCLUSION The study demonstrated that any strategy to control A. gossypii and manage pirimicarb resistance should target A. gossypii strains of all MLG types residing on any plant species and not just cotton

The critical threshold of Lawsonia intracellularis in pig faeces that causes reduced average daily weight gains in experimentally challenged pigs.

Serology indicates that Lawsonia intracellularis infection is widespread in many countries, with most pigs seroconverting before 22 weeks of age. However, the majority of animals appear to be sub-clinically affected, demonstrated by the low reported prevalence of diarrhoea. Production losses caused by sub-clinical proliferative enteropathy (PE) are more difficult to diagnose, indicating the need for a quantitative L. intracellularis assay that correlates well with disease severity. In previous studies, increasing numbers of L. intracellularis in pig faeces, quantified with a real time polymerase chain reaction (qPCR), showed a strong negative correlation with average daily gain (ADG). In this study, the association between faecal L. intracellularis numbers and PE severity was examined in two L. intracellularis experimental challenge trials (n1=32 and n2=95). The number of L. intracellularis shed in individual faeces was determined by qPCR on days 0, 7, 14, 17 and 21 days post challenge, and average daily gain was recorded over the same period. The severity of histopathological lesions of PE was scored at 21 days post challenge. L. intracellularis numbers correlated well with histopathology severity and faecal consistency scores (r=0.72 and 0.68, respectively), and negatively with ADG (r=-0.44). Large reductions in ADG (131 g/day) occurred when the number of L. intracellularis shed by experimentally challenged pigs increased from 10(7) to 10(8)L. intracellularis, although smaller ADG reductions were also observed (15 g/day) when the number of L. intracellularis increased from 10(6) to 10(7)L. intracellularis.

Can fatty acids and oxytetracycline protect artificially raised larvae from developing European foulbrood

A quantitative assay for the transmission of European foulbrood (EFB) in artificially raised larvae was developed. This assay was used to determine the concentration of oxytetracycline (OTC) required to prevent larvae from developing EFB and whether 8 fatty acids (undecanoic, lauric [dodecanoic], myristic, myristoleic, ricinoleic, ricinelaidic, homo-y-linolenic and 13,16,19-docosatrienoic acids) which had previously been demonstrated to inhibit the growth of Melissococcus plutonius cultures, could protect larvae from developing EFB. The larval assay involved grafting individual larva (less than 24 hours old) into a single well in a micro-titre plate. Each larva was fed 10 μL of basic larval diet (BLD) containing 500 000 M. plutonius organisms. After 3 days the larvae were also fed 60 000 Paenibacillus alvei spores (a common secondary invader associated with EFB) in 10 μL BLD. The combination of these two organisms was required to reliably produce symptoms typical of that seen in field cases of EFB. Most larvae infected using this protocol died from EFB. To determine the efficacy of OTC, EFB infected larvae were fed 0, 1, 2.5, 5 10 or 20 μg/mL of OTC. Treatment with 1 μg/mL lowered the mortality rate from 93.75% to 69.5%. Treatments with 2.5 μg/mL to 10 μg/mL reduced the mortality rate further whereas treatment with 20 μg/mL reduced the rate to the same as the negative control. Larvae fed 20 or 200 μg/mL of each of the eight fatty acids were not protected from developing EFB.ZusammenfassungDie Europäische Faulbrut (EFB) ist im Vergleich zur Amerikanischen Faulbrut (AFB) eine relativ schlecht untersuchte Bakterienerkrankung. Anders als bei AFB, bei der lediglich ein Erreger in den Krankheitsprozess involviert ist, sind bei EFB eine ganze Reihe von sekundären Eindringlingen bekannt, von denen Paenibacillus alvei einer der häufigsten ist (Bailey, 1960). Da die Übertragung von EFB durch unkontrollierbare äußere Bedingungen sowie durch das Ausräumen von infizierten Larven durch Adultbienen behindert wird (Bailey, 1960), ist es schwierig, eine EFB-Erkrankung in Honigbienenvölker verlässlich durch künstliche Infektion zu erzeugen. Um die Bedeutung von Melissococcus plutonius und Paenibacillus alvei für die Entwicklung von EFB zu untersuchen und um die unterschiedlichen Behandlungsmethoden ohne Störung durch Adultbienen beurteilen zu können, entwickelten wir einen quantitativen Biotest für die Übertragung von EFB auf künstlich aufgezogene Larven.Dieser Larventest beinhaltete die Übertragung individueller Larven (jünger als 24 h) in eine Vertiefung einer Mikrotiter-Platte. Jede Larve wurde mit einer Basisdiät gefüttert, die 500 000 M. plutonius Organismen enthielt. Nach 3 Tagen wurden die Larven zusätzlich mit 60 000 Sporen von P. alvei gefüttert, um den Verlauf von EFB besser zu simulieren. P. alvei ist ein verbreiteter sekundärer Eindringling im Zusammenhang mit EFB und eine Beimpfung mit M. plutonius allein ruft nicht die typischen klinischen Symptome einer EFB hervor.Oxytetrazyklin (OTC) ist derzeit das Antibiotikum der Wahl bei der Bekämpfung von EFB. Obwohl OTC seit Jahrzehnten zur Bekämpfung von EFB benutzt wird, ist nach wie vor unbekannt, welche Konzentration von OTC in einer Bienenlarve benötigt wird, um sie vor dem Ausbruch von EFB zu schützen. Um die Wirksamkeit von OTC zu bestimmen, wurden EFB-infizierte Larven mit 0, 1, 2,5, 5, 10 oder 20 μg/mL OTC gefüttert. Eine Behandlung mit lediglich 1 μg/mL reduzierte die Mortalität der Larven von 93,75 (Positivkontrolle) auf 69,53 %. Eine Konzentration von 20 μg/mL bot bereits einen Schutz, der vergleichbar mit der negativen Kontrolle war. Allerdings ist es unwahrscheinlich, dass eine solch hohe Konzentration benötigt wird, um EFB effektiv im Bienenvolk zu bekämpfen. Geringere Konzentrationen in Verbindung mit dem natürlichen Hygieneverhalten der Bienen würden vermutlich ausreichen, um klinische Symptome in Völkern mit EFB auszuschließen. Daher sollten Konzentrationen zwischen 2,5–10 μg/mL, für eine effektive Behandlung von EFB im Bienenvolk ausreichen.Falls Rückstandsprobleme vermieden werden müssen, sind alternative Methoden zur EFB-Kontrolle notwendig. Acht Fettsäuren (Undecan-, Laurin-, Myristin-, Myristolein-, Rizinol-, Ricinalaidin-, Linolen- und 13,16,19-Docosatrienoic-Säure), für die bereits eine Hemmung des Wachstums von M. plutonius- und P. larvae-Kulturen nachgewiesen wurden (Hornitzky, 2003), wurden im Larventest beurteilt. Keine der Fettsäuren bot einen Schutz der Larven gegenüber EFB. So war die Anzahl an toten Larven nach der Behandlung mit den Fettsäuren ähnlich hoch wie bei der unbehandelten Kontrolle. Die Tatsache, dass selbst hohe Dosen von 200 μg/mL an Fettsäuren keinen Schutz der Larven boten, während sehr kleine Konzentrationen von OTC (1 and 2,5 μg/mL) bereits die Larven schützte, untermauert die Unzweckmäßigkeit von Fettsäuren für die Bekämpfung.Diese Untersuchung zeigt klar, dass für den Test von Behandlungen sowohl in-vitro-Methoden (Laborkultur von M. plutonius) als auch in-vivo-Methoden (EFB-Larventest) verwendet werden sollten.

A significant fitness cost associated with ACE1 target site pirimicarb resistance in a field isolate of Aphis gossypii Glover from Australian cotton

The aphid Aphis gossypii Glover is an important pest of Australian cotton and has developed resistance to many chemicals used for its control. Its resistance management is partially based on chemical rotation that relies on insecticide resistance being associated with fitness costs. Therefore, understanding fitness costs associated with insecticide resistance is critical to its sustainable resistance management. We studied the fitness cost of pirimicarb resistance in A. gossypii caused by a single mutation in the acetylcholinesterase gene ACE1 by mixing different ratios of susceptible and resistant aphids. This was achieved by establishing A. gossypii populations of a known starting allele frequency in aphid proof cages and measuring allele frequency change over time via qPCR. Unlike traditional cohort fitness studies, we used competitive fitness as a measure of relative fitness of resistant versus susceptible aphids in the same environment. We demonstrate that competitive fitness measured in this study is an accurate predictor of overall relative fitness. We found that pirimicarb resistance had a significant fitness cost in the presence of susceptible aphids in the absence of insecticide pressure and that the fitness cost was related to the initial resistance allele frequency. By using the competitive fitness measure and knowing the initial allele frequency, it is possible to predict the likely time from resistant to an essentially susceptible population. As resistance was stable in the absence of susceptible competition, we recommend the use of resistance management tactics that do not completely eliminate the susceptible genotype such as complimentary integrated pest management.

A TaqMan qPCR method for detecting kdr resistance in Aphis gossypii demonstrates improved sensitivity compared to conventional PCR–RFLP

Cotton aphid, Aphis gossypii Glover, has emerged as a prominent pest in Australian cotton production, and monitoring pesticide resistance including pyrethroids in field populations is crucial for its sustainable management. We examined the distribution of kdr resistance in 35 field-collected A. gossypii populations and used TaqMan qPCR assays with pooled samples. The study demonstrated proof of concept that pooled insect qPCR methodology provided effective detection with better sensitivity than individual PCR–RFLP genotyping techniques for the kdr resistance allele. The practical outcome is that routine resistance monitoring can examine more sites while increasing the likelihood of detecting incipient resistance at those sites. More importantly, the method is adaptable to any genetically caused resistance and so not limited to A. gossypii or even insect control. It cannot be overstressed that the ability to detected resistance at very low frequencies is critical to all sustainable resistance management. Early detection of resistance provides critical time for the modification of chemical use prior to potential insecticide control failure.

A six-month-long assessment of the health of bee colonies treated with APITHOR™ hive beetle insecticide

The safety of APITHOR™ hive beetle insecticide on the health of honey bee colonies was assessed in a field trial in which 16 bee colonies that were exposed to two consecutive treatments each of three-months duration, were compared with 10 untreated (control) hives. Measurements of brood area, available hive frames occupied by bees (hive strength) and hive weight (as an indirect indicator of honey production) were recorded pre-treatment and after three- and six-months exposure to APITHOR™ treatment. Samples of honey and wax collected from six of the treated hives at the same times were independently tested for the presence of fipronil and its metabolites, and no residues were detected in any sample at either time. Mean net increases in the weights of the APITHOR™ treated and control hives were not significantly different (p > .05). Similarly, neither mean brood area nor the mean proportion of available hive frames occupied by bees in the control and APITHOR™ treated hives was significantly different from each other (p > .05) at both the three- and six-month post-treatment assessments. Compared to the control hives, however, significantly (p < .001) fewer live beetles were recorded in the APITHOR™ treated hives at these times.