Learnings from over a decade of increasing pesticide resistance in the redlegged earth mite, Halotydeus destructor (Tucker).

BACKGROUND: The redlegged earth mite, Halotydeus destructor (Tucker), is a destructive and economically important pest of winter grain crops and pastures in Australia. It is largely controlled by pesticides, but this mite has evolved resistance to pyrethroid and organophosphate chemicals. A national Resistance Management Strategy has been developed for pro-active management to delay further resistance evolution, though its success is reliant on a detailed understanding of the incidence, patterns of spread, current distribution and the nature of resistance in the field. Here, we report on a long-term resistance surveillance programme undertaken between 2006 and 2019 informed by resistance risk forecasting. RESULTS: By mapping the Australian distribution of resistance through time, we show that resistance is present across three Australian states and covers more than 3000 km. This current range includes a recently identified population exhibiting organophosphate resistance representing the most easterly location of resistance in H. destructor. Using field history information, we identify associations for the first time between crop management practices employed by farmers and the presence of pyrethroid resistance. Management strategies that could minimize the risk of further resistance include limiting local spread of resistance through farm hygiene practices, crop rotations and reducing pesticide usage. CONCLUSION: This study highlights the challenges of resistance in H. destructor but also indicates how quantitative resistance risk analysis can be developed to target field surveillance and delay further resistance. The management strategies highlighted in this study can help maintain the effectiveness of control options but will depend on farmer engagement and adoption. © 2021 Society of Chemical Industry.

A genomic approach to identify and monitor a novel pyrethroid resistance mutation in the redlegged earth mite, Halotydeus destructor.

Resistance mechanisms are typically uncovered by identifying sequence variation in known candidate genes, however this strategy can be problematic for species with no reference data in known relatives. Here we take a genomic approach to identify resistance to pyrethroids in the redlegged earth mite, Halotydeus destructor, a member of the Penthalidae family of mites that are virtually uncharacterized genetically. Based on shallow genome sequencing followed by a genome assembly, we first identified contigs of the H. destructor parasodium channel gene. By linking variation in this gene to known resistant phenotypes, we located a single nucleotide polymorphism in resistant mites. This polymorphism results in a leucine (L) to phenylalanine (F) amino acid substitution in the II6 region (predicted) of the gene (L1024F). This novel mutation has not previously been linked to pyrethroid resistance, although other polymorphisms have been identified in the two-spotted spider mite, Tetranychus urticae at the same locus (L1024V). The sequencing approach was successful in generating a candidate polymorphism that was then validated using laboratory bioassays and field surveys. A high throughput Illumina-based sequencing diagnostic was developed to rapidly assess resistance allele frequencies in pools of mites sourced from hundreds of populations across Australia. Resistance was confirmed to be widespread in the southern wheatbelt region of Western Australia. Two different resistance mutations were identified in field populations, both resulting in the same amino acid substitution. The frequency and distribution of resistance amplicon haplotypes suggests at least two, and probably more independent origins of resistance.

Discovery and characterisation of field resistance to organophosphorus chemicals in a major mite pest, Halotydeus destructor.

BACKGROUND: The redlegged earth mite (Halotydeus destructor) is an agricultural pest in Australia that attacks a wide variety of crops and pasture species. Chemicals remain an important part of control strategies for H. destructor, despite the existence of resistance to pyrethroid insecticides in this species. Recent chemical control failures involving a second insecticide class, organophosphates, were investigated using pesticide bioassays. RESULTS: We confirmed, for the first time, resistance to organophosphates in H. destructor, and show that resistance is not confined to a single property, or region. There was no evidence that resistance to organophosphorus chemicals has evolved in Australian states outside of Western Australia. CONCLUSION: These findings demonstrate the strong evolutionary capability of H. destructor and highlight the need for ongoing resistance surveillance within Australia. © 2017 Society of Chemical Industry.

The current status of pesticide resistance in Australian populations of the redlegged earth mite (Halotydeus destructor).

BACKGROUND: The redlegged earth mite, Halotydeus destructor Tucker, is an important pest of broad-acre farming systems in Australia, New Zealand and South Africa. In 2006, high levels of resistance to synthetic pyrethroids were discovered in this species in Western Australia. RESULTS: Between 2007 and 2010, the authors monitored resistance in field populations and found it had spread considerably within the state of Western Australia. Twenty-six paddocks from 15 individual properties were identified with resistance, and these paddocks ranged over 480 km. To date, resistance has not been detected in any other Australian state. Resistance in H. destructor appears to be found across the entire pyrethroid group, but not to other chemical classes such as organophosphates and carbamates, or other chemistries with novel modes of action. CONCLUSION: The high levels of resistance occurring in Western Australia have caused considerable economic losses due to ineffective chemical applications and mortality of crop plants at seedling establishment. These findings highlight the need for a comprehensive resistance surveillance programme to be developed for H. destructor within Australia. Growers need to consider non-chemical approaches for pest control and should be encouraged to implement pesticide resistance management programmes for H. destructor.

Direct spectrophotometric determination of hesperidin in pharmaceutical preparations.

A simple, rapid and reliable direct spectrophotometric method for the determination of hesperidin is proposed and validated. The influence of wavelength, solvent, the ionic strength, pH and temperature on hesperidin determination were investigated. Under the optimum conditions, λ = 283 nm, 60% methanol as the solvent, ionic strength, I = 2.5 × 10-5 mol L-1, pH = 6.4 and T = 37.0 °C, the Beer's law is obeyed in the concentration range 1.83-24.5 µg mL-1. The molar absorptivity and Sandells sensitivity were found to be 1.8 × 104 L mol-1 cm-1 and 0.03 µg cm-2, respectively. The sensitivity of the proposed method was 0.9 µg mL-1 (as limit of detection) and 3.2 µg mL-1 (as limit of quantification). Applicability of the proposed method to the direct determination of total flavonoids as hesperidin equivalents in pharmaceutical formulation (Vitamin C with citrus bioflavonoids & Rose Hips) was demonstrated. Although the presence of ascorbic acid may cause problem in identification and measurements, hesperidin has been determined successfully.

Isocratic RP-HPLC method for rutin determination in solid oral dosage forms.

A rapid and sensitive assay for quantitative determination of rutin in oral dosage forms based on isocratic reversed phase high performance liquid chromatography (RP-HPLC) was developed and validated. Using a C(18) reverse-phase analytical column, the following conditions were chosen as optimal: mobile phase methanol-water 1:1 (v/v), pH 2.8 (adjusted with phosphoric acid), flow rate=1 mL min(-1) and temperature T=40.0 degrees C. Linearity was observed in the concentration range 8-120 microg mL(-1) with a correlation coefficient of 0.99982 and the limit of detection (LOD)=2.6 microg mL(-1), and limit of quantification (LOQ)=8.0 microg mL(-1). Intra- and inter-day precision were within acceptable limits. Robustness test indicated that the mobile phase composition and pH influence mainly the separation. The proposed method allowed direct determination of rutin in pharmaceutical dosage forms in the presence of excipients, but is not suitable for preparations where compounds structurally/chemically related to rutin may be present.