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.

Predicting the timing of first generation egg hatch for the pest redlegged earth mite Halotydeus destructor (Acari: Penthaleidae).

Integrated pest management in Australian winter grain crops is challenging, partly because the timing and severity of pest outbreaks cannot currently be predicted, and this often results in prophylactic applications of broad spectrum pesticides. We developed a simple model to predict the median emergence in autumn of pest populations of the redlegged earth mite, Halotydeus destructor, a major field crop and pasture pest in southern Australia. Previous data and observations suggest that rainfall and temperature are critical for post-diapause egg hatch. We evaluated seven models that combined rainfall and temperature thresholds derived using three approaches against previously recorded hatch dates and 2013 field records. The performance of the models varied between Western Australia and south-eastern Australian States. In Western Australia, the key attributes of the best fitting model were more than 5 mm rain followed by mean day temperatures of below 20.5 °C for 10 days. In south-eastern Australia, the most effective model involved a temperature threshold reduced to 16 °C. These regional differences may reflect adaptation of H. destructor in south-eastern Australia to varied and uncertain temperature and rainfall regimes of late summer and autumn, relative to the hot and dry Mediterranean-type climate in Western Australia. Field sampling in 2013 revealed a spread of early hatch dates in isolated patches of habitat, ahead of predicted paddock scale hatchings. These regional models should assist in monitoring and subsequent management of H. destructor at the paddock scale.