The mitogenome of Halotydeus destructor (Tucker) and its relationships with other trombidiform mites as inferred from nucleotide sequences and gene arrangements

The redlegged earth mite, Halotydeus destructor (Tucker, 1925: Trombidiformes, Eupodoidea, Penthaleidae), is an invasive mite species. In Australia, this mite has become a pest of winter pastures and grain crops. We report the complete mitogenome for H. destructor, the first to represent the family Penthaleidae, superfamily Eupodoidea. The mitogenome of H. destructor is 14,691 bp in size, and has a GC content of 27.87%, 13 protein-coding genes, two rRNA genes, and 22 tRNA genes. We explored evolutionary relationships of H. destructor with other members of the Trombidiformes using phylogenetic analyses of nucleotide sequences and the order of protein-coding and rRNA genes. We found strong, consistent support for the superfamily Tydeoidea being the sister taxon to the superfamily Eupodoidea based on nucleotide sequences and gene arrangements. Moreover, the gene arrangements of Eupodoidea and Tydeoidea are not only identical to each other but also identical to that of the hypothesized arthropod ancestor, showing a high level of conservatism in the mitogenomic structure of these mite superfamilies. Our study illustrates the utility of gene arrangements for providing complementary information to nucleotide sequences with respect to inferring the evolutionary relationships of species within the order Trombidiformes. The mitogenome of H. destructor provides a valuable resource for further population genetic studies of this important agricultural pest. Given the co-occurrence of closely related, morphologically similar Penthaleidae mites with H. destructor in the field, a complete mitogenome provides new opportunities to develop metabarcoding tools to study mite diversity in agro-ecosystems. Moreover, the H. destructor mitogenome fills an important taxonomic gap that will facilitate further study of trombidiform mite evolution.

Origin of resistance to pyrethroids in the redlegged earth mite (Halotydeus destructor) in Australia: repeated local evolution and migration

BACKGROUND

Halotydeus destructor is a major pest of crops and pastures across southern parts of Australia. This invasive mite has been chemically controlled for over 50 years, but resistance to synthetic pyrethroids and organophosphates is developing. Understanding processes behind the emerging resistance is important for effective management efforts. We undertook a ddRAD pool-sequencing approach to analyse genome-wide single nucleotide polymorphism variation in H. destructor population samples at two scales: local resistance across a set of fields, and regional resistance across their Australian range, along with toxicology bioassays to screen for pyrethroid resistance.

RESULTS

Spatial patterns of genomic variation and resistance at a local scale indicated that genetic similarity among samples was more closely correlated with distance along roads and fence-lines than with straight-line geographic distance. This pattern was particularly strong in resistant samples, which were also more related than susceptible samples, suggesting local spread of resistance within an area after it emerged. By contrast, regional data suggest resistance has emerged repeatedly within parts of Australia. Our de novo annotation of the H. destructor draft genome sequence and Bayesian analysis identified several candidate loci strongly associated with population-level resistance to pyrethroids, located in genomic regions that code for transmembrane transport and signalling proteins that have previously been linked to insecticide resistance in other arthropods.

CONCLUSION

Our findings highlight multiple independent evolutionary events leading to resistance in H. destructor, and demonstrate the utility and cost-effectiveness of a cross-population, genome-wide association study to reveal processes underlying adaptive evolution in a non-model invasive species. © 2019 Society of Chemical Industry.

A method to investigate neonicotinoid resistance in mites

The redlegged earth mite (Halotydeus destructor) is an important agricultural pest in Australia, with a wide range of plant hosts. Halotydeus destructor has developed resistance to pyrethroid and organophosphorus insecticides as a consequence of the widespread use of these chemicals by farmers. Neonicotinoids are one of the few remaining insecticide classes registered against H. destructor in which resistance has not been detected, although there have been occasional reports of control difficulties experienced in the field. There is currently no reliable way to accurately test the response of H. destructor (or indeed any mite species) to neonicotinoid insecticides. Here, we developed a new bioassay to assess the response of mites against the neonicotinoid imidacloprid. The method provided consistent results and showed no variation when used by different operators. We generated base-line sensitivity data for imidacloprid across a number of field-collected populations of H. destructor. This is important for future monitoring of mite responses given the considerable selection pressure now being exerted across large areas of the Australian farming landscape through the widespread use of neonicotinoid seed treatments.

A cryptic diapause strategy in Halotydeus destructor (Tucker) (Trombidiformes: Penthaleidae) induced by multiple cues

BACKGROUND

The polyphagous mite pest, Halotydeus destructor, typically has three generations during the cool moist season in Australia and produces over-summering diapause eggs in spring. Diapause eggs have a distinct thick and dark chorion and can survive heat, desiccation and the application of pesticides. Farmers suppress mites producing diapause eggs by a carefully timed spring pesticide application using Timerite^® , which predicts the onset of diapause egg production based largely on day length. We investigated the association between diapause induction and other environmental factors that may complicate diapause predictions.

RESULTS

Diapause in H. destructor induction was influenced by three interacting environmental factors, namely day length, temperature and soil moisture. A cryptic type of diapause egg that lacked a thick chorion and was instead morphologically similar to non-diapause eggs was also discovered. Like diapause eggs, this newly discovered egg type could also survive hot and dry summer conditions.

CONCLUSIONS

There is an opportunity to refine the Timerite^® spring spray by incorporating knowledge of other environmental factors inducing diapause in H. destructor. Compared with typical diapause eggs, the production of cryptic diapause eggs could reflect a strategy to deal with diversifying environmental stresses and/or represent a bet-hedging strategy to adapt to unpredictable environments. © 2018 Society of Chemical Industry.

Challenges in devising economic spray thresholds for a major pest of Australian canola, the redlegged earth mite (Halotydeus destructor)

BACKGROUND

A key component for spray decision-making in IPM programmes is the establishment of economic injury levels (EILs) and economic thresholds (ETs). We aimed to establish an EIL for the redlegged earth mite (Halotydeus destructor Tucker) on canola.

RESULTS

Complex interactions between mite numbers, feeding damage and plant recovery were found, highlighting the challenges in linking H. destructor numbers to yield. A guide of 10 mites plant(-1) was established at the first-true-leaf stage; however, simple relationships were not evident at other crop development stages, making it difficult to establish reliable EILs based on mite number. Yield was, however, strongly associated with plant damage and plant densities, reflecting the impact of mite feeding damage and indicating a plant-based alternative for establishing thresholds for H. destructor. Drawing on data from multiple field trials, we show that plant densities below 30-40 plants m(-2) could be used as a proxy for mite damage when reliable estimates of mite densities are not possible.

CONCLUSION

This plant-based threshold provides a practical tool that avoids the difficulties of accurately estimating mite densities. The approach may be applicable to other situations where production conditions are unpredictable and interactions between pests and plant hosts are complex.

Estimating Densities of the Pest Halotydeus destructor (Acari: Penthaleidae) in Canola

Development of sampling techniques to effectively estimate invertebrate densities in the field is essential for effective implementation of pest control programs, particularly when making informed spray decisions around economic thresholds. In this article, we investigated the influence of several factors to devise a sampling strategy to estimate Halotydeus destructor Tucker densities in a canola paddock. Direct visual counts were found to be the most suitable approach for estimating mite numbers, with higher densities detected than the vacuum sampling method. Visual assessments were impacted by the operator, sampling date, and time of day. However, with the exception of operator (more experienced operator detected higher numbers of mites), no obvious trends were detected. No patterns were found between H. destructor numbers and ambient temperature, relative humidity, wind speed, cloud cover, or soil surface conditions, indicating that these factors may not be of high importance when sampling mites during autumn and winter months. We show further support for an aggregated distribution of H. destructor within paddocks, indicating that a stratified random sampling program is likely to be most appropriate. Together, these findings provide important guidelines for Australian growers around the ability to effectively and accurately estimate H. destructor densities.