The pest status and distribution of three cryptic blue oat mite species (Penthaleus spp.) and redlegged earth mite (Halotydeus destructor) in southeastern Australia

Population Genetic Diversity of Two Blue Oat Mite Species on Triticum Hosts in China

Blue oat mite species, including Penthaleus major and P. tectus, are pests widely distributed across China that cause damage to winter wheat. This study evaluated the genetic diversity of P. major and P. tectus on Triticum hosts collected from 23 geographic locations based on mitochondrial cytochrome c oxidase subunit I (COI) sequences. We identified nine haplotypes in 438 P. major individuals from 21 geographic locations and five haplotypes in 139 P. tectus individuals from 11 geographic locations. Meanwhile, P. major exhibits high values of haplotype diversity (Hd) and nucleotide diversity (Pi) (Hd = 0.534 > 0.5 and Pi = 0.012 > 0.005), representing a large stable population with a long evolutionary history. P. tectus shows low values of Hd and Pi (Hd = 0.112 < 0.5 and Pi = 0 < 0.005), which suggest recent founder events. Moreover, demographic analysis suggested that P. major and P. tectus have not undergone a recent population expansion. The lowest genetic variation was observed in Xiangzhou (XZ-HB), Zaoyang (ZY-HB), Siyang (SY-JS), and Rongxian (RX-SC), with only one species and one haplotype identified in over 30 individuals. Robust genetic differentiation was found in P. major compared to P. tectus, which provides a theoretical basis for the widespread distribution of P. major in China.

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.

Field associations of first generation densities of the pest mites Halotydeus destructor and Penthaleus major in pasture

Halotydeus destructor and Penthaleus major are species of earth mite commonly found at high densities in agricultural fields in Australia and other parts of the world. These mites pose a risk to a range of winter crops and pastures when seedlings emerge in autumn. In order to predict likely mite pressure, we investigated whether autumn densities in pastures can be determined from agronomic and environmental field variables. For H. destructor, field densities showed little association with a range of vegetation variables but could largely be explained using the variable field type, with high densities present when fields had mixtures of grass, clover and weeds. For P. major, we found a regional effect. In the region where most data were available, P. major field densities were associated with grass abundance, whereas an association with field type was significant but different to that found for H. destructor. For both species, densities were not associated with rainfall, but there was a weak association with soil moisture capacity. We discuss how these results can help in managing these important pest mites.

Influence of previous host plants on the reproductive success of a polyphagous mite pest, Halotydeus destructor (Trombidiformes: Penthaleidae)

In the evolution of phytophagous arthropods, adaptation to a single type of host plant is generally assumed to lead to a reduction in fitness on other host plant types, resulting in increasing host specialization. While this process is normally considered to be genetically based, short-term effects acting within one generation (plasticity) or across two generations (cross-generation variation) could also play a role. Here, we test these effects in the redlegged earth mite, Halotydeus destructor (Tucker) (Prostigmata: Penthaleidae), a major agricultural pest of multiple crop plants. Field populations of mites were collected from grasses, legumes, and broad-leaf weeds and placed into enclosures with different plant types. The survival, net reproductive output (Ro), and feeding damage of each mite population were assessed across two generations. The interaction between the origin of mites and plant type had a significant effect on parental survival, Ro, offspring development, and feeding damage. Mites collected from legumes showed higher parental survival on all host types; however, Ro, offspring development and feeding damage were all higher when mites were placed onto the same plant type from which they were collected. These patterns point to the ability of H. destructor to perform well on host plants even in the absence of genetically differentiated host races, but also the likelihood of performance trade-offs when populations are forced to rapidly change hosts within and across sequential generations.

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.

Development of "best practices" for sampling of an important surface-dwelling soil mite in pastoral landscapes

In this study, we analyzed 1145 vacuum samples of redlegged earth mites (RLEM) [Halotydeus destructor (Tucker) (Acari: Penthaleidae)] from 18 sampling events at six locations in pastoral landscapes of Western Australia during three growing seasons (2012-2014) (total of 228,299 RLEM individuals). The specific objectives were to determine: (1) presence/absence effects of a range of vegetation characteristics, (2) possible factors influencing RLEM sampling performance during the course of the season and day, (3) effects of size of area sampled and duration of sampling, (4) the spatial structure of RLEM counts in uniform pastoral vegetation, and (5) develop "best practices" regarding field-based vacuum sampling of surface dwelling soil mites in pastoral landscapes. We found that sampling of completely bare ground will lead to very low RLEM counts but spots with sparse vegetation (presence of bare ground) probably increases the presence of microhabitats for mites to shelter in and therefore lead to higher RLEM counts. RLEM counts were positively associated with the height of vegetation, at least up to about 15 cm in height. In early season (May-August), highest RLEM counts will be obtained in the afternoon hours (2-4 pm), whereas in late season sampling (August-November), highest RLEM counts will be obtained around noon. Higher RLEM counts should be expected from spots with grazed/mowed vegetation including cape weed and without presence of grasses and stubble. Variogram analyses of high-resolution data sets suggested that considerable range of spatial autocorrelation should be expected from fields with fairly uniform vegetation, especially if RLEM population densities are high. We are therefore recommending that samples are collected at least 30 m apart, if the objective is to obtain independent (spatially non-correlated) counts. The results from this study may be used to develop effective sampling protocols deployed in field ecology studies of soil surface dwelling mesofauna in pastoral landscapes and other ecosystems.

Impact of Halotydeus destructor on crop seedlings at different plant developmental stages and levels of moisture stress

Tolerance and compensatory ability after invertebrate damage are important components of plant defense and are affected by various endogenous and exogenous factors. Here, we examine the impact of feeding damage caused by different densities of the redlegged earth mite (Halotydeus destructor Tucker) on the performance and recovery of canola and wheat seedlings at different plant developmental stages, and when grown under three levels of moisture stress. Both canola and wheat were susceptible to feeding damage caused by H. destructor at early growth development stages (canola-cotyledon, first and second true leaf stage; wheat-GS10 and GS12), but tolerated damage at the later growth stages (canola-third true leaf stage; wheat-GS14). Wheat plants compensated completely for mite feeding damage, while canola plants showed partial compensation when feeding occurred at the cotyledon and first true leaf stages. Canola was susceptible to immediate feeding damage from H. destructor under all levels of moisture stress and did not compensate for this damage over time when grown in the unstressed and moderately stressed treatments. Under severe moisture stress conditions, plant performance traits and mite density often showed a nonlinear relationship, with growth initially increasing as mite density increased, but then decreasing markedly as densities exceeded 30 mites/100 cm(2). Wheat seedlings were susceptible to immediate feeding damage when grown in the unstressed and severe stress conditions, but were generally able to compensate for this damage regardless of moisture conditions. The implications of these results for the management of H. destructor are discussed.

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.

Survival and reproduction of the pest mites Balaustium medicagoense and Bryobia spp. on winter grain crops

Balaustium medicagoense and Bryobia spp. have recently been identified as emerging pests of winter crops and pastures in Australia. These mites have a high natural tolerance to currently registered pesticides, highlighting the need to develop alternative control strategies such as cultural controls which require an understanding of plant associations. In shade-house experiments, Bryobia spp. survived and reproduced successfully on pasture, lupins and oats, but progeny failed to reach the adult stage on canola and wheat. Balaustium medicagoense progeny failed to produce a generation on any crop but parental adults survived a few months on all crops, particularly wheat. Bryobia spp. damaged canola, pasture and lupins, but caused minimal damage to oats and wheat, whereas Ba. medicagoense caused considerable damage to wheat and lupins, but only moderate damage to canola, oats and pasture. Field survey data, taken from approximately 450 sites across southern Australia, combined with analysis of historical pest reports, suggest broadleaf crops such as canola, lucerne, lupins and weeds appear particularly susceptible to attack by Bryobia species. Balaustium medicagoense was more commonly found on cereals and grasses, although they also attacked broadleaf crops, particularly canola, lucerne and lupins. These findings show that the mites have the potential to be an important pest on several winter grain crops and pasture, but there are important differences that can assist in management strategies such as targeted crop rotations.

Pests of germinating grain crops in southern Australia: an overview of their biology and management options

Grain crops in southern Australia are subject to attack by countless pests, with greater than 40 invertebrate species threatening seedling establishment. Control tactics for crop establishment pests rely heavily on the application of pesticides, especially in canola, which is the most susceptible crop to invertebrate damage. There is genuine interest in integrated pest management (IPM) among growers, but relatively little adoption of classical IPM in broadacre farming in southern Australia. The driving forces behind the lack of adoption are unknown, although over-reliance on broad-spectrum pesticides – which are inexpensive and often applied prophylactically as a means of negating the need to monitor crops – is undoubtedly a key factor. Recent control failures against important pests due to pesticide resistance, increased restrictions on pesticide applications, environmental concerns about pesticide applications and strong support for grain quality assurance programs by exporters, highlight the need to consider IPM principles as a means of reducing chemical inputs. IPM guidelines for broadacre farming systems are limited in scope and there is a need to develop practical management tools that encompass a whole system approach. This paper provides an overview of the main invertebrate pests affecting crop establishment and identifies gaps hindering the wide-scale adoption of IPM.

Strategies for control of the redlegged earth mite in Australia

The redlegged earth mite, Halotydeus destructor, continues to be an intractable pest causing damage to most crop and pasture species in southern Australia. H. destructor feed on all stages of plants, but particularly damage seedlings in autumn. Research has aimed to develop new controls based on a better understanding of the biology and ecology of this pest. Chemicals remain the key tool to control H. destructor, despite the recent appearance of resistance to synthetic pyrethroids. A control package, Timerite, has been developed by which a single well-timed spray in spring can prevent H. destructor from developing diapause eggs. Field trials show this strategy provides effective control of H. destructor the following autumn, and protects plant seedlings, although mite populations build up again during winter. Non-chemical control strategies include grazing, the use of tolerant plants such as cereals, resistant legume cultivars and avoiding rotations where favourable host plants are available in the year before growing susceptible crops such as canola. Natural enemies can assist in mite control, and their numbers can be enhanced by methods including increasing landscape features like shelterbelts. Interspecific competition can occur between H. destructor and other pest mites, but the extent to which these interactions influence the structure of pest communities under different management regimes remains to be investigated.

Emerging pest mites of grains (Balaustium medicagoense and Bryobia sp.) show high levels of tolerance to currently registered pesticides

Balaustium medicagoense and Bryobia sp. (clover or pasture mite) have recently been identified as potential emerging pests of crops and pastures within southern Australia. Recorded damage by these mites has markedly increased in the past decade. There is limited information on the pesticide tolerance of these mites relative to other earth mite pests. This study examined the response of Ba. medicagoense and Bryobia sp., using the redlegged earth mite [Halotydeus destructor (Tucker)] as a comparison, to several currently registered pesticides against earth mites (omethoate, bifenthrin, chlorpyrifos, methidathion and α-cypermethrin). Ba. medicagoense had a much greater level of tolerance to all pesticides tested than H. destructor. Similarly, Bryobia sp. had a higher level of tolerance to bifenthrin, methidathion and α-cypermethrin than H. destructor. However, in the case of omethoate and chlorpyrifos, the tolerance levels were similar for Bryobia sp. and H. destructor. Ba. medicagoense had a higher level of tolerance than Bryobia sp. to the organophosphates tested (omethoate, chlorpyrifos and methidathion), but there was no difference for bifenthrin. We were unable to compare tolerance levels between Ba. medicagoense and Bryobia sp. to α-cypermethrin because of inconsistencies between replicate tests. These emerging pest mite species, therefore, have a high natural tolerance to currently registered pesticides and may prove difficult to control in the field. These findings suggest that other strategies that are not reliant on chemicals should be considered for the control of Ba. medicagoense and Bryobia sp.

The changing status of invertebrate pests and the future of pest management in the Australian grains industry

The Australian grains industry is dealing with a shifting complex of invertebrate pests due to evolving management practices and climate change as indicated by an assessment of pest reports over the last 20–30 years. A comparison of pest outbreak reports from the early 1980s to 2006–07 from south-eastern Australia highlights a decrease in the importance of pea weevils and armyworms, while the lucerne flea, Balaustium mites, blue oat mites and Bryobia mites have increased in prominence. In Western Australia, where detailed outbreak records are available from the mid 1990s, the relative incidence of armyworms, aphids and vegetable weevils has recently decreased, while the incidence of pasture cockchafers, Balaustium mites, blue oat mites, redlegged earth mites, the lucerne flea and snails has increased. These changes are the result of several possible drivers. Patterns of pesticide use, farm management responses and changing cropping patterns are likely to have contributed to these shifts. Drier conditions, exacerbated by climate change, have potentially reduced the build-up of migratory species from inland Australia and increased the adoption rate of minimum and no-tillage systems in order to retain soil moisture. The latter has been accompanied by increased pesticide use, accelerating selection pressures for resistance. Other control options will become available once there is an understanding of interactions between pests and beneficial species within a landscape context and a wider choice of ‘softer’ chemicals. Future climate change will directly and indirectly influence pest distributions and outbreaks as well as the potential effectiveness of endemic natural enemies. Genetically modified crops provide new options for control but also present challenges as new pest species are likely to emerge.

Pyrethroid resistance discovered in a major agricultural pest in southern Australia: the redlegged earth mite Halotydeus destructor (Acari: Penthaleidae)

BACKGROUND

The redlegged earth mite (Halotydeus destructor Tucker) is an important pest of field crops and pastures. Control of this pest relies heavily on chemicals, with few genuine alternatives presently available. Pesticide responses of H. destructor from the field with reported chemical control failures were compared with mites from susceptible 'control' populations. Toxicology bioassays were conducted on adult mites across multiple generations.

RESULTS

Very high levels of resistance to two synthetic pyrethroids, bifenthrin and alpha-cypermethrin, were detected in this species for the first time. For bifenthrin, LC(50) estimates showed a difference in resistance of greater than 240 000-fold. Resistance to alpha-cypermethrin was almost 60 000-fold. This resistance was shown to be heritable, persisting after several generations of culturing. There was no evidence that resistance to organophosphorus chemicals had evolved, which is likely to be a direct consequence of the history of chemical applications these mites have experienced.

CONCLUSION

These results highlight the need for more judicious management decisions in order to control pest species in a sustainable manner. The implications of these findings in regard to the management and future research of the redlegged earth mite are discussed.

Biology, ecology and control of the Penthaleus species complex (Acari: Penthaleidae)

Blue oat mites, Penthaleus spp. (Acari: Penthaleidae), are major agricultural pests in southern Australia and other parts of the world, attacking various pasture, vegetable and crop plants. Management of these mites has been complicated by the recent discovery of three cryptic pest species of Penthaleus, whereas prior research had assumed a single species. The taxonomy, population genetics, ecology, biology and control of the Penthaleus spp. complex are reviewed. Adult Penthaleus have a dark blue-black body approximately 1 mm in length, and eight red-orange legs. Within Australia, they are winter pests completing two or three generations a season, depending on conditions. The summer is passed as diapausing eggs, when long-distance dispersal is thought to occur. The Penthaleus spp. reproduce by thelytokous parthenogenesis, with populations comprising clones that differ ecologically. The three pest Penthaleus spp. differ markedly in their distributions, plant hosts, timing of diapause egg production and response to pesticides, highlighting the need to develop control strategies that consider each species separately. Chemicals are the main weapons used in current control programs, however research continues into alternative more sustainable management options. Host plant resistance, crop rotations, conservation of natural enemies, and improved timing of pesticide application would improve the management of these pests. The most cost-effective and environmentally acceptable means of control will result from the integration of these practices combined with the development of a simple field-based kit to distinguish the different mite species.