The Effect of Temperature, Relative Humidity, and Virus Infection Status on off-host Survival of the Wheat Curl Mite (Acari: Eriophyidae)

Removing grass clippings reduces bermudagrass mite (Acari: Eriophyidae) infestation during turfgrass regrowth

Bermudagrass mite (Aceria cynodoniensis Sayed) infestation stunts bermudagrass (Cynodon spp. [Poales: Poaceae]) growth, leading to thinned turf and lower aesthetic and recreational value. Bermudagrass mites cause characteristic symptoms called witch's brooms, including shortened internodes and leaves and the proliferation of tillers. Grass clippings produced by mowing or scalping bermudagrass harbor mites, which abandon the desiccating grass clippings and spread to surrounding turfgrass. Dropped grass clippings can lead to infestation of new turfgrass. Nursery experiments were conducted with potted bermudagrass to determine the effect of removing witch's brooms or grass clippings after scalping on witch's broom densities on the recovering bermudagrass. Additionally, laboratory experiments were conducted to assess the potential for mites to abandon detached witch's brooms and to evaluate mite survival after leaving their hosts. The number of initial witch's brooms and individually removing witch's brooms did not affect subsequent witch's broom densities, suggesting that infested but asymptomatic terminals later developed into witch's brooms. Removing grass clippings after scalping reduced witch's broom densities by over 65% in two trials. Most mites (96%) abandoned witch's brooms within 48 h after detaching witch's brooms, and adult mites survived an average of 5.6 h after removal from the host plant. Removing clippings after scalping may improve bermudagrass mite management and limit damage on the recovering turfgrass. Additionally, clippings resulting from regular mowing or scalping should be disposed of properly because this study demonstrates that mites abandon desiccating host plants and survive sufficiently long to infest surrounding turfgrass.

Influence of temperature on the population size of Aceria litchii (Acari: Eriophyidae) and the development of its galls

The lychee erinose mite, Aceria litchii (Keifer), is a tiny eriophyid mite known to induce the formation of open galls (erinea) on lychee plants, Litchi chinensis Sonn. In lychee infested by A. litchii, four stages of erineum are observed, based on erineum color: hyaline, white, amber and dark brown. The cause of the color change in erinea is unknown, but it might be linked to the extent of A. litchii infestation. Artificially infested lychee seedlings were used to determine the developmental time, trichome density, and mite population size in each of the four erineum stages. The effect of temperature (15, 25, and 35 °C) on A. litchii population size and erineum developmental time was also investigated. Overall, each erineum stage lasted approximately 50 days and the population size of A. litchii increased gradually through the hyaline, white and amber stages, and a strong decline was observed in dark brown erinea. Visual comparisons of the low-temperature scanning electron microscope (LT-SEM) images revealed that the trichome density was lower in the hyaline stage than in the other stages. Temperature influenced mite population size and amber erinea developmental time. Mite population was lower at 35 °C than at 15 and 25 °C, suggesting that high temperatures promote detrimental effects on A. litchii. Amber erinea developed slower at 15 °C than at 25 and 35 °C. Possible effects of the temperature on A. litchii population in lychee plants are discussed.

Wheat transcriptomic responses to extended feeding by wheat curl mites

The economic importance of wheat and its contribution to human and livestock diets has been already demonstrated. However, wheat production is impacted by pests that induce yield reductions. Among these pests, wheat curl mite (WCM, Aceria tosichella Keifer) impacts wheat all around the world. WCM are tiny pests that feed within the whorl of developing leaves, and their feeding causes leaf curling by preventing them from unfurling. The curling of the leaves provides a protective niche for the WCM. Additionally, WCM are also the vector of serious viruses in wheat. Little is known regarding the impact of the WCM on wheat transcriptome, and to date, only one article has been published describing the wheat transcriptomic changes after 1 day of WCM feeding. To better understand the wheat transcriptome variation after extended feeding by WCM [10 days post infestation (dpi)], we used an RNA-seq approach. We collected WCM-infested and uninfested leaves from two wheat cultivars: Byrd (WCM resistant) and Settler CL (WCM susceptible) at 10 dpi. Our transcriptomic analysis revealed the common and specific transcriptomic variations in WCM resistant and susceptible wheat cultivars, chromosome 3D specific location of the differentially expressed genes with functions involved in defense and stress response, and also identified the gene functions related to lipid signaling and membrane integrity, and phytohormone pathways potentially contributing to WCM resistance. Collectively, our study provides important insights on wheat defense mechanisms against WCM after extended feeding.

Hitchhiking or hang gliding? Dispersal strategies of two cereal-feeding eriophyoid mite species

Dispersal shapes the dynamics of populations, their genetic structure and species distribution; therefore, knowledge of an organisms' dispersal abilities is crucial, especially in economically important and invasive species. In this study, we investigated dispersal strategies of two phytophagous eriophyoid mite species: Aceria tosichella (wheat curl mite, WCM) and Abacarus hystrix (cereal rust mite, CRM). Both species are obligatory plant parasites that infest cereals and are of economic significance. We investigated their dispersal success using different dispersal agents: wind and vectors. We hypothesised that in both mite species the main mode of dispersal is moving via wind, whereas phoretic dispersal is rather accidental, as the majority of eriophyoid mite species do not possess clear morphological or behavioural adaptations for phoresy. Results confirmed our predictions that both species dispersed mainly with wind currents. Additionally, WCM was found to have a higher dispersal success than CRM. Thus, this study contributes to our understanding of the high invasive potential of WCM.

High Plains wheat mosaic virus: An enigmatic disease of wheat and corn causing the High Plains disease

BRIEF HISTORY

In 1993, severe mosaic and necrosis symptoms were observed on corn (maize) and wheat from several Great Plains states of the USA. Based on the geographical location of infections, the disease was named High Plains disease and the causal agent was tentatively named High Plains virus. Subsequently, researchers renamed this virus as maize red stripe virus and wheat mosaic virus to represent the host and symptom phenotype of the virus. After sequencing the genome of the pathogen, the causal agent of High Plains disease was officially named as High Plains wheat mosaic virus. Hence, High Plains virus, maize red stripe virus, wheat mosaic virus, and High Plains wheat mosaic virus (HPWMoV) are synonyms for the causal agent of High Plains disease.

TAXONOMY

High Plains wheat mosaic virus is one of the 21 definitive species in the genus Emaravirus in the family Fimoviridae.

VIRION

The genomic RNAs are encapsidated in thread-like nucleocapsids in double-membrane 80-200 nm spherical or ovoid virions.

GENOME CHARACTERIZATION

The HPWMoV genome consists of eight single-stranded negative-sense RNA segments encoding a single open reading frame (ORF) in each genomic RNA segment. RNA 1 is 6,981-nucleotide (nt) long, coding for a 2,272 amino acid protein of RNA-dependent RNA polymerase. RNA 2 is 2,211-nt long and codes for a 667 amino acid glycoprotein precursor. RNA 3 has two variants of 1,439- and 1,441-nt length that code for 286 and 289 amino acid nucleocapsid proteins, respectively. RNA 4 is 1,682-nt long, coding for a 364 amino acid protein. RNA 5 and RNA 6 are 1,715- and 1,752-nt long, respectively, and code for 478 and 492 amino acid proteins, respectively. RNA 7 and RNA 8 are 1,434- and 1,339-nt long, code for 305 and 176 amino acid proteins, respectively.

BIOLOGICAL PROPERTIES

HPWMoV can infect wheat, corn (maize), barley, rye brome, oat, rye, green foxtail, yellow foxtail, and foxtail barley. HPWMoV is transmitted by the wheat curl mite and through corn seed.

DISEASE MANAGEMENT

Genetic resistance against HPWMoV in wheat is not available, but most commercial corn hybrids are resistant while sweet corn varieties remain susceptible. Even though corn hybrids are resistant to virus, it still serves as a green bridge host that enables mites to carry the virus from corn to new crop wheat in the autumn. The main management strategy for High Plains disease in wheat relies on the management of green bridge hosts. Cultural practices such as avoiding early planting can be used to avoid mite buildup and virus infections.

Temperature-dependent development and survival of an invasive genotype of wheat curl mite, Aceria tosichella

Quantifying basic biological data, such as the effects of variable temperatures on development and survival, is crucial to predicting and monitoring population growth rates of pest species, many of which are highly invasive. One of the most globally important pests of cereals is the eriophyoid wheat curl mite (WCM), Aceria tosichella, which is the primary vector of several plant viruses. The aim of this study was to evaluate temperature-dependent development and survival of WCM at a wide range of constant temperatures in the laboratory (17-33 °C). The development time of each stage depended significantly on temperature and it was negatively correlated with temperature increase. At high temperatures (27-33 °C), individuals had shorter developmental times, with the shortest (6 days) at 33 °C, whereas at the lowest tested temperatures (17-19 °C), developmental time was almost 3× longer. Moreover, temperature had a clear effect on survival: the higher the temperature, the lower the survival rate. These data provide information promoting more efficient and effective manipulation of WCM laboratory colonies, and further our understanding of the ramifications of temperature change on WCM physiology and implications for the growth and spread of this globally invasive pest.

Off-host survival of Eriophyoidea and remarks on their dispersal modes

Dispersal of eriophyoid mites is crucial for the successful colonization of new plants. Literature suggests that their long-distance dispersal is through aerial transfer. During dispersal, eriophyoids might be captured in vapor or fine drops of water (perhaps most likely in clouds) where they might be protected against water loss and desiccation, but where they would have no food and be exposed to low temperatures and oxygen concentrations. Considerable resistance of these mites to these stressful environmental conditions is expected and has only partly been confirmed experimentally. The aim of the bioassays conducted here was to assess the survival of five eriophyoid species off their host plants, with poor oxygen availability under two temperature regimes. The bioassays were carried out on live mites dipped into two media used as microenvironments: (1) vaseline oil (used also as control treatment), and (2) water solution of Tween 80 (0.2%) and cycloheximide (50 mg/l). The bioassays were performed at 5 ± 1 and 25 ± 1 °C. The survival of mites was assessed weekly (5 °C) or daily (25 °C) by counting live and active specimens. The following species were subjected to the bioassays: Aceria caulobia (a stem gall mite), Aceria ficus (a vagrant mite), Cecidophyopsis hendersoni (a vagrant mite), Colomerus vitis (protogyne/male population and deutogyne morphs; a leaf gall mite) and Phytoptus avellanae (a bud gall mite). The survival rate of the mites was higher at 5 °C than at 25 °C under both experimental conditions. At 5 °C, the survival of almost all species was higher in the water solution (up to 6-7 weeks) than in vaseline oil (3-5 weeks). Longer survival was found for A. caulobia and P. avellanae (gall-making species) than for C. hendersoni and A. ficus (vagrant species). As expected, the deutogynes of C. vitis survived longer than its protogynes. The current results suggest that individuals of some of the tested species are well suited for withstanding cold, starvation and low oxygen rates, which could be found at higher atmospheric layers, within the clouds, allowing them an effective long-distance dispersal.

Differential Spatial Gradients of Wheat Streak Mosaic Virus into Winter Wheat from a Central Mite-Virus Source

The wheat curl mite (WCM), Aceria tosichella Keifer, transmits three potentially devastating viruses to winter wheat. An increased understanding of mite movement and subsequent virus spread through the landscape is necessary to estimate the risk of epidemics by the virus in winter wheat. Owing to the small size of WCMs, their dispersal via wind is hard to monitor; however, the viruses they transmit produce symptoms that can be detected with remote sensing. The objective of this study was to characterize the spatial dispersal of the virus from a central mite-virus source. Virus infection gradients were measured spatially by using aerial remote sensing, ground measurements, geostatistics, and a geographic information system between 2006 and 2009. The red edge position vegetation index as measured via aerial imagery was significantly correlated with in-field biophysical measurements. The occurrence of virus symptoms extended differentially in all directions from mite-virus source plots, and predictions from cokriging revealed an oval pattern surrounding the source but displaced to the southeast. The variable dispersal in different directions appeared to be influenced by the mite source density and wind direction and speed, but temperature also seemed likely to have affected mite spread. The spatial spread revealed in this study may be used to estimate the potential sphere of influence of mite-infested volunteer wheat in production fields. These risk parameter estimates require further validation, but they may potentially aid growers in making better virus management decisions regarding differential virus spread potential away from a central source.

An Intimate Relationship Between Eriophyoid Mites and Their Host Plants - A Review

Eriophyoid mites (Acari Eriophyoidea) are phytophagous arthropods forming intimate relationships with their host plants. These mites are associated with annual and perennial plants including ferns, and are highly specialized with a dominant monophagy. They can be classified in different ecological classes, i.e., vagrant, gall-making and refuge-seeking species. Many of them are major pests and some of them are vectors of plant pathogens. This paper critically reviews the knowledge on eriophyoids of agricultural importance with emphasis on sources for host plant resistance to these mites. The role of species belonging to the family Eriophyidae as vectors of plant viruses is discussed. Eriophyoid-host plant interactions, the susceptibility within selected crops and main host plant tolerance/resistance mechanisms are discussed. Fundamental concepts, subjects, and problems emerged in this review are pointed out and studies are suggested to clarify some controversial points.

Temperature and Alternative Hosts Influence Aceria tosichella Infestation and Wheat Streak Mosaic Virus Infection

Wheat streak mosaic, caused by Wheat streak mosaic virus (WSMV; family Potyviridae), is the most important and common viral disease of wheat (Triticum aestivum L.) in the Great Plains of North America. WSMV is transmitted by the wheat curl mite (WCM; Aceria tosichella). We evaluated how mean daily temperatures, cumulative growing degree-days, day of the year, and surrounding alternative host identity affected WCM infestation and WSMV infection of wheat from late summer through early autumn in Montana, United States. Cumulative growing degree-days, warm mean daily temperatures (i.e., >10°C), and surrounding alternative hosts interacted to alter risk of WCM infestation and WSMV infection. Wheat surrounded by Bromus tectorum L. and preharvest volunteer wheat had WCM infestation and WSMV infection rates of 88% in years when the mean daily temperature was 15°C in October, compared with 23% when surrounded by bare ground, and <1% when the temperature was 0°C regardless of surrounding alternative host. Mean daily temperatures in the cereal-growing regions of Montana during autumn are marginally conducive to WCM population growth and movement. As the region continues to warm, the period of WCM movement will become longer, potentially increasing the frequency of WSMV outbreaks.

Meteorological factors affect the epidemiology of hemorrhagic fever with renal syndrome via altering the breeding and hantavirus-carrying states of rodents and mites: a 9 years' longitudinal study

The incidence of hemorrhagic fever with renal syndrome (HFRS) in Qingdao, China was three times higher than that of the average national level. Here we characterized the epidemiology, ecological determinants and pathogen evolution of HFRS in Qingdao during 2007–2015. In this longitudinal study, a total of 1846 HFRS patients and 41 HFRS-related deaths were reported. HFRS in Qingdao peaked once a year in the fourth quarter. We built a time series generalized additive model, and found that meteorological factors in the previous quarter could accurately predict HFRS occurrence. To explore how meteorological factors influenced the epidemic of HFRS, we analyzed the relationship between meteorological factors and hantavirus-carrying states of the hosts (including rodents and shrews). Comprehensive analysis showed humidity was correlated to high host densities in the third quarter and high hantavirus-carrying rates of animal hosts in the third to fourth quarters, which might contribute to HFRS peak in the fourth quarter. We further compared the L segments of hantaviruses from HFRS patients, animal hosts and ectoparasites. Phylogenetic analysis showed that hantaviruses in gamasid and trombiculid mites were the same as those from the hosts. This indicated mites also contributed to the transmission of hantavirus. Furthermore, Hantaan virus from HFRS patients, hosts and mites in Qingdao formed a distinct phylogenetic cluster. A new clade of Seoul virus was also identified in the hosts. Overall, meteorological factors increase HFRS incidence possibly via facilitating hosts’ reproduction and consequent mite-mediated hantavirus transmission. New hantavirus subtypes evolved in Qingdao represent new challenges of fighting against HFRS.

Population Dynamics of the Wheat Curl Mite (Acari: Eriophyidae) During the Heading Stages of Winter Wheat

The wheat curl mite (Aceria tosichella Keifer) is the only known vector of three viruses in wheat-Wheat streak mosaic virus, Wheat mosaic virus, and Triticum mosaic virus. The economic impact of this disease complex is linked to the presence of suitable hosts prior to winter wheat maturing in early summer and the movement of wheat curl mite from wheat to oversummering hosts prior to wheat harvest. Previous research has documented the prevalence and density of mite populations on maturing wheat heads; however, these studies were limited to a few late stages of wheat. A study was conducted to evaluate mite population densities across all stages of head development to determine when wheat curl mites are most abundant and the relative increase in abundance over time. In addition, a study was conducted to evaluate the impact of rainfall on mite populations during wheat heading. A final study was conducted to determine the potential for direct infestation of seedlings germinating from wheat curl mite-infested wheat heads. Results showed a rapid buildup in mite populations from low densities in early heading and peaking at the hard dough stage, with nearly all wheat heads having some mite presence. In addition, high mite populations resulted in direct infestation of germinated seedlings from the early through hard dough stages. Rainfall applications had no observable impact on mite population densities in wheat heads. These results demonstrate the increased potential for mites to infest hosts prior to winter wheat maturing and illustrate the increased risk for these hosts to serve as oversummering hosts.

Spatial and Host-Related Variation in Prevalence and Population Density of Wheat Curl Mite (Aceria tosichella) Cryptic Genotypes in Agricultural Landscapes

The wheat curl mite (WCM), Aceria tosichella Keifer, is a major pest of cereals worldwide that also comprises a complex of at least 16 genetic lineages with divergent physiological traits, including host associations and specificity. The goal of this study was to test the extent to which host-plant species and landscape spatial variation influence WCM presence and population density across the entire area of Poland (>311,000 km2). Three important findings arose from the results of the study. (1) The majority of WCM lineages analyzed exhibited variation in patterns of prevalence and/or population density on both spatial and host-associated scales. (2) Areas of occurrence and local abundance were delineated for specific WCM lineages and it was determined that the most pestiferous lineages are much less widespread than was expected, suggesting relatively recent introductions into Poland and the potential for further spread. (3) The 16 WCM lineages under study assorted within four discrete host assemblages, within which similar host preferences and host infestation patterns were detected. Of these four groups, one consists of lineages associated with cereals. In addition to improving basic ecological knowledge of a widespread arthropod herbivore, the results of this research identify high-risk areas for the presence of the most pestiferous WCM lineages in the study area (viz. the entirety of Poland). They also provide insight into the evolution of pest species of domesticated crops and facilitate testing of fundamental hypotheses about the ecological factors that shape this pest community.

Thermal Niches of Two Invasive Genotypes of the Wheat Curl Mite Aceria tosichella: Congruence between Physiological and Geographical Distribution Data

The wheat curl mite (WCM), Aceria tosichella Keifer, is a major pest of cereals worldwide. It is also a complex of well-defined genetic lineages with divergent physiological traits, which has not been accounted for in applied contexts. The aims of the study were to model the thermal niches of the two most pestiferous WCM lineages, designated MT-1 and MT-8, and to assess the extent to which temperature determines the distribution of these lineages. WCM population dynamics were modeled based on thermal niche data from March to November on the area of Poland (>311,000 km²). The most suitable regions for population development were predicted and compared to empirical field abundance data. Congruence between modeled parameters and field data for mite presence were observed for both WCM lineages although congruence between modeled thermal suitability and mite field abundance was observed only for MT-8. Thermal niche data for MT-1 and MT-8 provide biological insights and aid monitoring and management of WCM and the plant viruses it vectors. The presented models accurately estimate distributions of WCM and can be incorporated into management strategies for both current and predicted climate scenarios.

Future Scenarios for Plant Virus Pathogens as Climate Change Progresses

Knowledge of how climate change is likely to influence future virus disease epidemics in cultivated plants and natural vegetation is of great importance to both global food security and natural ecosystems. However, obtaining such knowledge is hampered by the complex effects of climate alterations on the behavior of diverse types of vectors and the ease by which previously unknown viruses can emerge. A review written in 2011 provided a comprehensive analysis of available data on the effects of climate change on virus disease epidemics worldwide. This review summarizes its findings and those of two earlier climate change reviews and focuses on describing research published on the subject since 2011. It describes the likely effects of the full range of direct and indirect climate change parameters on hosts, viruses and vectors, virus control prospects, and the many information gaps and deficiencies. Recently, there has been encouraging progress in understanding the likely effects of some climate change parameters, especially over the effects of elevated CO2, temperature, and rainfall-related parameters, upon a small number of important plant viruses and several key insect vectors, especially aphids. However, much more research needs to be done to prepare for an era of (i) increasingly severe virus epidemics and (ii) increasing difficulties in controlling them, so as to mitigate their detrimental effects on future global food security and plant biodiversity.