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Wheat Curl Mite: A New Source of the Eriophyoid Mite in Wheat Fields Identified

    Affiliations
    Authors and Affiliations
    • Ken Obasa1
    • Olufemi J. Alabi2
    • Mamoudou Sétamou3
    1. 1Department of Plant Pathology and Microbiology, Texas A&M AgriLife Research and Extension Center, Amarillo, TX 79106
    2. 2Department of Plant Pathology and Microbiology, Texas A&M AgriLife Research and Extension Center, Weslaco, TX 78596
    3. 3Department of Agriculture, Agribusiness and Environmental Sciences, Texas A&M University-Kingsville Citrus Center, Weslaco, TX 78599

    Abstract

    Wheat curl mites (WCMs; Aceria tosichella) are an important global pest of cultivated wheat. Their feeding activities on epidermal cells of wheat leaves result in characteristic leaf curl symptoms that prevent the unfurling of affected leaves and impair the proper emergence of heads from the boot stage. The most significant economic impact of WCM infestation, however, is their ability to vector and transmit four important viruses of wheat, specifically, wheat streak mosaic virus, Triticum mosaic virus, High Plains wheat mosaic emaravirus, and brome streak mosaic virus. Being wingless, WCMs are almost completely dependent on air currents for their dispersal. In addition, because of their obligate lifestyle, wheat field infestations are thought to originate from sources such as volunteer wheat. Corn and other cultivated and noncultivated Poaceae hosts are also known to act as green bridges between successive wheat crops. Consequently, management practices mostly target these off-season host plants but also the use of resistant varieties and other cultural control methods. Here we report the discovery of seed-borne WCM eggs, a previously unknown method of their dispersal, as a possible source of new infestations in wheat fields. This discovery expands our understanding of the biology of WCMs, with potential implications for the development of more holistic and effective management strategies for this economic pest and virus vector.

    Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

    Wheat (Triticum aestivum L.) is one of the most important sources of human dietary calories and protein (Arzani and Ashraf 2017). In the United States, wheat is the principal food grain and ranks third among field crops, behind corn and soybean, both in total planted and harvested area, with a 2021 harvested area totaling 36.7 million acres (Tack et al. 2015; USDA-ERS 2021). One of the most significant pests affecting wheat production globally is microscopic arthropods about 0.2 mm in length known as wheat curl mites (WCMs), Aceria tosichella Keifer (Acari: Eriophyidae). Several other grass species are also infested by WCMs, including the cereal crops corn (Zea mays L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), rye (Secale cereale L.), and pearl millet (Pennisetum glaucum [L.] R. Br.), as well as cultivated (pasture) and noncultivated grasses (Navia et al. 2013). Eriophyoid mites are an assemblage of a mostly host-specific (Skoracka et al. 2010), cryptic species complex (Miller et al. 2013; Skoracka et al. 2013). WCMs, in particular, two genotypes designated as Type 1 and Type 2 (globally, MT-1 and MT-8), are highly polyphagous and infest over 80 species in the family Poaceae (Navia et al. 2013) with worldwide distribution (Skoracka et al. 2014). Feeding damage caused by WCMs on leaf epidermal tissues, especially thin-walled bulliform cells within the whorl of a developing leaf, prevents unfurling of affected leaves and results in the characteristic leaf curling associated with WCM-infested wheat plants (Royalty and Perring 1996). However, the greatest economic impact of WCM infestation of wheat is their ability to transmit four distinct viruses to wheat (Stenger et al. 2016), including wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) (Slykhuis 1955), Triticum mosaic virus (TriMV; genus Poacevirus, family Potyviridae) (Seifers et al. 2009), High Plains wheat mosaic emaravirus (HPWMoV; genus Emaravirus, family Fimoviridae) (Seifers et al. 1997), and brome streak mosaic virus (BrSMV; genus Tritimovirus, family Potyviridae) (Stephan et al. 2008).

    Wheat is a dual-purpose crop in Texas and the Midwestern United States, cultivated for grain and/or feeding cattle. Infection by WCM-transmitted viruses is often a major consideration in the decision by growers to divert a wheat crop from grain production to feed. Kansas, for instance, ranks top in wheat production and planted acreage in the United States. However, it also ranks among the top states, behind Oklahoma and Texas, whose number of acres planted but not harvested averages more than 30%, likely due to crop diversion to grazing informed by poor crop conditions (Obembe et al. 2021).

    WCMs are wingless, crawl slowly, and consequently are almost completely dependent on wind for passive dispersal. WCMs also constantly need a host for food and survival (Townsend et al. 1996). Volunteer wheat is a known major reservoir for mite population buildup. In the absence of wheat, corn is another important WCM reservoir (Navia et al. 2013; Styer et al. 1996) and serves as a green bridge between successive wheat crops in the field. Reproduction and wind dispersal on actively growing, green, wheat plants, alternative hosts such as corn, as well as cultivated and noncultivated Poaceae hosts primarily are the known survival and dispersal mechanisms of WCMs in fields. However, during spring 2021, a wheat sample at the soft dough stage of development was collected from the Texas High Plains with symptoms of prematurely bleached glumes and submitted for diagnosis at the Texas High Plains Plant Disease Diagnostic Laboratory (Amarillo, TX). An initial inspection of the sample showed the presence of adult WCMs on the glumes of the developing heads. Further inspection revealed several adult wheat curl mites on the developing kernels, still at the soft dough stage, within the glumes and especially toward the base of the developing heads (Fig. 1A). Follow-up inspections of five additional diagnostic wheat samples, including three from a variety of evaluation research plots of the Texas A&M AgriLife Research wheat-breeding program located in Dumas in the Texas High Plains, and wheat plants in a USDA research field located in Bushland, Texas, also in the Texas High Plains region, with similar symptoms of prematurely bleached heads at the early hard dough stage of development also revealed the presence of adult WCMs within the glumes of affected heads, albeit at lower densities compared with those observed in samples at the soft dough stage. Closer examinations of the developing kernels further revealed the presence of egg-like structures in clusters and singly (Fig. 1B, C, and D). A sample of kernels from infested heads with these egg-like structures was collected and maintained in the laboratory for observation or germinated in sterile potting media in mite-proof cages under greenhouse conditions. After 1 week, most of the eggs had hatched into nymphal stages of WCMs. Samples of the adult mites were subsequently collected for laboratory identification.

    FIGURE 1

    FIGURE 1 Adult wheat curl mites (WCMs; red arrowheads) and WCM eggs (yellow arrowheads) on wheat kernels at A, the soft dough stage of development, B, the early hard dough stage of development, and C and D, the hard dough stage of development; E, still-image of motile larvae in the recessed crease areas of the seeds; and F, wheat plants grown from kernels with surface-attached eggs showing leaf-curling symptom (black arrowhead) characteristic of WCM infestation.

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    Morphological identification of the adult mites was conducted at the Entomology Lab of Texas A&M University-Kingsville Citrus Center (TAMUK-CC), Weslaco, Texas, by comparing them with the morphological characteristics of eriophyoid mites described by Lindquist (1996). Ten adult females were randomly selected from the specimens and mounted on slides for examination with a phase contrast microscope. Females were distinguished based on body dimensions (i.e., body length and width). Females are much larger than males and those in immature stages. Measurements of body length and number of dorsal and ventral annuli were taken. Voucher specimens were kept at the Entomology Lab at TAMUK-CC. Molecular identification was carried using total nucleic acids (TNA) extracted from four samples, THP-WCM_1-4, according to the Dellaporta et al. (1983) extraction protocol, with each sample consisting of cohorts of five or more mites. A 2-µl aliquot of each stock TNA was used as the template in a 25-μl polymerase chain reaction (PCR) with the reagents and rapid protocol described for the PrimeSTAR GXLDNA Polymerase (Takara Bio USA, Mountain View, CA). The primer pair LCO1490 (5′-GGTCAACAAATCATAAAGATATTGG-3′) and HCO2198 (5′-TAAACTTCAGGGTGACCAAAAAATCA-3′) was used to target an approximately 700-bp fragment of the mitochondrial cytochrome c oxidase subunit I (mtCOI) gene from diverse metazoan invertebrates (Folmer et al. 1994). DNA extracts from a laboratory-reared arthropod species were included as positive controls, and a plant nucleic template extract was used as a negative control. The amplified PCR products were subsequently visualized on a 1% agarose gel relative to a DNA ladder. The target-specific DNA bands of the correct sizes from two randomly chosen mite samples were individually excised and gel-eluted using the Zymoclean Gel DNA Recovery Kit (Zymo Research, Irvine, CA). The recovered DNA were ligated individually into the pJET1.2/blunt vector using the CloneJET PCR Cloning Kit (Thermo Fisher Scientific, Waltham, MA) according to the manufacturer's recommended protocol. The ligation products were used to transform chemically competent DH5α Escherichia coli cells. Two recombinant plasmids with PCR-verified correct size inserts per cloned DNA amplicon were isolated from the respective transformed E. coli cells using the GenElute Plasmid Miniprep Kit (Sigma-Aldrich, St. Louis, MO). Each plasmid sample was sequenced in both directions with the pJET1.2F and pJET1.2R primers by the Sanger method in a commercial facility (Elim Biopharm, Hayward, CA). The raw sequences were analyzed with VecScreen (https://www.ncbi.nlm.nih.gov/tools/vecscreen/) and trimmed to remove the pJET1.2 vector sequence. The CAP contig assembly program of the BioEdit software (Hall 1999) was used to derive a consensus sequence for each plasmid-specific sequence, which was subsequently subjected to BLASTN analysis (Altschul et al. 1990) for species identification purposes. A phylogenetic relationship between the sequences derived in this study and the corresponding sequences of hits produced in GenBank was inferred using the neighbor-joining method (Saitou and Nei 1987), implemented in MEGA X (Kumar et al. 2018).

    An initial microscopic examination of the mites indicated that they belonged to the Eriophyidae family based on their annulated and wormlike bodies. The presence of prodorsal tubercles and setae that are directed to the rear confirmed their assignment into the genus Aceria Keifer (Halawa 2016). The adult females were whitish in color and varied from 185 to 245 μm in length. They had an 8-rayed empodium on leg I, a small lobe over the gnathosoma, and a prodorsal shield with longitudinal median line restricted to the posterior half, consistent with features described for Aceria tosichella (Keifer) (Lindquist 1996).

    The expected approximately 700-bp fragment was amplified from each of the four mite TNA samples. Two of the four amplicons from mite samples THP-WCM_1 and THP-WCM_2 were randomly selected, gel-purified, and cloned into the plasmid vector. The generated Sanger sequences (GenBank accession nos. ON720276 and ON720277) of the cloned inserts shared 100% nucleotide (nt) identity with each other. BLASTN analyses of the 658 nt consensus sequence, post-trimming off primer sequences, produced hits only to isolates of A. tosichella, with percentage nucleotide identities ranging from 87.86 to 100% and query coverage ranging from 68 to 90% for the top 100 hits of this mite species in GenBank. In pairwise comparisons, the A. tosichella sequences derived in this study from Texas shared 87.6 to 100% nt identity with corresponding sequences of global isolates of the WCMs. Phylogenetic analysis also showed that they clustered closely with WCM isolates belonging to the previously identified MT-1 clade (Fig. 2).

    FIGURE 2

    FIGURE 2 A neighbor-joining cladogram depicting the evolutionary relationships of the mitochondrial cytochrome c oxidase subunit I (mtCOI) gene sequences of wheat curl mite (WCM; Aceria tosichella Keifer) isolates identified in this study (shaded; GenBank accession nos. ON720276 and ON720277) with some global isolates of WCM species groups retrieved from GenBank. The percentages of replicate trees in which the associated taxa clustered together in the bootstrap test (1,000 replicates) are shown next to the branches. The tree was rooted with isolates belonging to the MT-8 species group. The evolutionary analyses were conducted in MEGA X (Kumar et al. 2018).

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    WCMs have previously been documented and collected from maturing wheat heads in the soft to hard dough stage (Mahmood et al. 1998; Seifers et al. 2002) but not on wheat kernels within spikelets of developing heads. Because this WCM lifestyle of egg-laying on wheat kernels within spikelets had not previously been documented, replicated studies were conducted with 30 seeds each of three wheat varieties (WB 4595, Deliver, and TAM W-101) from 2021 harvests, arranged in a flat single layer in a 5-mm-diameter Petri dish, to investigate the viability and development of the eggs on the kernels. Treatment 1 consisted of a set of approximately 50 to 70 WCM eggs that were left attached on wheat kernel surfaces and monitored for their development on germinating seeds. Treatment 2 comprised a second set of surfaced-sterilized wheat kernels from which all WCM eggs had been physically removed and that had been confirmed to be WCM egg-free by microscopy. Subsequently, approximately 50 to 60 previously collected WCM eggs, obtained from wheat kernels with eggs attached to their surfaces, were placed on these egg-free treatment 2 seeds using a pipette. Treatment 3 had a third set of surface-sterilized and WCM egg-free wheat kernels to serve as controls. The three sets of wheat kernel treatments were placed separately on wet sterile paper towels in sterile Petri dishes and incubated at room temperatures (∼23°C) to germinate. After 48 h of incubation, and with the radicle barely starting to push out of the germinating seeds, observation with a dissecting microscope revealed that the WCM eggs in treatments 1 and 2 had hatched, with larvae visibly moving around in the pool of water that collected in the recessed crease areas of the seeds (Fig. 1E). Adult WCMs observed on wheat plants, from the germinated seeds, showing the typical leaf-curling symptom associated with WCM infestation (Fig. 1F) were morphologically similar to those observed initially on the field-collected wheat kernels. No WCM larvae were observed in treatment 3. In addition to demonstrating the viability of the eggs, the observation revealed a synchronization of the timing of egg hatch with that of the host seed germination, probably in response to germinating seed-associated chemical stimuli.

    This finding adds to the current understanding of the biology of WCMs. Whether this is a recently adopted behavior of the WCMs is currently not known and beyond the scope of this study. The length of time and the conditions under which such seed-attached WCM eggs can remain viable is also not currently known. However, follow-up examinations of stored (dry and ∼21°C), 2- to 5-year-old, wheat seeds that were germinated as described above and observed after 48 h with a dissecting microscope, similarly revealed the presence of hatched mite larvae visibly moving around in the pool of water that collected in the recessed crease areas of the seeds. This finding suggests that mite eggs can survive on wheat seeds and remain viable for at least 2 years. Additionally, examinations of a batch of a newly purchased Texas farmers’ wheat seeds for the 2021/22 cropping season similarly revealed the presence of viable WCM eggs on the seeds, indicating that seeds for planting could serve as vehicles for long-distance dispersal of WCMs. Given the above findings, it is conceivable that the WCM infestation of wheat heads, beyond mere surface colonization, leads to infestation of the kernels within the spikelets of infested heads and ultimately to the deposition of eggs on such infested kernels. These findings add to our understanding of the sources of field WCMs with implications for their management. Considering previous reports of seed transmission of WSMV (Dwyer et al. 2007; Jones et al. 2005; Lanoiselet et al. 2008), the implications of these findings for the epidemiology of the four WCM-transmitted viruses of wheat, if any, are currently being investigated.

    The author(s) declare no conflict of interest.

    LITERATURE CITED

    The author(s) declare no conflict of interest.