Location: Peanut and Small Grains Research Unit
2025 Annual Report
Objectives
The long-term goal of this project is to improve integrated pest management (IPM) practices for cereal aphids in wheat, barley, and sorghum in the United States. To achieve this goal, we will increase the knowledge of aphid host plant resistance and natural enemies for IPM programs and provide fundamental knowledge of cereal aphid biology and ecology. Over the next 5 years we will focus on the following objectives:
OBJECTIVE 1: Determine regional distribution and significance of invasive aphid species on cereals and alternate grass hosts, discover changes or shifts in the populations, generate relevant phenotypic information, and work with breeders or geneticists to map and pyramid resistant genes.
Subobjective 1A: Determine the biotype diversity of the S. Maydis known as the hedgehog aphid (HGA) in cereals and native grasses in advance of discovering and utilizing resistant sources used in breeding for resistance.
Subobjective 1B: Identify wheat germplasm resistant to HGA, as well as wheat accessions resistant to all U.S. RWA biotypes.
Subobjective 1C: Characterize RWA and HGA resistance genes and develop molecular markers for their introgression into locally adapted breeding lines and cultivars.
OBJECTIVE 2: Determine the biological differences and interactions among available sugarcane aphid resistance genes, including comparisons of cross-resistance genes, elucidate resistance mechanisms, and work with breeders and geneticists to map, pyramid, and deploy resistant genes.
Subobjective 2A: Develop forage sorghum germplasm resistant to sugarcane aphids.
OBJECTIVE 3: Develop effective multi-scale aphid infestation monitoring and sampling technology, develop geographically explicit cereal aphid pest infestation risk models, and elucidate and integrate field and landscape-level components of the natural enemy populations into improved management systems for invasive cereal aphids.
Subobjective 3A: Use field research methods and statistical modeling and hypothesis testing to develop a detailed understanding of spatial and temporal factors that determine colonization, population dynamics, and community development of aphid natural enemies in grain sorghum fields and their importance for sugarcane aphid biological control.
Subobjective 3B: Develop and disseminate new, time-efficient, and statistically accurate and precise methods for sampling aphid natural enemies in sorghum.
Approach
The combined annual economic value of wheat, sorghum, and barley within the United States is over $16 billion. Cereal aphids are major pests of world agriculture that vector numerous plant viruses and remove photoassimilates by inserting their mouthparts into the sieve elements of the phloem tissue. The direct loss from feeding and the transmission of viral diseases during the process of feeding make cereal aphids the most significant threat to U.S. cereal crop producers. Although aphids in general are small in size, the reproductive potential is at the higher end of the spectrum when compared to all arthropods, with exponential increases and short generation times. Significant yield reductions occur from the direct effect of endemic or epidemic aphid populations. New information on the factors affecting insect populations and new integrated pest management (IPM) tools, control technologies, and aphid-resistant germplasm are needed in order to advance sustainable IPM programs for cereals. The objectives of this project that will deliver the associated products to the consumer are to: 1) determine the biotypic diversity in aphid populations which threatens deployment of aphid-resistant cereals; 2) identify new sources of resistance to aphids in wheat and sorghum and barley; 3) determine the extent and severity of new aphid pest species attacking cereal crops; and 4) develop or refine methods for detecting and monitoring aphid infestations to optimize biocontrol in cereals. The specifically designed research provided in this plan will increase our knowledge on the genomics of virulent cereal aphid biotypes within the U.S. and increase the knowledge of available genetic sources of resistance in wheat, barley, and sorghum. Once identified these resistant sources will be introgressed into available breeding lines. In addition, the benefits and ecological associations of beneficial insects in the agricultural landscape will be better understood and utilized as a result of areawide studies on these organisms.
Progress Report
Under Objective 1, Sub-objective 1A, ARS scientists from Stillwater, Oklahoma, and Tifton, Georgia, identified the host plants, the geographical distribution, and if differing biotypes of the hedgehog grain aphid (HGA) in the United States. They determined that only a single clone of the HGA exists within the United States and that the geographic distribution of HGA was not expanding from its initial extent, and furthermore, that economically significant populations of the aphid did not occur. Therefore, the threat of this aphid species as a pest of small grains appears limited. This is positive for U.S. small grain producers. Further study by ARS scientists in Stillwater, Oklahoma, indicated that HGA is a minimal threat to wheat and other small grains and further study in not required and will be discontinued.
Under Objective 1, Sub-objective 1B, ARS scientists in Stillwater, Oklahoma, evaluated a set of preselected wheat germplasm for responses to all five U.S. Russian wheat aphid (RWA) biotypes - RWA1, RWA2, RWA3/7, RWA6, and RWA8 - and 33 accessions resistant to multiple biotypes were identified, 14 of which were resistant or highly resistant to all five biotypes. A set of over 1000 wheat germplasms were also evaluated for HGA resistance and two of them showed resistance. The identified RWA and HGA resistance sources were confirmed and purified, and are available for use by plant breeders.
Under Objective 1, Sub-objective 1C, ARS scientists in Stillwater, Oklahoma, discovered a set of seven newly identified RWA resistant accessions and two HGA resistant lines were also chosen to develop mapping populations from which three RWA resistance genes were discovered. The gene in PI 625139, designated Dn625130, conditions resistance to all U.S. RWA biotypes. Dn625139 was delimited to a 2.6 cM interval in the proximal region of chromosome arm 7DS, and a marker co-segregating with Dn625139 was developed for tagging Dn625139 in wheat breeding. The RWA resistance genes in PI 625134 were also characterized using an F2:3 population from the cross PI 625134 × Jagalene. Linkage analysis indicated that two RWA resistance genes on chromosome arms 1DS and 7DS condition resistance to RWA biotype 2 (RWA2). Of the known RWA2 resistance genes, none reside in chromosome 1D, indicating that the gene on 1DS is a new gene. Moreover, the RWA resistance gene in PI 626575, designated Dn626575, was mapped to the long arm of chromosome 7D, where Dn10, a new RWA resistance gene ARS scientists in Stillwater, Oklahoma, recently identified from PI 686275, resides. To determine whether Dn626575 is a novel resistance gene or a new allele of Dn10, an F2 population from the cross PI 626575 × PI 686275 was further developed for allelism tests. The scientists developed two mapping populations to identify HGA resistance genes. However, a recent field survey indicated that HGA unlikely to infest wheat at levels causing economic injury. Therefore, we shifted our focus from HGA to greenbug (GB) and bird cherry-oat aphid (BCOA), which are serious insect pests that not only damage cereal plants but also spread viruses. A novel, efficient BCOA evaluation procedure that yields reproducible results has been developed and used in germplasm screening leading to the discovery of the first set of BCOA resistant synthetic hexaploid wheat (SHW) lines that can be used in wheat improvement. A project aimed at introgression of the BCOA resistance genes in these SHW lines into locally adapted cultivars has been initiated. The scientists have evaluated over 600 translocation lines and SHW lines for GB resistance, and over 100 resistant lines have been identified. Two GB resistance genes have been discovered from the newly identified resistance sources with one officially designated as Gb9 and another one temporarily named as Gb49921. The sum total of this work paves the way for greater flexibility and breadth in breeding for aphid resistance in wheat improvement programs.
Under Objective 2, Sub-objective 2A, ARS scientists at Stillwater, Oklahoma, Tifton, Georgia, and Brazil, identified the clonal diversity of the sugarcane aphids from 121 collections from the United States and Brazil. We determined using 8-9 Melanaphis spp. microsatellite markers that the multi-locus-lineage designated F (MLL-F) that remains a threat to sorghum production in the United States from 2013 to the present was the same lineage that invaded Brazil 6 years later. Genotyping results showed that all samples from the United States in 2019 and Brazil in 2020 had alleles identical to the predominant superclone MLL-F, and these were not the same as the MLL-C lineage that effects sugarcane. ARS scientists in Stillwater, Oklahoma, and Lubbock, Texas, incorporated host plant resistance from grain sorghum sources into forage sorghum resistance.
Under Objective 3, Sub-objective 3A, ARS scientists in Stillwater, Oklahoma, in collaboration with Oklahoma State University scientists evaluated coexistence mechanisms including differences in host specificity, and how parasitoid density, arrival time, and host discrimination ability impact the ability of parasitoids to suppress aphid populations. These studies led to the conclusion that one parasitoid species, A. nigritus, deters the ubiquitous parasitoid L. testaceipes from utilizing the same host patches. ARS scientists in Stillwater, Oklahoma determined interrelationships among co-occurring host aphid species on parasitism of sorghum aphid (SA) by A. nigritus in sorghum. Through a series of experiments designed to measure host use by A. nigritus they found that preference for SA was reduced when it co-occurred on sorghum with other aphid species. Experiments indicated that biological control of SA by A. nigritus is less effective when GB occurs with SA on sorghum. We also showed that co-occurrence of SA with Rhopalosiphum maidis was beneficial to SA biological control because A. nigritus readily switches from parasitizing the earlier occurring R. padi to SA. We estimated predation rates by important natural enemies of aphids on SA in order to determine their importance as natural enemies of SA in grain sorghum and found that the natural enemies have significant impact on SA and knowledge of their density can be used to assess likelihood of successful biological control of SA in grain sorghum. ARS scientists in Stillwater, Oklahoma, in collaboration with a multi-institutional group of scientists developed and tested a natural enemy importance index that incorporated cumulative degree days of first occurrence of a natural enemy taxa in a field, the average density of the natural enemy in a field, and natural enemy voracity. Differences occurred in biological control of SA by particular natural enemies and in biological control by the complex of natural enemies among widely separated geographic locations. It was determined that patch composition, arrangement, and shape impacted aphid natural enemy communities and levels of aphid mortality caused by parasitoids in wheat, canola, and sorghum. Both the landscape and weather affect the abundance and effectiveness of natural enemies to suppress SA infestations. SA population dynamics in the context of landscape and weather factors was evaluated using simulation modeling. Results indicate that various aspects of landscape composition, landscape configuration, and weather affect key beneficial insect groups and aspects of their population dynamics in the field. Specific characteristics of landscape structure were particularly informative in determining potential for biological control of SA. It was determined that interactions of SA and natural enemy population dynamics in both space and time can be harnessed to control the SA on an areawide basis.
Under Objective 3, Sub-objective 3B, ARS scientists in Stillwater, Oklahoma, conducted multi-year fields studies to evaluate the statistical accuracy and precision of population estimates of natural enemies of aphids in sorghum fields using suction sampling, sweepnet sampling, visual counts of natural enemies for a fixed time, and counting numbers of natural enemies per sorghum leaf. These sampling methods provide estimates of relative population.
The methods were compared to population estimates from complete enumeration of natural enemies on sorghum plants. Each of the four sampling methods were biased for at least some natural enemies and bias was either positive or negative depending on the category or species of natural enemy. Overall, 2-leaf counts and timed counts were the best sampling methods in terms of high precision and low bias for estimating natural enemy populations in sorghum. ARS scientists in Stillwater, Oklahoma, developed statistical models for converting relative population estimates of aphid natural enemies to estimates of absolute population density. They found that sorghum plant growth stage, aphid population intensity, and several meteorological variables were related to relative population and population intensity estimates for natural enemies. Statistical models were constructed to relate absolute population density of natural enemies to relative population density or population intensity. Useful estimates of absolute population were obtained using time and cost-efficient relative sampling methods and will contribute to both more effective and efficient ecological research on natural enemies and aphid biological control in sorghum, and more cost effective and environmentally sound insect pest management.
Accomplishments
1. Discovery of three novel wheat greenbug resistance genes Gb8, Gb9 and Gb49921. The greenbug (GB) is a worldwide pest that poses a serious threat to wheat production, and annual losses of up to $135 million were reported in Oklahoma alone when outbreaks occurred. New GB resistance genes that can be readily used in wheat breeding are urgently needed because of the limited resistance sources available for wheat breeding and the occurrence of new greenbug biotypes in fields. Three novel greenbug resistance genes (Gb8, Gb9 and Gb49921) have been identified in the last funding period, two of which have been officially designated as Gb8 (Xu et al. 2020) and Gb9 (Xu et al. 2024). Gb8 and Gb9 were identified in a wheat reselection line PI 697274 and synthetic hexaploid wheat (SHW) line CWI 76364, respectively, whereas Gb49921 was discovered in wheat breeding line BW 49921 (Li et al. 2025). Each gene confers resistance to a few economically important GB biotypes, and Gb49921, the only gene identified in an elite spring wheat breeding line, is ideal for enhancing GB resistance in spring wheat.
2. Five new barley greenbug resistance genes discovered. Two greenbug (GB) resistance genes (Rsg1 and Rsg2) had been previously identified in barley, with each one conferring resistance to a limited number of GB biotypes. However, the genomic location of Rsg2 was previously unknown, and both Rsg1 and Rsg2 lack resistance to GB biotype H, making it urgent to map Rsg2 and identify novel GB resistance genes. ARS scientists at Stillwater, Oklahoma, discovered five new greenbug resistance genes. Of these, Rsg1-a2 and Rsg1-a3 are novel alleles of Rsg1, whereas Rsg2-a3 is a new allele of Rsg2. Rsg3 and Rsg4 represent novel GB resistance loci. These new genes make it feasible for breeders to develop barley cultivars resistant to all economically important GB biotypes. Kompetitive allele-specific polymerase chain reaction markers were also developed and will facilitate the introgression of these genes into elite barley cultivars by breeders.
3. Sorghum aphid (SA) spatiotemporal modeling and population dynamics. The SA causes millions of dollars in losses annually to grain sorghum producers. ARS researchers in Stillwater, Oklahoma, developed mathematical and statistical models to evaluate the effect of weather and other abiotic and biotic factors on SA infestation of sorghum in Southern and Central Plains States. The models identified meteorological variables at multiple levels of the atmosphere as directly impacting seasonal migration of sorghum aphid and colonization of sorghum fields, and in addition, biotic factors such as natural enemies of SA influenced SA population growth in fields. Models accurately predict the timing of occurrence of migrant aphids at test sites in north Texas and Oklahoma and form a basis for understanding and predicting sorghum aphid colonization and population dynamics in sorghum fields to assist sorghum producers in production planning and insect pest management.
4. Cereal aphid biological control by natural enemies. Variation in composition, patch arrangement, and patch shape in landscapes impact aphid natural enemy communities and levels of aphid mortality caused by parasitoids in wheat, canola, and sorghum. ARS researchers at Stillwater, Oklahoma, discovered that landscape structure plays an important role in determining levels of biological control by natural enemies of aphid pests in agricultural fields of several crops (including wheat, sorghum, and canola) and indicates that crop and habitat management are important in determining the extent of aphid pest infestations and levels of biological control of aphid pests. This knowledge can be used to improve the economic viability, human health, and environmental health of crop production systems.
5. A new gene conferring resistance to all U.S. Russian wheat aphid biotypes discovered. Russian wheat aphid (RWA) is a highly invasive and destructive aphid pest of wheat that has evolved rapidly to overcome host plant resistance. Most RWA resistance genes have lost effectiveness to new RWA biotypes. Novel genes conferring resistance to multiple RWA biotypes are needed to sustain wheat production. ARS researchers at Stillwater, Oklahoma, identified a new gene designated as Dn625139 in the landrace PI 625139, which confers resistance to all five U.S. RWA biotypes. Dn625139 can be widely used by breeders to enhance RWA resistance in wheat.
6. Natural enemy biology predicts aphid biological control in sorghum. Research conducted by ARS scientists at Stillwater, Oklahoma, demonstrated that differences in host specificity, host preference, density, arrival time into fields, host discrimination, competitive ability, and predation rate by predatory arthropods and parasitoids impact the ability of the natural enemies to suppress sorghum aphid and greenbug populations. These factors determine the overall structure, function, and ecological resilience of aphid and natural enemy communities in sorghum and are critically important for understanding interactions leading to successful greenbug and sorghum aphid biological control in sorghum.
7. Efficient methods for sampling and monitoring aphid natural enemies in sorghum. Multi-year field studies conducted by ARS researchers in Stillwater, Oklahoma, compared population estimates obtained by four relative methods for sampling aphidophagous arthropods in sorghum for their reliability and efficiency to estimate natural enemy population densities to estimates of absolute population achieved by complete enumeration of natural enemies on sorghum plants. All relative sampling methods were biased for at least some natural enemies and statistical precision varied among methods. Overall, 2-leaf counts and timed counts were the best sampling methods in terms of high precision and low bias for estimating natural enemy populations in sorghum. Meteorological and biological variables were used in statistical models to correct for sampling bias in the methods. The research indicated that useful estimates of absolute population for natural enemies can be derived using relative sampling methods, which is important to researchers and pest managers for understanding and predicting levels of biological control of aphids in sorghum, and the need for pest management intervention.
8. The diversity of sorghum aphids from the United States and Brazil. The clonal diversity of sugarcane aphids (Melanaphis spp.) including the SA from 121 collections from the United States and Brazil was determined using 8-9 Melanaphis spp. microsatellite markers. The multi-locus-lineage designated F (MLL-F) includes the SA, which remains a threat to sorghum production in the United States from 2013 to the present is the same lineage that invaded Brazil 6 years later. Genotyping showed that all samples from the United States in 2019 and Brazil in 2020 had alleles identical to the predominant superclone MLL-F, and these were not the same as the MLL-C lineage that effects sugarcane. This knowledge is important to plant breeders and pest managers developing solutions for managing sugarcane aphids and SA in sorghum and sugarcane.
9. An efficient bird cherry-oat aphid assay protocol and wheat resistance source. The bird cherry-oat aphid (BCOA) poses a persistent threat to wheat production in the United States. However, an efficient BCOA resistance assay was not available and BCOA resistance sources that can be used in cultivar development have not been found. ARS scientists at Stillwater, Oklahoma, developed a BCOA assay protocol that yields reliable and reproducible results for differentiating responses to BCOA leading to the identification of a few BCOA resistant wheat lines. These lines can be widely used in wheat breeding to enhance BCOA resistance.
Review Publications
Xu, X., Mornhinweg, D.W., Bai, G., Li, G., Bian, R., Bernardo, A.E. 2024. Characterization of a new barley greenbug resistance gene Rsg4 in the Chinese landrace CI 2458. The Plant Genome. 17(4). https://doi.org/10.1002/tpg2.20527.
Li, G., Watira, T., Bai, G., Bian, R., Bernardo, A.E., Carver, B., Wu, Y., Xu, X. 2024. Characterization of a gene conferring resistance to U.S. Russian wheat aphid biotypes in the Iranian wheat landrace PI 625139. Crop Science. 65(1). Article e21398. https://doi.org/10.1002/csc2.21398.
Xu, X., Li, G., Bai, G., Kolmer, J.A., Xu, Y., Bernardo, A.E., Carver, B.F., Tan, C. 2025. Characterization of a new Lr52 allele for leaf rust resistance in the Iranian wheat landrace PI 622111. The Plant Genome. 18(1). Article e70003. https://doi.org/10.1002/tpg2.70003.
Cavalet-Giorsa, E., Gonzalez-Munoz, A., Athiyannan, N., Holden, S., Salhi, A., Gardener, C., Quiroz-Chavez, J., Rustamova, S., Elkot, A.F., Patpour, M., Rasheed, A., Kolmer, J.A., Lazo, G.R., Xu, S.S., Gu, Y.Q., Xu, X. 2024. Origin and evolution of the bread wheat D genome. Nature. 633:848-855. https://doi.org/10.1038/s41586-024-07808-z.
Elliott, N.C., Giles, K.L., Butler, H., Vazquez, N., Coackley, L. 2025. Parasitism of three species of aphid by aphelinus nigritus on sorghum. Southwestern Entomologist. 50(1):340-357. https://doi.org/10.3958/059.050.0135.
Giles, K.L., Elliott, N.C., Royer, T.A., Butler, H., Rudin, N. 2024. Ecology of aphid parasitoids in winter wheat habitats of the Southern Plains: How latitude and crop diversity influence pest management. In: Brewer, M.J. Hein, G.L., editors. Arthropod Management and Landscape Considerations in Large-scale Agroecosystems. Boston, MA:CAB International. p. 119-132.
Elliott, N.C., Giles, K.L., Baum, K.A., Brewer, M.J., Elzay, S.D., Royer, T.A. 2024. Aphid biological control in winter wheat and canola in heterogeneous agricultural landscapes. In: Brewer, M.J. Hein, G.L., editors. Arthropod Management and Landscape Considerations in Large-scale Agroecosystems. Boston, MA:CAB International. p.104-118.
Brewer, M.J., Umina, P.A., Elliott, N.C. 2024. Pest management for spatially-variable arthropod pests in large-scale agroecosystems. In: Brewer, M.J. Hein, G.L., editors. Arthropod Management and Landscape Considerations in Large-scale Agroecosystems. Boston, MA:CAB International. p. 27-43.
Elliott, N.C., Jones, W.L. 2025. Predation on Sorghum aphid (Melanaphis sorghi) by Hippodamia convergens and Chrysoperla rufilabris. Southwestern Entomologist. 50(1):72-83. https://doi.org/10.3958/059.050.0120.
Faris, A.M., Brewer, M.J., Elliott, N.C. 2024. Natural enemy suppression supplemented by regional pest management for the invasive Melanaphis sorghi, Sorghum aphid, on Sorghum. In: Brewer, M.J. Hein, G.L., editors. Arthropod Management and Landscape Considerations in Large-scale Agroecosystems. Boston, MA:CAB International. p. 151-167.
Hayashida, R., Williams, E., McKay, L., Mornhinweg, D.W., Elliott, N.C., Hoback, W. 2025. Responses of resistant and susceptible barley and wheat genotypes to simultaneous infestation by multiple aphid species. American Journal of Plant Sciences. 16(5):634-650. https://doi.org/10.4236/ajps.2025.165046.
Hayashida, R., Carey, C., Springer, T.L., Knighten, B.A., Armstrong, J.S., Hoback, W. 2024. Interactions between Sudangrass lines selected for differing nitrate expression and sorghum aphid. Agronomy Journal. 14(10). Article 2250. https://doi.org/10.3390/agronomy14102250.
Lhamo, D., Li, G., Song, G., Li, X., Sen, T.Z., Gu, Y.Q., Xu, X., Xu, S.S. 2024. Genome-wide association studies on resistance to powdery mildew in cultivated emmer wheat. The Plant Genome. 18(1). Article e20493. https://doi.org/10.1002/tpg2.20493.
Koralewski, T.E., Brewer, M.J., Deleon, L., Elliott, N.C., Giles, K.L., Szczepaniec, A., Faris, A. 2025. Activity of sorghum aphid and its natural enemies in the context of agro-ecological and weather conditions. Frontiers in Insect Science. 5. Article 1503044. https://doi.org/10.3389/finsc.2025.1503044.