1a. Objectives (from AD-416):
The long-term objective of this project is to improve integrated pest management (IPM) practices for cereal aphids in wheat, sorghum, and barley in the United States. Achieving this objective will result in tools and knowledge to enhance the role of host plant resistance and natural enemies in IPM programs for cereal aphids, and fundamental knowledge of cereal aphid biology and ecology required for more effective crop management. Over the next 5 years we will focus on the following objectives: Objective 1: Determine the genetic and biochemical basis for biotype formation in the Russian wheat aphid and greenbug by identifying salivary proteins, genes, and quantitative trait loci associated with virulence. Subobjective 1A. Produce a systematic revision of greenbug, Schizaphis graminum (Rondani) sensu lato (i.e., in the wide sense), based on molecular methods. Subobjective 1B. Map virulence and avirulence loci to wheat resistance genes in the Schizaphis graminum genome. Subobjective 1C. Identify and characterize the salivary proteins injected during feeding by the Russian wheat aphid. Subobjective 1D. Identify physiological and biochemical attributes that confer plant resistance to cereal aphids. Objective 2: Monitor and characterize the biotypic structure of cereal aphid populations in the United States, and develop methods to efficiently determine their biotypic status. Subobjective 2A. Determine the biotypic diversity in Russian wheat aphid populations at a regional level. Subobjective 2B. Characterize holocyclic reproduction in the Russian wheat aphid and its role in biotype evolution. Subobjective 2C. Characterize the role of non-cultivated grass species on the biotypic composition of greenbug populations and their occurrence, along with other cereal aphids, in small grain cropping systems. Objective 3: Assess the effectiveness of key biological control agents in traditional and emerging small grain crop production systems and elucidate the intra- and extra-field processes that influence their population dynamics. Subobjective 3A. Elucidate the effect of landscape factors on the population dynamics of the key greenbug parasitoid, Lysephlebus testaceipes. Subobjective 3B. Elucidate the effect of co-occurring cereal aphids on parasitism of greenbug by Lysephlebus testaceipes. Subobjective 3C. Assess the effect of non-cultivated hosts and wide-spread plantings of switchgrass (Panicum virgatum) on the biology, ecology, and tritrophic relationships of Hymenopterous parasitoids of cereal aphids. Objective 4: Expand the existing Greenbug Decision Support System to include Russian wheat aphid IPM decision support.
1b. Approach (from AD-416):
Field and laboratory experiments will be conducted to: (1) produce a systematic revision of greenbug based on molecular methods; (2) map virulence and avirulence loci to wheat resistance genes in the greenbug genome; (3) identify and characterize the salivary proteins injected during feeding by the Russian wheat aphid; (4) identify physiological and biochemical attributes that confer plant resistance to cereal aphids; (5) determine the biotypic diversity in Russian wheat aphid populations at a regional level; (6) characterize holocyclic reproduction in the Russian wheat aphid and its role in biotype evolution; (7) characterize the role of non-cultivated grass species on the biotypic composition of greenbug populations and their occurrence, along with other cereal aphids, in small grain cropping systems; (8) elucidate the effect of landscape factors on the population dynamics of the key greenbug parasitoid, Lysephlebus testaceipes; (9) elucidate the effect of co-occurring cereal aphids on parasitism of greenbug by L. testaceipes; (10) assess the effect on non-cultivated hosts and wide-spread plantings of switchgrass on the biology, ecology, and tritrophic relationships of Hymenopterous parasitoids of cereal aphids; and (11) develop sophisticated decision support computer software for greenbug and Russian wheat aphid pest management decision support that is comprehensive in scope yet easy for end users to understand and operate.
3. Progress Report:
1C. Salivary proteins of Russian wheat aphid (RWA) and greenbug (GB) biotypes were identified and revealed that saliva greatly differs between species and between biotypes within a species. This research confirms that aphid species feed differently on cereals, and the information will be used to develop aphid-plant interaction models. The genome assembly of the RWA has been completed and estimated to be 422 Mbp. The genome will provide gene sequences to generate dsRNA to test RNAi approaches for aphid control. 2A. Biotyping of RWA samples via plant screening was completed on the 2012 samples to describe variation within-fields in 7 western states. The 2012 results confirmed a shift in RWA biotype composition away from RWA2 and revealed RWA1 and RWA6 were now predominant. Over 350 RWA collections were made in the spring 2013 to determine regional biotypic diversity in the western U.S. The study will continue another two years to examine biotype change over time. This information is vital to plant breeders developing RWA resistant cereals. 2B. Laboratory studies on induction of RWA sexuals determined the environmental conditions needed to induce sexual forms and hatch eggs. Only RWA females could be produced in the laboratory. In contrast, Diuraphis tritici produced males, females, and fertile eggs. Eggs hatched when subjected to cold treatments for 6 weeks. RWA 8 and D. tritici co-inhabit wild grasses and were successfully hybridized in the lab. The impact hybridization between these aphid species will have on agriculture is being evaluated. 2D. Barley germplasm from the National Small Grains Collection was evaluated for resistance to the greenbug (GB) biotypes, and we found two resistant germplasms with excellent resistance to the major greenbug biotypes E, I, and K that will be useful in developing winter and spring barley. Greenbug resistance was identified in two sorghum varieties that respond differently to GB biotypes and will help in GB identification. 3A. Eighteen fields were studied in each of four geographic areas in central Oklahoma. Over the 3 years, we studied a total of 72 wheat fields. Landscape data were obtained from USDA, NASS, RDD. Cropland data was supplemented with ground survey data to verify the accuracy of the NASS data. Landscape metrics were calculated for the area at hierarchically increasing spatial extents with radii from one-half mile to 3 miles. 3B. Lysiphlebus testaceipes (LT) were introduced into arenas constructed with wheat plants growing in pots and infested with known, varying, numbers of 3rd-4th instar bird cherry-oat aphids (BCOA) and greenbugs (GB) to determine aphid preference. Data indicate no evidence of LT preference for GB or BCOA. 4A. A RWA pest management decision support system was constructed to deliver pest management information and decision support functionality via the internet, and was integrated with an existing greenbug decision support system to achieve inter-functionality with that system.
1. Sequencing RWA genome to advance understanding of aphid-plant interactions and develop new aphid control technologies. Scientists at ARS-Stillwater, Oklahoma, led a team effort to sequence the Russian wheat aphid (RWA) genome and have arrived at a genome assembly that represents 93% of its total estimated genome size of 422 Mbp. The genome assembly represents the second known genome assembly for aphids and, through bioinformatics, will provide new information on how differently aphids interact with their host plants. The genome is also being used in conjunction with proteomics data on salivary proteins of RWA to develop gene sequences useful in developing an RNAi approach to aphid control. The RNAi approach was presented at scientific and industry meetings as a new technology for developing resistant cereals that can genetically control aphid pests without expressing foreign or natural proteins.
2. Discovered invasive RWA can hybridize with related native U.S. species, posing a new threat to agriculture. Through ecological studies of Russian wheat aphid (RWA) distribution and host preferences, scientists at ARS-Stillwater, Oklahoma, discovered RWA occupies the same ecoregions and co-occupies some of the same native grasses with native Diuraphis species in the western U.S. Most significant was the discovery that RWA co-occupies wheat grasses with native western wheat aphid, D. tritici, during the fall when sexual reproduction occurs to produce overwintering eggs. Laboratory methods were developed in order to cross breed and hatch eggs of Diuraphis species, and we found that RWA and D. tritici can successfully hybridize. The hybrid progeny were found to be more virulent to cereals but also had lower fitness values than both parents. This finding was presented to the entomologists and the cereal breeders as a potential new threat to the agricultural industry, and it emphasizes the need to monitor occurrences of hybrid aphid species.
3. Wheat field image analyses accurately identifies RWA infestations to improve pest management. Multispectral remote sensing has not been widely adopted in agricultural pest management because of inability to distinguish among various factors causing crop stress in multispectral imagery. The new methodology developed by a research team led by scientists from ARS-Stillwater, Oklahoma, overcomes this limitation and can assist growers in making management recommendations at field, sub-field, and multi-field scales at a much lower cost than traditional pest scouting and sampling methods. The team developed novel methodology that combines field-scale multispectral imagery acquired from an airborne platform with spatial pattern recognition algorithms to differentiate wheat fields infested by Russian wheat aphids (RWA) from non-infested fields. Validation showed the methodology to be more than 95% accurate at differentiating infested from non-infested fields. The methodology has been presented at professional meetings as a new precision agricultural approach for pest management where specific problem areas within a larger landscape can be managed more effectively. On a larger, regional scale, this method provides the ability to assign relative pest risk to managed fields and to target field-level activities accordingly.
Elliott, N.C., Kieckhefer, R.L., Phoofolo, M.W. 2012. Foraging by Hippodamia convergens for the aphid Sitobion avenae on wheat plants growing in greenhouse plots. Southwestern Entomologist. 37(4):467-474.