Project : USDA ARS

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Research Project: Management of Aphids Attacking Cereals

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2019 Annual Report

Objectives
The long-term objective 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 objective enhancing the role of aphid host plant resistance and natural enemies for IPM programs and providing fundamental knowledge of cereal aphid biology and ecology is required. Over the next 5 years we will focus on the following objectives: Objective 1: Determine the distribution and diversity of resistance-breaking biotypes of cereal aphids in the Great Plains states, identify new sources of resistance for wheat and sorghum, and transfer into suitable genetic backgrounds, to facilitate development of new aphid resistant cereal varieties. Subobjective 1A: Characterize the biotypic structure of Russian wheat aphid (RWA) populations in wheat and non-cultivated grasses to address biotypic diversity to provide knowledge needed to develop and deploy durable RWA resistance in wheat and barley. Subobjective 1B: Identify, characterize, and introgress greenbug resistance sources/genes into wheat germplasm. Objective 2: Determine the distribution and severity of sugarcane aphid in sorghum in the Southwest United States, identify resistant germplasm in sorghum, and evaluate population dynamics to assess the potential for development of resistance-breaking biotypes in this aphid species. Subobjective 2A: Identify sorghum germplasm with resistance to sugarcane aphid and determine the mechanisms of resistance. Subobjective 2B: Determine if biotypes exist in sugarcane aphid populations that can overcome sugarcane aphid resistance in sorghum. Objective 3: Develop and refine methods for field, landscape, and area-wide scale approaches for detecting and monitoring invasive aphid infestations, and optimizing invasive aphid biological control methods in wheat and sorghum. Subobjective 3A: Develop and refine methods for aphid infestation detection and monitoring in wheat and sorghum based on spatial pattern analysis of multispectral remotely sensed imagery. Subobjective 3B: Assess resource availability and diversity for the aphid parasite Lysephlebus testaceipes across a range of landscape/agroecosystem diversity levels. Objective 4: Apply knowledge obtained from aphid genome and transcriptome sequencing to develop plant mediated or other delivery methods for RNAi silencing of critical genes for aphid survival in a broad range of aphids affecting cereals.

Approach
Field and laboratory experiments will be conducted to : (1) characterize the biotypic structure of Russian wheat aphid (RWA) populations in wheat and non-cultivated grasses to address biotypic diversity to provide knowledge needed to develop and deploy durable RWA resistance in wheat and barley; (2) identify, characterize, and introgress greenbug resistance sources/genes into wheat germplasm; (3) identify sorghum germplasm with resistance to sugarcane aphid and determine the mechanisms of resistance; (4) determine if biotypes exist in sugarcane aphid populations that can overcome sugarcane aphid resistance in sorghum; (5) develop and refine methods for aphid infestation detection and monitoring in wheat and sorghum based on spatial pattern analysis of multispectral remotely sensed imagery; (6) assess resource availability and diversity for the aphid parasite Lysephlebus testaceipes across a range of landscape/agroecosystem diversity levels; and (7) apply knowledge obtained from aphid genome and transcriptome sequencing to develop plant mediated or other delivery methods for RNAi silencing of critical genes for aphid survival in a broad range of aphids affecting cereals.

Progress Report
Under Objective 1A, in FY 2019, the 26 Russian wheat aphid (RWA) samples that were collected from wheat during 2016-2018 in the Rocky Mountain and Southern Plains states were found to be primarily biotype 70% RWA6 and 21% RWA2. RWA6 is the least virulent biotype while RWA2 was the most virulent to wheat. Aphid populations, including RWA, were extremely low from 2016-2018 throughout the multi-state study area in contrast to an 2011-2013 study due to extreme temperature and rainfall conditions. However, Sipha maydis, a recently introduced cereal pest, was more abundant and widespread than RWA on both wheat and wild grasses. Additional research was conducted on S. maydis resistance in barley, wheat and sorghum. Strong resistance to S. maydis was identified in barley (cv 'Post 90'), however, only moderate resistance was found in wheat and sorghum. Results of the S. maydis distribution and host range have been published in 2019, and two manuscripts are being prepared on the results for S. maydis resistance in barley, wheat, and sorghum. Under Objective 1B, the greenbug (GB) resistance gene in wheat reselection line PI 595379-1 has been characterized and permanently designated as Gb8. Gb8 was mapped to an interval of 3.1 Mb in the terminal bin of chromosome 7DL (7DL3-082-1.0), spanning 595.6 to 598.7 Mb in the Chinese Spring IWGSC RefSeq v1.0 reference sequence. An SSR marker co-segregating with Gb8 was also developed for marker-assisted selection. Allelism tests indicated that Gb8 is a new gene different from three permanently named genes on the same chromosome arm. Gb8 confers a broader range of resistance than other known genes, and can be directly used in wheat breeding to enhance resistance to greenbug biotypes B, C, E, H, I, and FL. Under Objective 2A, several sources of resistant sorghums have been identified, registered, and released as parental lines that have helped the sorghum breeding Industry with many choices from the public access and incorporated into breeding programs. A total of 38 parental lines are available from the release in association with USDA-ARS Laboratories and in collaboration with Texas A&M and Kansas State University breeding programs. Evaluations for host plant resistance are continuing from the converted world sorghum line collections and this could conceivably take another 12 months to complete. The mechanisms of host-plant resistance in sorghum germplasm to the sugarcane aphid has recently been discovered and published. Under Objective 2B, different host races (biotypes) of the sugarcane aphid were discovered within the U.S. as a result of phenotyping and genotyping. The two host races are identified as being the sugarcane strain from clonal lineage MLL-D, while the economically threatening MLL-F lineage that is currently threatening sorghum production in the U.S. is also known as the sorghum type. We evaluated over 35 clonal collections from all known hosts and locations after identifying a reliable source of host differentials. A continuation of evaluations will continue as host races are known to change in monoculture agricultural production systems. Under Objective 3A, imagery was analyzed for eight sugarcane aphid infested grain sorghum fields in Oklahoma and Kansas. A time sequence of imagery of selected fields was analyzed to determine change over time in reflectance and spatial pattern of sugarcane aphid infestations in grain sorghum fields. Objective 3B, nine greenhouse grown plants (wheat or canola) infested with bird cherry-oat aphids (wheat) or turnip aphids (canola) were placed in each of eight study fields of wheat and canola. The study was repeated twice during the growing season. The plants were left in the field for three days and then caged and returned to the greenhouse to allow parasitoids to develop. Adult parasitoids from plants were counted and identified to species. A DVAC sampler was used to estimate the density of aphids in each field. Parasitoid exclusion cage experiments were conducted in each wheat field in late October and again in mid-March in each wheat and canola field. At the end of four weeks the foliage from within each cage was cut and transported to the laboratory where numbers of aphids and mummified aphids were counted. Under Objective 4, testing of dsRNA did not identify new constructs that could provide at least 75% mortality in feeding trials. To identify new salivary protein candidates, salivary analysis of sugarcane aphid strains from sorghum and sugarcane were initially analyzed broaden our database on aphid salivary proteomes, identify new protein targets with conserved sequences across aphid species and target with RNAi, and to determine is biotypes exist between sorghum and sugarcane populations. Analyses will continue in FY2019 to conclude this comparison and publish the results. Information provided will be used to develop and screen dsRNA constructs that will be suitable for a virus-induced gene silencing (VIGS) for wheat. For the subordinate project, 'Areawide Pest Management of the Invasive Sugarcane Aphid in Grain Sorghum', the following progress was made. Objective 3, the myfields network was tasked reporting of sugarcane aphid using unified methods to achieve multi-state reporting to a national distribution map. A webservice between myFields.info and EDDSMaps.org was created and modified as needed, and 114 presence reports were submitted via this webservice. New county reports trigger email alerts to specialist users on myFields. Under Objective 4, two sorghum growing areas in South Texas, one of susceptible sorghum consisting of 8,240 acres and the other of resistant sorghum consisting of 2,850 were intensively monitored weekly for sugarcane aphid and natural enemies. The resistant area showed lower sugarcane aphid populations and reduced damage. Under Objective 5-a, more than 3,000 sorghum lines were screened in the greenhouse, several of those were evaluated in the field. Over 40 sorghum germplasm lines with resistance to sugarcane aphid were registered and released to breeders. Phenotyping studies confirmed the presence of two biotypes of sugarcane aphid in the United States. Quantitative trait loci (QTL) were identified that are associated with the resistance to sugarcane aphid. A total of 22 crosses were made with the newly identified sources in order to move resistance genes into adapted backgrounds. Field trials of 44 selected breeding lines and 12 commercially available varieties were made at multiple locations in Oklahoma and Texas to evaluate their responses to sugarcane aphid. In addition to advancing a set of breeding crosses initially made in 2015 for new germplasm development, the sorghum association panel (SAP) with 296 inbred lines was evaluated for aphid resistance for the second year under the field conditions. Under Objective 5-b, ARS scientists developed the initial version of a spatially-explicit, individual-based, stochastic model that integrates the life cycle and aeroecology of sugarcane aphid to forecast regional infestations of sorghum fields. We developed a methodology to couple a meteorological model (HYSPLIT) to our custom-made sorghum/sugarcane aphid simulation model. Under Objective 5-c, an experiment on synergism of aphid-resistant hybrid and insecticide application to simulate spatio-temporal patterns of aphid infestation with a large sorghum plot size was conducted in 2018 in the Southern Plains. The phenomenon of devastating aphid damage on grain/forage/silage sorghum in a duration of 6-week infestation was examined. The experiment utilized the commercial resistant/susceptible hybrids and chemical/microbial insecticides to accelerate epizootics of entomopathogens. Under Objective 6, nine presentations about Sugarcane Aphid management and programs were made to grower groups, Extension Educators, and other professionals. The sorghum component of an existing web site was restructured for use in the areawide project. A brief description is given for each available publication at https://betteryield.agrilife.org/publications/. Resources were made available for linking to individual state websites. We also participated in State Varity Trials to deliver timely information on the best choices of grain/forage/silage sorghum hybrids with sugarcane aphid resistance. Under Objective 7, We began the development of a spatially explicit economic model coded in the GAMS environment but with substantial interface and linkages with GIS ArcMap software.

Accomplishments
1. Greenbug is a worldwide insect pest that poses a serious threat to wheat production. New greenbug resistance genes that can be readily used in wheat breeding are urgently needed. ARS scientists at Stillwater, Oklahoma, discovered a novel greenbug resistance gene, designated Gb8, in wheat resection line PI 595379-1 and placed it to a 3.1 Mb interval in the terminal bin of chromosome 7DL (7DL3-082-1.0), spanning 595.6 to 598.7 Mb in the Chinese Spring IWGSC RefSeq v1.0 reference sequence. An SSR marker co-segregating with Gb8 was also developed for marker-assisted selection. Gb8 confers a broader range of resistance than other known genes, and can be directly used in wheat breeding to enhance resistance to greenbug biotypes of agronomical importance, including biotypes B, C, E, G, H, I, and FL.

2. Developing sugarcane aphid resistant sorghum save producers millions of dollars. The most significant advantage in breeding sorghum for resistance to sugarcane aphid is knowledge of the mechanisms of resistance within the sorghum germplasm. In some instances, there exists more than one form of expression (antixenosis, antibiosis, and tolerance) within the same plant. These single or multiple forms of resistance were identified in several sorghum germplasm and the heritability of these forms is transferred from parental lines used in traditional breeding by ARS scientist in Stillwater, Oklahoma. Sorghum producers across the sorghum belt of the U.S. now have a multitude of choices to plant sugarcane aphid resistant sorghums from this work.

3. Monitoring and predicting economically threatening sugarcane aphid populations can literally save millions of dollars in preventative measures. The U.S. typically produces over 400 million bushels of sorghum valued at over $1.5 billion on more than 6 million acres. Sugarcane aphid is an invasive pest of sorghum, and is a severe threat to the economic viability of sorghum production in Kansas, Texas, and Oklahoma, which accounts for about 82% of grain sorghum production nationwide. Effective insecticides have been identified, helping to reduce damage, but monitoring sugarcane aphid infestations in sorghum fields is essential to economically optimal management. ARS scientist in Stillwater, Oklahoma, developed sugarcane aphid infestation monitoring methods based on acquisition and analysis of multispectral imagery obtained from an aerial platform, in this case a fixed wing aircraft, which can delineate spatially variable infestations of sugarcane aphid in sorghum fields and detect change in spatial extent and infestation intensity. The ARS research facilitated novel methods for monitoring sugarcane aphid infestations in sorghum fields that are applicable for making insecticide control decisions at the whole field and intrafield scales. Further development and application of the research could reduce the number of improperly timed or unnecessary insecticide applications, which would be an economic benefit to sorghum growers.

U.S. DEPARTMENT OF AGRICULTURE

Plains Area