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:
Two SY's retired in FY12 which has left two critical vacancies, and thus Objectives 1A&B, 2C, and 3C were not met. A Research Entomologist transferred to ARS-Stillwater in July 2012 and will be developing new objectives addressing the greenbug problem on cereals and sorghum. Identification of salivary proteins among biotypes within Russian wheat aphid (RWA) and greenbug (GB) has been completed. Major salivary proteins common to all RWA biotypes were considered general plant feeding proteins while proteins unique to a biotype are thought to play a role in gene-for–gene aphid-plant interactions. Salivary analysis in greenbug biotypes confirmed the role of several proteins as general salivary proteins that enable cereal aphids to feed on plant. A number of salivary proteins were unique to this species. Many of the salivary proteins of RWA and GB differed considerably from those reported for nonphytotoxic aphids such as Acyrthosiphon pisum Harris. Based on this information a new phytotoxic aphid-plant interactions model for cereal pests is being developed, as well as new technologies for aphid control (i.e., RNAi based plant resistance). A partial DNA genome assembly for RWA has been completed and will be used to determine the DNA sequences of these salivary proteins for use in a plant mediated RNAi approach to silence these genes in the aphid and prevent infestations. In 2011, over 350 RWA collections from Texas, Colorado, New Mexico, Oklahoma, Kansas, Utah, and Wyoming were made and biotypes were identified. We found that RWA biotype composition has shifted to a new biotype that matches RWA6. These results confirmed data from 2010 where the biotype shift was first detected in Texas. In 2012, about 400 RWA samples (30-50/site) from 10 sites across Texas, Colorado, and Wyoming were collected this summer to determine biotypic variation within fields to better understand biotype distributions. This research is partially supported by a RAMP grant in multi-state research effort to develop an information system (iWHEAT) to inform growers of the current pest status in wheat so that they can make better wheat variety and IPM choices. Sexual morphs of Diuraphis species have been reared under fall greenhouse conditions and photographed for publication. Field plots with wheat and wheatgrass have been established in western Colorado to better characterize the overwintering ecology and biology of this pest in an effort to better document the development on biotypic diversity and the sexual morphs. This research determined that agroecosystems and obligate asexual reproduction heavily influences which biotype is prevalent in the US. Available literature and unpublished data were evaluated to determine the form of acceptable yield loss model to relate the intensity of an RWA infestation in a wheat field to the quantity of wheat yield lost. A quantitative infestation/yield loss model was developed for the Texas High Plains and will serve as an acceptable model for describing RWA population growth and wheat yield loss relationships in the decision support system.
1. Russian wheat aphid biotype composition shifts in the Wheat Belt. A study was initiated to determine the current status of Russian wheat aphid (RWA) biotypes infesting wheat in the western US. Aphid samples were collected from Texas, Colorado, and Wyoming and screened on nine resistant and susceptible wheat and barley entries to determine their biotype. Since 2005, RWA biotype 2 was thought to be the dominant wheat pest in the western states, with RWA biotype 1 resistance no longer useful. Researchers at ARS-Stillwater, Oklahoma, have determined the biotype composition in 2010 and 2011 has shifted back to mainly RWA1, and those varieties carrying RWA1 resistance are again useful in managing this significant pest, thereby reducing insecticide usage.
2. Russian Wheat Aphid (RWA) saliva key to new control approaches. Russian wheat aphid salivary proteins are vital in allowing this aphid to feed on plants. ARS-Stillwater, Oklahoma, researchers, have identified the proteins and their gene sequences, which opens the way to apply new molecular technologies to make wheat, and crops in general, resistant to aphid attack. Plants can be modified to silence the salivary genes in aphids when they feed on the plant, which will prevent further feeding and plant damage.
3. Hungry lady beetles make better hunters. Laboratory and greenhouse experiments were conducted on lady beetles, Hippodamia convergens, to better understand hunting and feeding bahaviors of an important predator of aphids that infest wheat. The experiments showed that moderate starvation typically observed in lady beetles during the spring has two important effects on predation. On one hand, hungry beetles tend to more efficiently search for aphids on individual wheat plants. On the other hand, hunger reduces the ability of beetles to traverse the wheat field in search of cereal aphids. This research is the first to report the dynamics of foraging of hungry beetles in wheat fields, and mathematical models of predator-prey interactions between lady beetles and aphids must include the effects of starvation to accurately describe the biological control potential of these important predatory insects.Elliott, N.C., Kieckhefer, R.W., Phoofolo, M.W. 2011. Functional response of Hippodamia convergens to Sitobion avenae on wheat plants in the laboratory. Southwestern Entomologist. 36(4):423-431.