OKLAHOMA STATE UNIVERSITY
Department of Plant and Soil Sciences, 368 Ag Hall, Stillwater, OK 74078-6028, USA.
The 1998-99 Oklahoma wheat crop averaged 34 bu/acre, but because less wheat was planted (6.4 million acres) than in recent years (6.8 million acres) and far less was harvested for grain (4.3 million acres compared to 5.1 million acres in 1998), the total production was down 26 % from 1998. These differences were due partially to 1) a very wet planting conditions, 2) low price projections, and 3) fields with heavy cheat being hayed or grazed out.
The wheat crop started extremely slow. Following a record-setting hot, dry summer, little volunteer wheat had germinated and many producers were not able to incorporate the previous crop's residue or apply fertilizer. Rains occurred from 13-20 September, depending on the area of the state. By 28 September, only 17 % of the wheat was planted, compared to 37 % for the 5-year average.
Heavy rains fell throughout the autumn across most of the state. Temperatures remained above normal until mid-December. Favorable temperature and moisture resulted in more wheat growth than typical for later planted wheat. Temperatures and rainfall during grain filling were very good, except in places where too much water damaged the wheat. In some areas, test weights were low because the wheat died prematurely from excessive water.
Many of the 'dusted in' wheat fields, or fields where early rain allowed emergence by 20 September, were damaged by armyworms. Many fields had heavy infections of leaf rust in December. Tan spot infections were common in fields where wheat straw was not incorporated during the summer. Speckled leaf blotch was also common in the autumn.
In late winter, aphid numbers increased just before natural predators built to sufficiently high levels to severely reduce the aphid population. Soilborne mosaic virus was severe in many locations. Septoria leaf blotch symptoms were more severe than in several years. Leaf rust, BYDV, and root rots were present in many fields very late in the season. Cheat was present in a very high percentage of the fields and frequently caused yield loss, lodging, and severe dockage.
Harvest looked like it would start early; however, rain started after a few loads were harvested and slowed harvest for the remainder of the season except in the panhandle. Sprout-damaged wheat was a problem in some areas.
Wheat breeding research at Oklahoma State University brings together 10 university and adjunct USDA-ARS faculty, with expertise in wheat breeding and genetics (Brett Carver); wheat management, variety testing, and extension (Gene Krenzer); plant pathology and genetics (Bob Hunger and Jeanmarie Verchot); aphid resistance and genetics (David Porter); wheat transformation (Arron Guenzi); stress physiology (Bjorn Martin); cereal chemistry (Patricia Rayas); and germ plasm development (Art Klatt). The Wheat Improvement Team welcomed Dr. Guihua Bai in August, 1999, formerly with USDA-ARS at Peoria, IL, working as a research molecular biologist in wheat scab resistance. Dr. Bai's research will focus on the use of molecular approaches to improve acid-soil tolerance and disease resistance and to modify plant stature and coleoptile elongation in winter wheat.
During the 1998-99 breeding cycle, the core breeding program increased its emphasis on selection under early-planting conditions, with the intent of developing hard wheat varieties with improved adaptation and end-use quality under a dual-purpose management system. This crop year marked the first in which all early-generation bulk populations (F2 through F4) were planted, evaluated, and selected using a dual-purpose management system. Evaluation of breeding material at Marshall, involves a tri-partnership of resources derived from USDA federal appropriations, state allocations through the Oklahoma Agricultural Experiment Station and the Oklahoma Wheat Commission. Head rows and head-row bulk progenies were placed under observation using a simulated dual-purpose system at Stillwater, OK.
The 1998-99 crop year also marked the beginning of an intensive testing program designed to gather relevant production data on candidate cultivars. This program will allow direct comparison of OSU elite breeding material with cultivars currently produced in the state, under a wider range of production conditions than afforded in the wheat breeding program. Of the 193 breeding lines evaluated in replicated field trials, five lines were tested as candidate cultivars and included in the OSU Wheat Variety Trials conducted by Gene Krenzer. Detailed results of forage and grain production were published in two reports of the Oklahoma Cooperative Extension Service, PT99-2 and PT99-18. These trials identified three lines with competitive performance characteristics, leading to superior grain and forage yields. Some of the key agronomic features considered in rendering a release decision are summarized in Table 1 for those lines, along with their pedigrees.
|Name and pedigree||Leaf rust||Soilborne mosaic virus||Low pH||Dual purpose compatibility *||Test weight|
| OK94P549-2C (HRWW)
|* Dual-purpose compatibility represents several measures of vegetative and reproductive growth.|
The Wheat Improvement Team will seek permission to release OK95G701 and OK95571in the spring of 2000. Both lines were placed under large-scale increase in 1999-2000 and appear to have acceptable milling and bread-making characteristics based on tests conducted by commercial labs and cooperators of the Wheat Quality Council. OK95G701is adapted to western dryland production areas in Oklahoma, whereas OK95571 has potentially broader adaptation by providing better protection against leaf rust, wheat soilborne mosaic virus, and acid soils. OK95G701 or OK95571 was the highest ranking entry for grain yield at four of the eight sites in which they were included in the OSU Wheat Variety Trials conducted by Gene Krenzer. OK94P549-2C, a T1B·1R translocation line, is under reselection to purify this line for plant type and to identify candidates with suitable dough characteristics.
Additional information concerning activity of OSU's Wheat Improvement Team can be found on the Internet at http://clay.agr.okstate.edu/wheat/wit.html.
Wheat forage data were collected from two trials conducted in 1998-99. Autumn forage data can be found in PT 99-2 'Fall Forage Yield Wheat Variety Trials 1998'. Grain yield results were published in PT 99-18 'Wheat Grain Yield from Variety Trials 1998-99'. Variety trial data also can be found on the internet at http://clay.agr.okstate.edu/wheat/wit.html.
We also summarized data concerning when cultivars reach the first hollow stem (FHS) stage of growth and its relationship with heading. Cultivars appear to differ in earliness to FHS and, therefore, how late they can be grazed before reducing grain yield. Current research indicates that most cultivars are consistent from year to year in earliness to FHS, with some exceptions (average rank correlation of 0.69). The cultivar range in the FHS stage may be as much as 22 days within a location. As expected, varieties were quite consistent in ranking for heading (average rank correlation of 0.84). Averaged across the 4 years, the earliest and the latest varieties differed in heading date by 8 days. No relationship between earliness to FHS and earliness in heading is apparent.
More than 1,000 new varieties and advanced lines have been introduced into the wheat improvement program at OSU in the past year. These new genetic materials include 178 advanced lines from the winter wheat program at Vernon, TX; 395 winter wheat and 455 spring wheat materials from CIMMYT, Mexico; 105 primary synthetics from CIMMYT; and materials from Nebraska, Austria, and China. These materials will be utilized as sources of traits such as increased yield potential, better disease and insect resistance (especially stable leaf rust resistance), longer coleoptiles, improved sprouting resistance (for white wheats), acid-soil tolerance, better leaf characteristics, and improved industrial quality. A program to introgress these characteristics into adapted winter wheats is currently in progress.
The wheat improvement program also has initiated a coöperative
wheat breeding program with CIMMYT in Mexico. OSU will serve as
the coördinator for this program, which also will include
the HRWW research programs in Texas, Kansas, and Nebraska. The
specific objectives of the program as related to winter wheats
- enhanced genetic variability,
- improved industrial quality,
- increased yield potential,
- stable leaf-rust resistance,
- improved drought and heat tolerance,
- better resistance to BYDV, and
- broader adaptation of winter wheat varieties.
The program will involve the exchange of germ plasm and widespread testing of materials. The crossing program will be conducted jointly in Oklahoma and by CIMMYT in Mexico. For further details contact Art Klatt or Brett Carver at Oklahoma State University.
Entomology & Plant Pathology Department, 127 Noble Research Center, Stillwater, OK 74078, USA.
Bob Hunger, Kris L. Giles, Larry J. Littlefield, Tom A. Royer, Larry L. Singleton, Jeanmarie Verchot, and Mark E. Payton (Department of Statistics).
Bob Hunger and Jeanmarie Verchot.
BYDV caused some losses in Oklahoma in 1999; however, these losses were sporadic and were significant only in scattered fields. In autumn 1999, incidence of aphid vectors of BYDV was high across much of Oklahoma, and BYDV was identified in many samples as early as December. Severe BYDV is expected to occur in Oklahoma in the spring of 2000.
Growth-chamber research has indicated that bird-cherry oat aphids can significantly reduce wheat seedling root and shoot growth. Current research is determining if seedlings with reduced root and shoot growth recover from this reduction after aphids are removed, and the seedlings are vernalized and grown to maturity.
We also have been working to develop methods to assay for BYDV in wheat, including development of a method to mechanically inoculate wheat leaves with BYDV. Using the hand-held Agun®, we tried to inject virus directly into wheat leaves using helium pressure. We have successfully infected plants using this method; however, only approximately 10 % of the plants became infected, and the virus did not spread systemically to other leaves. Because BYDV is phloem-limited, this method may adequately deliver virus to the vascular tissue.
We also worked to develop an assay to determine the proportion of aphids that transmit BYDV in the field. First, a PCR assay was developed to detect BYDV in aphids and leaves. This assay effectively detected low quantities of virus, but was not highly reproducible and may not be the preferred method. Therefore, we developed an amplified ELISA assay to test for BYDV in individual aphids. This assay is similar to the standard DAS-ELISA, and utilizes an alkaline phosphatase antibody conjugate. The enzyme reaction relies on NADH conversion to NAD, which then stimulates diaphorase to interact with a substrate molecule to produce a red product. The reaction provides a 10-fold amplification of the routine DAS-ELISA. This translates into a more sensitive assay that can detect minimal quantities of virus. Thus, this assay appears to be ideal for studying BYDV transmission by aphids, and we are currently working to optimize the assay conditions.
Kris L. Giles and Tom A. Royer.
Aphid sampling in wheat (Kris Giles, Tom Royer, Norm Elliott, and S. D. Kindler). The autumn of 1999 saw a severe outbreak of greenbug in winter wheat. Much of the infestation occurred north of highway I-40. In general, late-planted fields sustained more injury. We completed data collection for development of presence-absence sampling plans for greenbug and bird cherry oat aphid and are developing two research papers for publication. Research suggests that different plans will be needed to accurately assess fall populations of greenbugs relative to spring populations.
Refinement of injury thresholds in winter wheat (Kris Giles, N.C. Elliott, Frank Tao, Tom Royer, and Gerrit Cuperus). This research is in its second year. Research plots are located at four sites, and two cultivars are being evaluated at each site. Once finished, this work will be integrated with the sampling information to develop a new set of guidelines for managing greenbug infestations in winter wheat.
Development of natural enemy thresholds in winter wheat (Kris Giles, Doug Jones, Norm Elliott, and Tom Royer). The research is in its second year. The first year of cage studies, designed to evaluate natural enemy impact on aphids, was completed at Chickasha. The second year of these cage studies is now underway, and results are being validated with 10 1-acre plots located in five different counties. This information will be integrated with the injury thresholds and sampling plans for greenbug to provide a comprehensive management program for greenbug in winter wheat.
Interaction of planting date and aphid management strategies in wheat (Tom Royer, Kris Giles, and Eugene Krenzer). Two years of research have been completed. Experiments conducted at Lahoma and Perkins suggested that seed treatments with imidacloprid and top-dress applications of a foliar insecticide can reduce aphid numbers. Results of this research currently are being written for publication.
Revision of B-831, Wheat Production Handbook (Tom Royer, Eugene Krenzer, and Jeff Miller). At present, the handbook has been revised extensively and is waiting for placement of photographs and editing. We are setting a target date of late spring for printing.
Effects of grazing on aphids in winter wheat (Kris Giles, Gene Krenzer, Gerald Horn, Tom Peeper, Jerry Michaels, and Tom Royer). A grazing study is being initiated to understand the effects of grazing on greenbug population dynamics and injury to winter wheat.
Evaluation of tri-trophic interactions among varieties of resistant and susceptible winter wheat, cereal aphids, and natural enemies of cereal aphids (Kris Giles and Roger Fuentes). Studies have been initiated to evaluate the interaction of host plant resistance, cereal aphids, and their natural enemies. Results will allow for a greater understanding of how and why natural enemies sometimes can control aphids.
Total yield losses from foliar diseases in Oklahoma in 1998-99 were minimal, although losses in specific fields were significant. Leaf rust and Septoria leaf blotch/Stagonospora glume blotch accounted for the major yield reductions. Usually, Septorial leaf/glume blotch is a minor problem in most of Oklahoma; however, abundant moisture and cool temperatures in April and May resulted in severe Septorial leaf/glume blotch in eastern and northcentral Oklahoma. The reactions of some popular cultivars planted in Oklahoma to leaf rust and Septorial leaf/glume blotch as observed in field plots in 1999 are given in Table 1.
|Cultivar||Leaf rust IT||SLB/SGB IT||Cultivar||Leaf rust IT||SLB/SGB IT|
|Agseco 7853||3||4||Tam 302||2||5|
The seedling and adult plant reactions of the 1999 SRPN to wheat leaf rust were determined and published in volume 15 (1999) of Biological & Cultural Tests for Control of Plant Diseases, which is published by American Phytopathological Society (APS) Press. The SRPN is a compilation of advanced breeder lines from university and private breeding programs in the Great Plains.
Bob Hunger and Jeanmarie Verchot.
Wheat soilborne mosaic incidence and severity were back to normal in Oklahoma in 1999 after being extremely mild in 1998. The WSBMV reactions of the wheat varieties most commonly cultivated in Oklahoma are presented in Table 2.
|Cultivar||Visual rating *||ELISA **||Cultivar||Visual rating *||ELISA **|
|Big Dawg||1||-||Tam 110||4||+|
|* Ratings were taken in one rep of a variety-demonstration
plot near Stillwater, OK, on 24 February, 1999. Ratings are on
a scale of 1-4, where 1 = no stunting or mosaic, 2 = slight stunting
and/or mosaic, 3 = moderate stunting and/or mosaic, and 4 = severe
stunting and/or mosaic. Values of 1 and 2 are considered resistant;
values of 3 and 4 are considered susceptible.
** A - value from ELISA indicative no WSBMV, a + indicates a value indicates presence of WSBMV, and a ± indicates a value indicates WSBMV at the positive/negative threshold.
Experiments were initiated to compare the effectiveness of host resistance in Tonkawa, Newton, and Hawk (HRWWs) against WSBMV. Results indicate that Newton is the most resistant to soil inoculation of the virus, whereas Tonkawa appears to be tolerant. Newton shows stunted growth after inoculation with the pathogen, but the virus does not accumulate in aerial plant parts. On the other hand, Tonkawa is more vigorous and does not produce symptoms even when low levels of virus are detected in the leaves. We also have developed a technique to mechanically inoculate wheat leaves with virus. All varieties are equally susceptible to virus inoculation, suggesting that resistance functions in the roots.
Currently, we are conducting experiments to identify the barrier to infection in the roots of these varieties. We have established a hydroponic system to provide high levels of root inoculation of wheat plants. First, wheat seeds are planted in growth pouches that provide a window for studying root development. Roots then are inoculated with purified virus or with viruliferous fungus. The response then can be observed and measured through the window. Resistance may appear as a necrotic response. Second, we have established a more elaborate hydroponic ebb and flow system that will serve to cultivate large quantities of fungus and provide optimal levels of inoculation. This system will allow us to study virus-host and host-vector interactions. Currently, experiments are being conducted to determine if a resistance response occurs in the roots to WSBMV and which tissues provide a block to virus infection. Electron microscopic research on the ultrastructure of P. graminis and immunogold localization of WSBMV in host and vector cells is being continued.
Hard red winter wheat germ plasms developed from crosses between HRWW adapted to Oklahoma and emmers collected in Turkey and Israel currently are being grown in the field as single plants in a WSBMV nursery. These crosses were made to combine leaf rust resistance from the HRWW with resistance to WSBMV identified in the emmers. Plants identified as having resistance to both diseases will be increased and tested for possible release as germ plasm.
Seed of five winter wheats from Italy were obtained and will
be tested for reaction to leaf rust and WSBMV. The reactions of
these winter wheats will be compared to the reactions of 12 winter
wheats sent to Italy for similar testing.
Other lines involving crosses with various sources of resistance to leaf rust (e.g., germ plasm from South Africa and Eastern Europe) are at various stages of testing in the greenhouse and the field.
Larry Singleton and Bob Hunger.
Commercial wheat produced in Oklahoma in 1999 was examined for the presence of teliospores of T. indica. Testing was conducted using methods and following protocols approved by the Animal and Plant Health Inspection Service (APHIS). In 1999, 69 samples collected from elevators in 18 counties were tested, which satisfied the APHIS testing program. Testing has been conducted every year since 1996, but no positive samples have been found.
Wheat root-rot report 1998-99 crop season. Isolates of R. cerealis (sharp eyespot) and R. solani are being tested in microplots in the field for their influence on stand establishment, forage production, and yield. Other parameters included in the study are planting date (early, 10 September and late, 20 October) and soil fumigation with methyl bromide. Analysis of first-year results indicated an isolate by planting date interaction. However, reductions in stand counts due to pre- and postemergence seedling death were higher with some isolates in the early planted plots where soil temperatures were higher than at the later planting date. The test is being repeated this year.
At Perkins, OK, nine fungicide seed treatments ((Metalaxyl + Triticonazole (3 rates) + (EXP80472J and EXP80991J (1 rate); (maneb +Triticonazole), (Triticonazole + thiram), (raxil - thiram), (difenoconazole - mefanoxam), (V200 - IMAZ) (1 rate)) were tested in the field for control of Rhizoctonia and Fusarium spp. Disease and yield parameters were not significantly different at this location. Root and crown rot damage was intensified greatly by mild spring temperatures and excessive moisture conditions through harvest.
Variety and planting date effects on sharp eyespot and Fusarium root rot were studied at two locations in 1997-98. These studies included six HRWW varieties (Custer, Chisholm, TMP 64, TAM101, TAM202, and TAM107) treated with Gaucho (Gustafson, Inc.) at 3 oz/cwt. A split-plot design with four replications was used with the main plots being planting dates (11 September, 29 September, 10 October, and 22 October, 1997) and varieties being subplots. Results indicated that planting dates did not have a consistent effect on the severity and incidence of sharp eyespot and Fusarium root rot. Later planting reduced the severity and incidence of sharp eyespot and Fusarium root rot at one location but not at the other. No clear relationship was observed between varieties and root rot disease parameters, but Triumph 64 had the lowest disease severity and incidence values of the six varieties regardless of location. Fusarium root rot was the major disease at these locations.
Continued recommendations for areas of chronic root rot disease pressure include cultural control by delayed planting (15th October) as an effective alternative to early planting, especially in known root-rot-prone fields. Because these are management types of decisions, these methods are cost effective and do not require major dollar investments for the producer. Planting in mid-October when soil temperatures at planting depth are lower than 77°F offers a mechanism for escaping seedling infection by this group of root rot pathogens. By contrast, early planting (September 1st) when soil temperatures can be 87°F or greater can result in greater potential for root rot infection and damage. For a more detailed discussion of control measures, see OSU Extension Fact Sheet. No. 7622.
Brian Olson, an M.S. student working with Bob Hunger to determine the effects of bird-cherry oat aphids on wheat seedlings, recently accepted the position of Plant Disease Diagnostician at O.S.U. Brian will continue his M.S. research while in this position.
PLANT SCIENCE RESEARCH LABORATORY, USDA-ARS
1301 N. Western St., Stillwater, OK 74075, USA.
C.A. Baker, J.D. Burd, M.H. Greenstone, S.D. Kindler, D.R. Porter, K.A. Shufran, and J.A. Webster.
Efforts are ongoing to develop RWA-resistant wheat germ plasm. Selfed seed from 143 segregating winter wheat lines and 164 spring wheat lines derived from 13 different RWA-resistance sources were planted for seed increase and evaluation in preparation for germ plasm release. Individual plants were harvested separately in order to allow the identification of homozygous resistant lines through progeny testing. Homozygous RWA-resistant lines will be selected for germ plasm release. Less advanced material has been developed from at least 24 different resistance sources and involves five different market classes of wheat. Field evaluations of RWA-resistant spring lines were done in Pullman, WA. Over 175 advanced winter lines were planted for 1998-99 field evaluation in Stillwater, OK. Replicated yield trials of nine promising winter lines were planted in irrigated and dryland field tests in Goodwell, OK. An additional 80 lines were planted under irrigation in Goodwell for observation. Development of 'resistant x resistant' populations continued in order to study genetic diversity of RWA resistance.
Aphid biology and ecology. Research has shown that aphids collected from noncultivated hosts are genetically more variable than those from crop hosts (based on mtDNA sequences). The large genetic variation within biotype I shows that it is probably polyphyletic in origin. In addition, the greenbug species appears to be divided into genetically divergent races that have different host preferences.
A molecular phylogenetic analysis was conducted on nine greenbug biotypes (B, C, E, F, G, H, I, J, and K); a new biotype from Canada wild rye (CWR); a probable example of biotype A from New York; an isolate from Germany (EUR); and S. rotundiventris (Signoret). Based on a 1.2-kb portion of the cytochrome oxidase I mitochondrial gene, three clades within S. graminum were identified. Clade 1 contained the wheat and sorghum biotypes (C, E, K, I, plus J) and was the most homogeneous group. The rarer and more virulent biotypes were placed in two other clades; Clade 2 (F, G, and NY) and Clade 3 (B, CWR, and EUR). Biotype H fell outside the above clades with the outgroup and may represent another Schizaphis species. Greenbug biotypes include a mixture of genotypes belonging to three subspecific groups and may have diverged as host-adapted races on wild grasses.
Field studies to monitor the demographic range of the sexual reproductive cycle of greenbugs are continuing. Field plots have been established in South Dakota, Nebraska, Kansas, Oklahoma, and Texas. Locally collected greenbugs and an Oklahoma biotype I population are being used to assess reproductive success.
The Cereal Insect Genetics Resource Library (CIGRL) is accepting collections of all cereal aphids and their parasitoids for inclusion into the library. Many acquisitions have been cataloged and are available for research purposes. Anyone who would like to contribute to the library or request specimens for research should contact Melissa Burrows (email@example.com).
Economic thresholds. Data from a 4-year study of the effects of greenbugs on yield of Karl (1994) and Karl 92 (1996, 1997, and 1998) winter wheats have been compiled and partially analyzed. Greenbug feeding-days generally were more strongly negatively correlated with yield than the maximum number of greenbugs per stem, for wheat infested with greenbugs during the autumn and during spring. Correlations between greenbug feeding-days and yield ranged from -0.25 to -0.84 for autumn infestations and from 0.23 to -0.74 for spring infestations. The positive correlation of 0.23 was for spring infestations in 1994 when we were unable to establish high greenbug infestations on wheat.
Evaluation of aphid natural enemy effectiveness. PCR primers have been developed successfully to distinguish six members of the U.S. Great Plains cereal aphid complex: greenbug, RWA, bird cherry-oat aphid, corn leaf aphid, yellow sugarcane aphid, and English grain aphid. We used the primers to detect corn leaf aphid and bird cherry oat aphid in extracts of fed Hippodamia convergens and Chrysopa plurabunda. The half-lives for detection of DNA of a single corn leaf aphid are 3.95 hrs for C. plorabunda and 8.78 hrs for H. convergens.
We used previously developed PCR primers to demonstrate that the aphelinid Aphelinus hordei was recovered following release in South Africa.
Preliminary taxonomic analysis of predators that were quantitatively collected (by D-vac from the vegetation and by hand at the soil surface) from wheat fields in Lahoma, OK, during the spring 1998 field season has been completed. The most abundant group was spiders, which constituted 39 % of individuals. The remaining groups were staphylinids 21 %, nabids 13 %, carabids 12 %, coccinellids 9 %, and chrysopids 6 %. Of the spiders, 39 % were linyphiids and 38 % tetragnathids, with no other family representing more than 7 % of the total. The linyphiids were represented by Tennesseellum formicum, Erigone autumnalis, and species of Ceratinopsis and Glenognatha; the staphylinids by Tachyporus jocosus, T. maculicollis, and undetermined species of Astenus and Stenus; and the carabids by Bembidion nigripes, B. castor, Elaphropus dolosus, Stenolophus conjuctus, Lebia atriventris, Microlestes linearis, Discoderus parallelus, and Harpalus faunus. The nabids were predominately Nabis roseipennis.
Ecology and modeling. Previous studies have demonstrated that certain widely distributed cool-season grasses are inhabitable during summer (after wheat harvest), but often can be considered sink habitat for the RWA in which mortality exceeds reproduction. A spatially explicit simulation model of RWA population dynamics was developed using an object-oriented design. The structure of existing landscapes throughout the High Plains region was determined by analysis of classified Landsat Multispectral Scanner imagery. Simulated landscapes with structure similar to that of real landscapes were generated. Analysis of RWA population dynamics by the model on simulated landscapes indicated that both the abundance and spatial distribution of patches of sink habitat were important in determining the timing of colonization of cultivated winter wheat by RWAs in autumn and the size of populations achieved prior to the onset of winter. In the absence of a necessary sink habitat, autumn RWA populations in cultivated winter wheat are reduced greatly by high mortality during the period between wheat harvest and planting and poor success in locating and colonizing patches of cultivated wheat in autumn. Insufficient cool-season grass habitats in the southern High Plains may explain the general absence of economically important RWA infestations in winter wheat in this region.