OKLAHOMA
OKLAHOMA STATE UNIVERSITY
Entomology & Plant Pathology Department, 127 Noble Research Center, Stillwater, OK 74078, USA.
Bob Hunger, Larry L. Singleton, Jeanmarie Verchot, and Mark E. Payton (Department of Statistics).
Bob Hunger and Mark Payton.
Aphids were present in fields across Oklahoma during the autumn
of 1999 and early winter of 2000, but BYDV was present sporadically
and losses due to the aphid/BYDV complex were not severe. Replicated
field trials testing Gaucho (imidacloprid, Gustafson Corp., Dallas,
TX) insecticide on Karl 92 wheat at rates of 03 oz/cwt were
planted at two location in Oklahoma. Results from these trials
demonstrated aphid control with increasing Gaucho rates at both
the Stillwater and Chickasha locations (Table 1). However, controlling
aphids did not translate into control of BYDV incidence or severity
or to an increased yield, test weight, or 1,000-kernel weight.
| Gaucho rate (oz/cwt) | Number aphids/ft of row | Incidence * (%) | Severity ** (1-50) | Yield (bu/A) | Test weight (lb/bu) | 1,000-kernel weight (gm) | ||
|---|---|---|---|---|---|---|---|---|
| 28 Oct 99 | 30 Nov 99 | 16 Feb 00 | ||||||
| Stillwater | ||||||||
| 0.0 | < 1.0 | 27.0 a *** | 17.1 a | 47.1 a | 9.1 a | 40.4 a | 60.4 a | 30.1 a |
| 0.5 | < 1.0 | 12.3 b | 13.5 b | 48.3 a | 11.2 a | 39.9 a | 60.3 a | 30.1 a |
| 1.0 | < 1.0 | 8.0 c | 6.3 c | 43.3 a | 8.4 a | 42.7 a | 60.2 a | 29.9 a |
| 1.5 | < 1.0 | 5.6 d | 5.3 c | 44.2 a | 7.8 a | 42.5 a | 60.0 a | 30.0 a |
| 2.0 | < 1.0 | 2.5 e | 2.0 d | 49.2 a | 4.5 a | 40.4 a | 59.9 a | 28.9 a |
| 3.0 | < 1.0 | 0.4 e | 0.4 d | 53.3 a | 9.4 a | 38.8 a | 60.0 a | 28.9 a |
| Chickasha | ||||||||
| 29 Oct 99 | 18 Nov 99 | 22 Feb 00 | ||||||
| 0.0 | 35.7 a | < 1.0 | 52.4 a | 60.0 a | 8.7 a | 27.2 a | 52.4 ab | 23.0 a |
| 0.5 | 21.9 b | < 1.0 | 30.2 b | 58.3 a | 7.5 a | 25.5 a | 51.9 a | 22.6 a |
| 1.0 | 15.3 c | < 1.0 | 20.8 c | 63.8 a | 6.8 a | 27.5 a | 53.0 b | 23.5 a |
| 1.5 | 2.2 d | < 1.0 | 19.6 c | 53.3 a | 7.9 a | 26.8 a | 52.5 b | 23.3 a |
| 2.0 | 0.9 d | < 1.0 | 4.6 d | 55.4 a | 6.6 a | 27.0 a | 52.9 b | 23.2 a |
| 3.0 | 0.2 d | < 1.0 | 0.7 d | 58.3 a | 6.3 a | 27.6 a | 52.9 b | 23.2 a |
| * Incidence indicates the percentage of
the plot showing symptoms of BYD, i.e., purpling, yellowing,
and/or stunting. ** Severity was determined using a scale of 050 where 0 = no disease; 1 = trace of BYD symptoms observed; 5 = scattered BYD symptoms observed; 15 = moderate to high incidence of BYD symptoms, but no or very little stunting and/or plant death; 25 = high incidence of BYD symptoms, with some stunting and/or plant death; 50 = high incidence of BYD symptoms, with the majority of plants stunted and/or dead. *** Means within columns followed by the same letter are not significantly different using Fisher Least Significant Difference procedure (LSD) and alpha = 0.5. |
||||||||
Bob Hunger.
Wheat leaf rust was sporadic across Oklahoma in 2000 and caused only minimal yield reductions. By comparison, the incidence and severity of wheat stripe rust was the highest seen in Oklahoma in at least the last 20 years. However, even at this elevated incidence, stripe rust still did not cause major reductions in yield. This higher incidence of stripe rust was attributed to the extremely mild and moist winter, which provided conditions favorable for stripe rust to overwinter and increase in Oklahoma. Other foliar diseases observed in Oklahoma included leaf and glume blotch, tan spot, and powdery mildew.
Bob Hunger and Jeanmarie Verchot.
The incidence and severity of WSBMV in Oklahoma for the 1999-00 season was typical, with losses occurring only in fields in which WSBMV was present that were planted with varieties susceptible to WSBMV.
Bob Hunger and Jeanmarie Verchot.
Six winter wheat germ plasm lines were approved for release by the Oklahoma Agricultural Experiment Station in February, 2001, that carry resistance to wheat leaf rust from winter wheats developed in Hungary and Bulgaria. Five of these germ plasm lines also have resistance to WSBMV from winter wheats developed in the U.S. These six germ plasms are designated as OAES-01, OAES-02, OAES-03, OAES-04, OAES-05, and OAES-06.
Development of winter wheat germ plasm using wheat leaf rust resistance from South African wheats and resistance to WSBMV from emmer wheats collected in Turkey and Israel also is continuing. With both of these sets of germ plasms, lines currently are being tested in field trials.
Bob Hunger and Larry Singleton.
Commercial wheat produced in Oklahoma in 2000 was examined for the presence of teliospores of T. indica, the causal agent of Karnal bunt. Testing was conducted using methods and following protocols approved by the Animal and Plant Health Inspection Service (APHIS). In 2000, 64 samples collected from elevators in 22 counties were tested, which satisfied the APHIS testing program. Testing has been conducted every year since 1996 in Oklahoma, with no positive samples found.
Ms. Aswathy Sreedharan completed her M.S. degree in Plant Pathology in December, 2000. Bob Hunger and Larry Singleton served as her co-advisors. The title of Ms. Sreedharan's thesis was 'A Characteristics of Rhizoctonia spp. isolated from wheat and peanut and their pathogenicity and virulence on hard red winter wheat'.
Department of Plant and Soil Sciences, 368 Ag Hall, Stillwater, OK 74078-6028, USA.
2000 Oklahoma winter wheat crop.
E.G. Krenzer.
The 199900 wheat crop was the fourth consecutive crop with above average yield (2.9 megagrams/ha). The planted and harvested area was down to 2.4 and 1.7 million hectares, respectively.
Although soils remained dry in early September, on 12 September, 7 % of the wheat was planted and 34 % was plantedby 27 September, compared to 28 % for the 5-year average.
After September rains, many areas of the state turned dry again before the wheat was planted and hopes for forage to graze cattle dwindled. Temperatures remained above normal. In the northern half of the state, rainfall was above normal in November. The temperature and moisture combined to result in more wheat growth than expected by 1 January. The southern half of the state remained dry throughout the autumn. Wheat in many fields had not emerged by mid-December.
The primary pest problems during the autumn of 1999 were aphids and the brown wheat mite. Greenbugs and/or oat-bird cherry aphids were present in sufficient numbers that fields were sprayed. In particular, greenbugs were a severe problem in two- to three-leaf wheat in November. A few fields were verified to have BYDV as early as mid-December. Root rots became apparent in January. Fields looking unusually purple were quite common in northcentral Oklahoma in January, but no common cause could be identified. Low pH, low phosphorus, and BYDV were eliminated as causes in most of the fields. We concluded that the purpling was probably caused by anthocyanin accumulation. However, we were not able to identify why this occurred in some fields and not others. Strawbreaker caused severe lodging in many acres in northcentral Oklahoma and into Kansas.
Winter and spring were unusually warm and wheat development was well ahead of normal at first hollow stem and heading. We were very fortunate not to have a freeze in March or April. Late May temperatures were over 38°C for several days in parts of the state, hastening maturity and harvest. Rains curtailed harvest about 13 June with about 80 % harvested. The remainder of harvest was exceptionally difficult because of wet conditions. Sprouting in the head was common in the late-harvested wheat, irrespective of kernel color.
Gaucho is a seed treatment that controls aphids early in the season. By controlling aphids, it reduces or eliminates early infections of BYDV that can strongly reduce wheat yield. Trials at a few locations in recent years indicate economic benefits to Gaucho. Therefore, we wanted to investigate the benefits over many locations and included a 2174 treated with Gaucho at 0.85 g/kg wheat seed to compare with untreated 2174 at all locations. Averaged across 22 trials this year, wheat yield was increased 54 kg/ha with Gaucho. This yield was not statistically significant or economically profitable.
B.F. Carver.
The Oklahoma Agricultural Experiment Station and USDA-ARS jointly announced in 2000 the release of Intrada HWWW. Initial development of this cultivar was facilitated by Dr. Joe Martin with the Kansas Agricultural Exp. Stn. Selected from the cross 'Rio Blanco/TAM 200' and tested under the experimental name OK95G701, Intrada is an awned, white-glumed, semidwarf cultivar with plant and kernel type resembling TAM 200, with exception of its white kernel color. Intrada appears most suited for dryland production systems in western and northwestern Oklahoma and similar areas in adjacent states. The cultivar is particularly adapted to a dual-purpose management system, because its recovery from grazing exceeds that of many cultivars currently in production and because it shows good stand establishment with early planting. Grazing is recommended, if placed under irrigation, to prevent excessive accumulation of vegetation and subsequent risk of lodging. Intrada is expected to be a suitable replacement for any HWWW cultivar currently in production. Assuming a shift in market class would be desired, it also would replace Custer in areas limited by soil-borne mosaic or marginally acidic soils. Intrada would be preferred over Jagger for its much higher test weight, greater yield performance in western Oklahoma, and later arrival at first-hollow-stem stage.
Intrada has outstanding test weight patterns inside and outside its primary zone of adaptation. Kernel hardness is acceptable, while kernel size is uniform but smaller than average, or about 2-3 mg less than that of 2174 in kernel weight. Grain protein concentration is intermediate, or about 0.5-1.0 percentage units lower than that of 2174. Standard deviations of single-kernel crushing characteristics have been within acceptable limits. Intrada has a medium-short mixing time, good mixing tolerance, and excellent loaf volume and texture. Grain samples from the 1998 SRPN showed good to very good milling and baking composite scores in three of the four intraregional production zones of the southern Great Plains. Grain samples of Intrada from the 1999 crop year received a favorable score for baking quality (3.5 on 1-6 scale) by the Hard Winter Wheat Technical Board of the Wheat Quality Council. Marketing of this cultivar is initially targeted for domestic bread production and international grain markets that traditionally purchase hard red winter wheat.
The Oklahoma Agricultural Experiment Station and USDA-ARS also jointly announced in February, 2001 the release of Ok101 HRWW. Tested under the experimental name OK95571, its pedigree is 'OK87W663/Mesa//2180'. The pedigree of OK87W663 is 'Chisholm*3/Newton-Largo//Chisholm'. Neither the wheat-rye translocation nor greenbug resistance were retained in this cultivar.
Six years of OSU breeder trials, conducted from 1995-00, generated 35 environments for comparing Ok101 with the four currently leading cultivars in the state (Custer, Jagger, 2137, and 2174). The yield potential of Ok101 was similar to those of 2174, 2137, and Custer, but greater than that of Jagger. Over the years, Ok101 has shown its best relative performance under irrigation in the High Plains, where it has appeared in the highest yielding group of entries (within one LSD value of the highest yielding entry) in all years except 2000. Grain-yield performance was extensively evaluated in OSU's Wheat Variety Trials across an additional 26 environments not normally available to the breeding program. Expression of the yield of Ok101 relative to the average yield of 2174, Jagger, 2137, and Custer produced values exceeding 0.99 at the majority of sites, indicating equality or superiority of performance for Ok101 versus prevalent cultivars.
Adaptation to low-pH soils is critical for cultivars grown in north-central Oklahoma. Ok101 is one of the more tolerant candidates to emerge from the OSU breeding program and surpasses the tolerance level of any OSU wheat cultivar released thus far. The acid-soil tolerance rating, based on visual ratings on a scale of 1 (most tolerant) to 5, averaged 1.7 in 1999 and 2000 at Enid, OK, where the soil pH is < 4.2. No other cultivar in the test exceeded that tolerance rating, i.e., had a lower value.
To many Oklahoma producers, forage yield can be equally or more important in making cultivar choices than grain yield. Unfortunately, environmental effects often mask genetic differences, but in a few cases, some cultivars can be identified with unusually high dormancy or tendency toward prostrate vegetative growth habit, often expressed as low forage yield. One example of that growth pattern and history is 2137. Autumn forage yields averaged across 5 site-years did not place Ok101 in a low-growth category. Conversely, Ok101 should be considered similar to 2174, often perceived as having good autumn forage potential. Given its desirable forage capability and acceptable grain yield potential after grazing, Ok101 appears well suited for a dual-purpose management system. The shorter coleoptile length might cause concern, but several other cultivars appear to be much shorter, such as AgSeco 7853, Oro Blanco, and Ike.
Ok101 shows a susceptible seedling reaction to leaf rust, an intermediate adult-plant reaction to leaf rust, and an intermediate reaction to tan spot. Ok101 is resistant to stem rust and WSBMV, moderately susceptible to WSMV, BYDV, and Septoria leaf blotch, and highly susceptible to stripe rust and powdery mildew. Ok101 has large kernel size but average test weight compared to most cultivars in production, being intermediate to that of 2137 (lower test weight) and that of 2174 (higher test weight).
Small-scale milling and baking analytical tests were performed by the USDA-ARS Hard Winter Wheat Quality Laboratory on grain samples collected from 1998 and 1999 SRPN sites throughout the Great Plains. Milling scores for Ok101 were rated good to very good in all southern megaenvironments, and average to very good in the northern megaenvironments. Across environments and years, Ok101 milled exceptionally well relative to the nursery average, exceeding it by 10 percentage points. Baking scores were more variable across environments than milling scores, ranging from very poor to very good. Further quality evaluation was provided by industry cooperators of the Wheat Quality Council in 1998 and 1999. No differences in overall baking quality were detected between Ok101 and the check cultivars, Karl 92 and 2174. Specific deviations for Ok101 were larger kernel size, but lower flour protein, lower farinograph stability, less bake absorption, and lower mixing tolerance. In summarizing 2 years of Wheat Quality Council evaluation, Ok101 drew consistent comments on its very good internal loaf appearance, but poor water absorption.
In summary, Ok101 is expected to be a suitable replacement for 2174 in critically acidic soils (pH<5.0), or in a dual-purpose production system, due to its better germination capability in hot soils, equivalent autumn forage production, and a vegetative growth habit better suited for forage removal and regrowth capacity in the spring. With few exceptions, Ok101 should provide greater yield and test weight performance than those of Jagger or 2137, and offer equal or better grazing potential in the autumn. Ok101 may avoid premature dormancy release in the winter by reaching first-hollow-stem stage at a later date than Jagger, and will likely produce more autumn forage than 2137. Ok101 should replace Custer in areas limited by WSBMV and by acid soils, in addition to offering improved dough and bread quality. Distribution of foundation seed is expected for the summer of 2001. The contributions of Ed Smith, retired OSU wheat breeder and former occupant of the Wheat Genetics Chair in Agriculture, are duly acknowledged and were central to the development of Ok101 through final stages of selection. Identification of Ok101 as a candidate cultivar was accomplished through OSU's Wheat Improvement Team, which includes Brett Carver (lead scientist), Gene Krenzer, Art Klatt, Arron Guenzi, Guihua Bai, and Bjorn Martin, Department of Plant and Soil Sciences; Bob Hunger and Jeanmarie Verchot, Department of Entomology and Plant Pathology; Patricia Rayas-Duarte, Department of Biochemistry and Molecular Biology; and David Porter, USDAARS Plant Science Research Laboratory, Stillwater.
The germ plasm enhancement program within the Wheat Improvement Program at Oklahoma State University concentrated on the introduction and evaluation of more than 1,000 spring and winter wheat materials during the 2000 greenhouse cycle. These materials originated from CIMMYT, Mexico, China, Turkey, Austria, Texas, Nebraska, and many other countries. Crosses were made to selected materials with special emphasis for durable-type, leaf rust resistance and to synthetic wheats obtained from CIMMYT. All retained materials are being evaluated for leaf rust and SBMV resistance during the 200001 crop cycle.
Additional materials were recently introduced, including advanced materials from the Kansas and Texas breeding programs, plus more than 1,350 spring and winter wheat materials from CIMMYT. The CIMMYT materials are currently undergoing a quarantine grow-out in the greenhouse. A limited number of crosses will be made to these materials. Further evaluation for promising parental materials will be conducted during the 200102 crop season.
For additional information contact Dr. Art Klatt at (405) 744-9604 or E-mail at aklatt@mail.pss.okstate.edu
G. Bai and B. Carver.
To strengthen the wheat breeding program, a new wheat molecular breeding laboratory has been established in Oklahoma State University. This laboratory has been set up for functional genomics research to discover new genes in wheat, and for molecular mapping of important agronomic traits to develop high-throughput molecular markers for marker-assisted selection. An automated DNA sequencer from LI-COR and a DNA fast preparation instrument were used for rapid DNA isolation and molecular marker analysis.
An international collection of wheat cultivars with various level of resistance to scab was analyzed with AFLP and SSR markers linked to a major QTL in the resistant cultivar Ning 7840 from China. Results indicated that these AFLP and SSR markers can be detected in almost all resistant or moderately resistant cultivars with Ning 7840 or Sumai 3 as the resistant parent. AFLP and SSR analysis indicated that Ning 7840 and Sumai 3 contain the same major QTL for scab resistance and the major QTL derived from a Chinese land race Taiwan Wheat, not from the Italian cultivar Funo. These AFLP and SSR marker alleles linked to the major scab QTL in Ning 7840 were not detected in Wangshuibai and a few other Chinese land races, indicating Wangshuibai and some other Chinese land races may have different QTL's for scab resistance. The results also demonstrate that these SSR and AFLP markers are good indicators of the major QTL for scab resistance in Ning 7840 and Sumai 3. Therefore, they can be widely used for marker-assisted selection in breeding programs.
Aluminum toxicity is one of the major constraints for wheat
production in Oklahoma and many other areas of the USA. To identify
molecular markers linked to aluminum tolerance from Atlas 66,
four pairs of NILs contrasting in Al tolerance were screened with
both SSR and PstI-AFLP markers. One AFLP marker was identified
in the resistant parent Atlas 66 and all four Al-tolerant near
isolines, but not in the susceptible parents Century and Chisholm
and their sensitive NILs. Several other putative AFLP markers
also were identified in some of the tolerant lines, but not all.
These markers will be further verified in the populations derived
from the crosses between tolerant isolines and sensitive NILs.
No SSR markers were identified to associate with Al-tolerance
genes, after about 100 SSR primers were screened. To isolate Al-tolerance-specific
ESTs, one pair of NILs derived by using Atlas 66 as the donor
and Century as the recurrent parent were challenged with Al solution
for 48 hours. Roots were excised for mRNA isolation. Differentially
expressed ESTs were isolated by suppression subtractive hybridization
(SSH). So far, about 300 EST clones have been isolated and twenty
of them have been sequenced. Among the sequenced ESTs, some are
known key enzymes responsible for plant resistance to biotic and
abiotic stresses and some are new genes without similar sequences
in the Genebank. These clones will be further sequenced and gene
expression profiling will be conducted by using new array technology.
PLANT SCIENCE RESEARCH LABORATORY, USDA-ARS1301 N. Western St., Stillwater, OK 74075, USA.
http://www.csrl.ars.usda.gov/pswcrl/pswcrl.htm
C.A. Baker, J.D. Burd, N.C. Elliott, M.H. Greenstone, S.D. Kindler,
D.R. Porter, K.A. Shufran, and J.A. Webster.
Russian wheat aphid. Efforts are ongoing to develop RWA-resistant wheat germ plasm. Seed (from self-pollination) from 61 HWWW, 91 HRWW, 35 HRSW, 46 HWSW, and 63 HRSW RWA-resistant wheat lines were grown for greenhouse evaluation. Plants were either harvested as individual plants or bulk harvested, depending on whether segregation for agronomic characteristics was apparent. Screening for RWA resistance was done on selected lines. RWA-resistant populations selected for increase and further evaluation included 40 HWWW lines, 35 HRWW lines, 38 HRSW lines, 44 HWSW lines, and 40 SWSW lines. Each of these market class selections include lines derived from 13 different RWA-resistance sources. Homozygous RWA-resistant lines will be selected for germ plasm release. Field evaluations and yield trials of RWA-resistant spring lines were done in Pullman, WA. Over 125 advanced winter lines were planted for 199900 field evaluation in Stillwater and Goodwell, OK. Replicated yield trials of the most promising winter lines were planted in irrigated and dryland field tests in Goodwell, OK.
Durum wheats are currently being evaluated for RWA resistance. We have tentatively identified some level of resistance in 10 winter durum PIs and four spring durum PIs. Most of these lines are land race populations heterogeneous for RWA resistance. Over 100 lines derived from individual resistant plants have been screened; resistant progeny were again transplanted and increased. True-breeding RWA-resistant durums will be resubmitted to the GRIN system. Collaborative screening was done for breeding/genetics programs associated with Oklahoma State University, Washington State University, the University of California at Riverside, and the Regional Germplasm Observation Nursery (RGON).
Greenbug. Breeding efforts have continued for the development
of greenbug-resistant winter wheats. Approximately 200 lines are
currently being evaluated and increased in the greenhouse.
Bryna Donnelly, a Ph.D. graduate student in the Entomology and
Plant Pathology Department at Oklahoma State University, is working
with our laboratory on the application of proteomics to understanding
aphid/wheat interactions and host-plant resistance.
Collaborative screening for greenbug resistance was done in the breeding/genetics programs associated with Oklahoma State University, the University of Nebraska, and RGON.
Aphid biology and ecology. James A. Anstead completed
the requirements for a M.S. degree in the Department of Entomology
and Plant Pathology at Oklahoma State University. The title of
his thesis was 'Genetic and biotypic diversity of greenbug Schizaphis
graminum (Rondani) on non-cultivated hosts'.
Scope and method of study. The purpose of the study was to determine
the biotypic and genetic diversity of S. graminum on noncultivated
hosts and compare this to the diversity on crop hosts. Collections
made from cultivated hosts were screened against resistant crop
lines to determine their biotype. A 1,043-bp region of the mitochondrial
DNA, cytochrome oxidase I gene, was sequenced to measure genetic
diversity. The evolutionary and taxonomic status of biotype also
was investigated.
Findings and conclusions. There was greater biotypic diversity on noncultivated grasses. We found biotypes E, I, and K on noncultivated grasses, but we also found biotype G at higher than previously recorded densities. An isolate with a unique virulence profile was also found (i.e., a new biotype). Our study indicates noncultivated grasses are a reservoir of biotypic diversity. This study supports previous conclusions that biotype formation was not driven by the development of resistant crop varieties. Biotypes, as defined in the greenbug (by virulence against resistant hosts), do not appear to have any evolutionary and taxonomic status. Rather, this study confirms the presence of three genetically distinct clades in S. graminum. The distances between clades indicate one to two million years have passed since they had shared a common mitochondrial ancestor. These clades are likely to represent host-adapted races, with only one of the three containing greenbugs commonly found on crop hosts.
The Cereal Insect Genetics Resource Library (CIGRL). The 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 (mjburrows@pswcrl.ars.usda.gov).
Cereal Insect Report Oklahoma. We studied the effects of greenbug feeding on the yield of the cultivars Karl, Karl 92, and 2163. Our objectives were to determine the effect of different greenbug densities during the autumn or spring on yield and develop models to describe the effect of greenbugs on yield loss in autumn- and spring-infested wheat. A regression model estimated yield loss for greenbug in relation to the number of greenbug-days that accumulate per tiller at 0.51 kg/ha of yield per greenbug-day in near normal years, and a loss of 1.17 kg/ha under severe drought conditions. The three greenbug susceptible winter wheat cultivars exhibited similar yield losses, as indicated by a common slope parameter, in relation to the intensity of greenbug infestation, suggesting that the model is robust for predicting yield loss for susceptible cultivars.
Greenbugs were a serious pest on winter wheat in the central Oklahoma wheat growing area (north of I-40) in the autumn of 1999. The intensity of infestations appeared to increase as one headed north towards the Kansas border. Early planted fields appeared to suffer less damage than later-planted fields, suggesting that fields were colonized by greenbugs primarily from mid-September through mid-October. Seriously damaged fields first became evident in late-October. Autumn weather was mild and dry, making conditions ideal for greenbug population growth. The majority of winter wheat fields in central Oklahoma were treated with insecticide in autumn of 1999 (mostly during November). Greenbug populations in untreated fields in central Oklahoma declined during winter, primarily because of the activity of parasitoids. Parasitism levels by L. testaceipes exceeded 90 % in some fields. Most such fields had probably already suffered economic damage. Wheat fields in the Oklahoma panhandle also developed serious greenbug infestations, but they developed later than in central Oklahoma, peaking in January or February. This was the result of low parasitism levels and mild weather. Much of the wheat acreage in the panhandle was treated with insecticide during January and February, but we believe a small proportion of fields were treated than in central Oklahoma. Parasitism levels increased from late January through February and greenbug populations in untreated fields eventually crashed as the result; however, not before economic damage occurred. Greenbugs were sparse in southwestern Oklahoma (south of I-40). The bird cherry-oat aphid was present in fields throughout the state, but generally occurred at low levels.
An obscure aphid found on wheat in Oklahoma was identified as the rice root aphid. Rather high numbers of this aphid were found on wheat beginning in October and continued through the middle of January. During October this was the most numerous aphid found on wheat. The aphid feeds at the base of the wheat plant. The aphid is believed to cause economic damage to wheat, although losses have apparently never been quantified. The rice root aphid may be a particularly important pest of cereals in the southern Great Plains (Susan Halbert, personal communication), where in addition to direct feeding damage, it can also transmit barley yellow dwarf disease. Interspecies similarities between the bird cherry-oat aphid and the rice root aphid often lead to visual misidentification, and damage to cereals often attributed to the bird cherry-oat aphid may sometimes be caused by the rice root aphid. A key distinguishing feature is that the bird cherry-oat aphid has six antennal segments, whereas the rice root aphid only has five antennal segments. Other, nondiagnostic characteristics can often be used to separate the species, including presence of dense setae on the antennal segments, and the slight differences in coloration.
Evaluation of aphid natural enemy effectiveness. We completed a 2-year study of aphid and predator populations in wheat in Chickasha and Lahoma, OK. In collaboration with ARS, museum, and university workers, we developed a collection of identified carabid, staphylinid, and spider species. One finding of this study is that linyphiid spiders are significantly negatively correlated, and staphylinids positively correlated, with aphid densities.
We developed a definitive, two-step PCR and restriction digest
assay to distinguish two Aphelinus hordei and A. varipes,
two parasitoid species that are barely distinguishable morphologically.
We developed a 16-s DNA phylogeny for the Aphelinus species
imported against the RWA, and, by comparison, with a companion
phylogeny developed for aphidiine parasitoids, showed that a species
complex of very similar, recently diverged Aphelinus species
exists.