PLANT SCIENCE RESEARCH LABORATORY, USDA-ARS

1301 N. Western St., Stillwater, OK 74075, USA.

C.A. Baker, J.D. Burd, N.C. Elliott, M.H. Greenstone, S.D. Kindler, J.H. Lee, D.W. Mornhinweg, D.R. Porter, K.A. Shufran, and J.A. Webster.

Host-plant resistance/germplasm enhancement.

Topcrossing of Russian wheat aphid-resistant wheat selections to high performance recurrent parents continued. Efforts are ongoing to develop RWA-resistant wheat lines for all market classes that are grown in areas where RWA is a problem. Over 400 winter wheat lines derived from crosses with seven different sources of RWA resistance were planted in the field for evaluation in Stillwater prior to germplasm release. Genetic diversity studies continued, utilizing intercross populations. Thirty-five advanced lines developed from four different RWA resistance sources were planted for seed increase and evaluation in preparation for entering the germplasm release stage. New sources with very high levels of RWA resistance now are being incorporated into the crossing and genetic evaluations programs. Several cooperative projects are ongoing to provide screening and evaluation tests for various public and commercial wheat breeders. RWA resistance testing continues on `wheat x triticale' cross populations. Several agronomically acceptable wheat lines with high levels of resistance have been identified and are being characterized cytologically.

Biological control.

The natural enemy impact evaluation studies initiated in southeastern Colorado in 1994 were continued in 1995. The scope of the studies was expanded to include the RWA-resistant winter wheat cultivar Halt, in addition to the susceptible cultivar Lamar used in 1994. Results from 1995 suggest that exotic parasitoids that were established in the region in 1993 still occur in the field at very low population densities, and thus, exert minimal impact on RWA populations. However, native parasitoids and aphid predators may exert greater impact on RWA populations in Halt than in the susceptible wheat variety.

Progress toward development of a computer model for simulation of the region-wide population dynamics of the RWA included acquisition and classification of Landsat MSS data for the portion of the Great Plains from south Texas to Wyoming and development of process models for simulating RWA population dynamics in fields of winter wheat and intermediate and crested wheatgrasses. Preliminary results of validation studies suggest that temperature, rainfall, and strong winds are important abiotic factors, and that wheat-plant growth stage is an important biotic factor contributing to the aphids' population dynamics.

We compared removal with quadrat sampling to determine if removal sampling provided useful estimates of population density of adult and larval coccinellids in winter wheat. We also determined the utility of timed count and sweepnet sampling for estimating adult and larval coccinellid densities. Removal sampling provided accurate estimates of population density for adults of most species, but consistently underestimated larval density. Regression models were developed to convert estimates of relative to estimates of absolute population density. A 25-sweep sample with a standard sweepnet was the most efficient unit (smallest coefficient of variation per unit cost) for estimating adult coccinellid density.

Integrated pest management.

Aphid-plant interactions. The effect of RWA on nonstructural carbohydrate constituents and their partitioning patterns for winter wheat were studied using HPLC. Total nonstructural carbohydrate levels of aphid-infested plants were significantly reduced. The greatest reductions were observed in root and leaf tissue, which resulted primarily from diminished fructan pools. Partitioning patterns of nonstructural carbohydrates in infested plants were altered during feeding, which indicated that the sink to source transition of infested leaves was delayed significantly.

Population genetics.

A genomic library of RWA was constructed from which the ribosomal RNA cistron was cloned. The rRNA cistron was subcloned into a Bluescript vector and currently is being mapped using restriction enzyme digests and hybridization with greenbug probes. Much homology seems to exist in the coding regions between the greenbug and RWA. However, as expected, the intergenic spacer (IGS) shows no homology. Thus far, we have identified an unusual EcoR1 polymorphism in the 18S gene in RWA. This polymorphism has been confirmed by probing genomic DNA isolated from RWAs maintained in a laboratory colony that was originally established from field collections in the USA. However, limited surveys of field populations in 1994 and 1995 did not reveal the polymorphism. This may be a rare or poorly fit genotype. Because of the parthenogenetic nature of RWA populations in the USA, finding a polymorphism such as this indicates that more than one RWA genotype has been introduced into the USA. Selected subclones of the RWA rRNA cistron will be sequenced, and PCR primers will be developed for the EcoR1 polymorphic site and the IGS. These will be used to further evaluate RWA populations in the USA for genetic diversity.

Personnel.

Dr. Matthew H. Greenstone recently joined us as Research Entomologist and will be using monoclonal antibodies and DNA hybridization probes to monitor natural enemy-aphid interactions.

Dr. Jang Hoon Lee has completed his Foreign Research Associate assignment here and has returned to Korea. Tel. 82-32-772-4859.

Publications.

Baker CA, Webster JA, and Porter DR. 1995. Inheritance and mechanisms of Russian wheat aphid (RWA) resistance in wheat PI 245462. Agron Abstr p. 88.

Burd JD, Webster JA, Puterka GJ, Hoxie RP, and Welso SG. 1996. Effect of Russian wheat aphid on nonstructural carbohydrate profiles of wheat seedlings. Southwest Ent (accepted).

Cushman MA, Guenzi AC, Veal RA, Porter DR, and Smith EL. 1995. Characterization of genes differentially expressed among greenbug-susceptible and -resistant lines of wheat. Agron Abstr p. 177.

Elliott NC, Burd JD, and Hein GL. 1995. Russian wheat aphid ecology in the Great Plains. Proc Southwest and Rocky Mountain Div, Am Assoc Advance Sci 31:32-33.

Elliott NC, Burd JD, Armstrong JS, Walker CB, Reed DK, and Peairs FB. 1995. Release and recovery of imported parasitoids of the Russian wheat aphid in eastern Colorado. Southwest Ent 20:125-129.

Elliott NC, Hein GL, Burd JD, and Lee JH. 1995. The population dynamics of the Russian wheat aphid in Great Plains agricultural landscapes. Suppl Bull Ecol Soc Am 76:74.

Friebe B, Zhang W, Raupp WJ, Gill BS, and Porter DR. 1995. Non-homologous wheat-rye chromosomal translocations conferring resistance to greenbug. Euphytica 84:121-125.

Hein GL, Elliott NC, Michels Jr. GJ, and Kieckhefer RW. 1995. A general method for estimating cereal aphid populations in small grain fields based on frequency of occurrence. Can Ent 127:59-63.

Kindler SD, Springer TL, and Jensen KB. 1995. Detection and characteristics of the mechanisms of resistance to Russian wheat aphid (Homoptera: Aphididae) in tall wheatgrass. J Econ Entomol 88:1503-1509.

Merkle OG, Hatchett JH, Porter DR, and Webster JA. 1995. Registration of wheat germplasm (PI 510693) resistant to Hessian fly. Crop Sci 35:294.

Porter DR, Nguyen HT, and Burke JJ. 1995. Genetic control of acquired high temperature tolerance in winter wheat. Euphytica 83:153-157.

Porter DR, Nguyen HT, and Burke JJ. 1995. Genetic control of acquired high temperature tolerance in winter wheat. Agron Abstr p. 88.

Reed HC, Tan SH, Haapanen K, Killmon M, Reed DK, and Elliott NC. 1995. Olfactory responses of the parasitoid Diaeretiella rapae (Hymenoptera: Aphidiidae) to the odor of plants, aphids, and aphid-plant complexes. J Chem Ecol 21:407-418.

Sebesta EE, Smith EL, Young Jr. HC, Porter DR, and Webster JA. 1995. Registration of Teewon wheat germplasm. Crop Sci 35:294.

Sebesta EE, Wood Jr. EA, Porter DR, and Webster JA. 1996. Development of a triticale resistant to the greenbug: An historical perspective. Euphytica 87:65-67.

Sebesta EE, Wood Jr. EA, Porter DR, Webster JA, and Smith EL. 1995. Registration of Amigo greenbug-resistant wheat germplasm. Crop Sci 35:293.

Sebesta EE, Young Jr. HC, Porter DR, and Webster JA. 1995. Registration of two wheat streak mosaic virus-resistant wheat germplasms. Crop Sci 35:1238.

Wood Jr. EA, Sebesta EE, Webster JA, and Porter DR. 1995. Resistance to wheat curl mite, Eriophyes tulipae Keifer (Acari: Eriophyidae) in greenbug-resistant `Gaucho' triticale and `Gaucho' x wheat crosses. J Econ Entomol 88:1032-1036.