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The Russian wheat aphid (RWA), diuraphis noxia, was first observed in North America, in Mexico, in 1980. It spread across the western United States and was found in the dryland area of the PNW in fall 1987. Since that time it has spread widely throughout wheat producing regions east of the Cascades.
This insect can inflict severe damage on wheat and barley crops. In addition the presence of this insect has indirectly increased the potential for soil erosion, where residue levels are low and seeding is delayed to avoid fall infestations.
The RWA is a small, somewhat elongated aphid with fairly short antennae (Fig. 1). It is light green and has short cornicles (tail appendages). Typically the aphid? inhabits the base of leaves just above the collar. Symptoms associated with RWA feeding include white, yellow or purple to red streaks in leaves accompanied by inward rolling of the leaf. Rolling of the leaves protects the aphid by reducing the efficiency of predators and parasites as well as contact insecticides.
Since the RWA has arrived in the PNW, several strategies have been developed to lessen its impact on cereal production. Immediate concerns have centered on control of the insect by cultural and chemical methods. This includes such practices as delayed seeding, developing spray threshold levels, improved insecticide formulations and fine-tuning timing of applications.
Longer term management strategies will rely on biological control and host plant resistance. STEEP researcher Pamela Zwer, wheat breeder at the Columbia Basin Agricultural Research Center at Pendleton, has initiated a program to assess wheat resistance and tolerance to RWA. Her ultimate goal is to introduce resistance or tolerance into club wheat varieties. STEEP is a tri-state program involving Idaho, Washington and Oregon. It is an interdisciplinary research program, designed to address the cropland production and erosion problems of the region.
Having varieties that are resistant or tolerant to RWA has both economic and environmental consequences. Currently the only option for control of RWA is through the application of insecticides. These materials are costly and pose health and environmental risks. Tolerant or resistant varieties would eliminate the need for insecticides and delayed seeding, thus reducing production costs and lessening health, environmental and erosion risks. Although development of new varieties may take several years, experience from South Africa has shown RWA to be a persistent problem. The aphid was introduced into that country in the late 1970's and still insecticide spraying is required on much of their wheat crop.
Developing Host Plant Resistance or Tolerance
The development of a wheat plant with tolerance to RWA is a tedious and lengthy process. The initial step is screening of advanced breeding lines and cultivars for tolerance or resistance. Zwer has developed a greenhouse procedure to evaluate seedling susceptibility to RWA. The procedure is adapted from methods used in South African research. Aphids are reared in confined plastic cylinders. Seedlings of varieties or cultivars to be evaluated are infected with 3 to 5 aphids and placed in a wooden insect cage covered with fine polyester mesh for confinement. Aphids are allowed to feed on the seedlings for 21 days and then plants are assessed for tolerance to the aphid, Zwer uses an evaluation scale developed by a South African researcher to screen for RWA tolerance or resistance (Table 1). Increasing scores indicate greater susceptibility or intolerance to the aphid.
During the period of October 1988 through May 1989, Zwer screened seedlings of 995 advanced lines or cultivars for aphid tolerance. Appaloosa oats served as a resistant control, while Gus barley served as a susceptible control. Oats are not a host for the RWA, whereas barley is very susceptible.
Table 2 shows the results of the seedling evaluations. Of the 995 lines evaluated, 975 were found to be susceptible and only 20 lines were found to have some tolerance to the RWA. Tolerant lines are those with scores of 2 or 3.
None of the tolerant lines are varieties currently grown in the PNW. The tolerant lines consisted of 13 advanced breeding lines of club wheat, two advanced lines of common soft white winter, two of primitive wheats, a hard white spring wheat from Iran, a hard white winter from Russia and hard red winter wheat from Hungary. The two primitive wheats, the hard white spring and the hard red winter, had ratings of 2 while the others had ratings of 3.
Zwer explains that work in South Africa has shown that tolerance to the RWA is conferred by single, dominant genes in two of the three lines. Initially, tolerance to the RWA will be transferred into adapted varieties using a backcross program. Her strategy is to incorporate multiple genes for tolerance to provide durable resistance.
The RWA is a recent and serious pest of cereal grains in the PNW. Currently, options for control of this insect are to plant winter cereals in the late part of the seeding window and close scouting for early detection of aphid population, followed by timely insecticide application when aphid numbers exceed threshold levels. Late seeding of winter cereals has the potential for increasing erosion if residue levels are low. If the RWA is a problem in fall plantings and delayed seeding is practiced, residue levels should be adequate to protect against erosion.
In spring cereal crops, early seeding is recommended followed by scouting and spraying. From past experience it is likely that spring cereal crops will suffer more heavily from aphids than winter cereals.
Work being done by Zwer has identified sources of tolerance in some wheats. Tolerance can likely be introduced into new varieties of club and common wheat for the PNW. However, new varieties will require several years to develop and test before they are available to producers.
us: Hans Kok, (208)885-5971
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