Distribution and diversity of Russian wheat aphid (Hemiptera: Aphididae) biotypes in South Africa and Lesotho

Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae) was recorded for the first time in South Africa in 1978. In 2005, a second biotype, RWASA2, emerged, and here we report on the emergence of yet another biotype, found for the first time in 2009. The discovery of new Russian wheat aphid biotypes is a significant challenge to the wheat, Triticum aestivum L., industry in South Africa. Russian wheat aphid resistance in wheat, that offered wheat producers a long-term solution to Russian wheat aphid control, may no longer be effective in areas where the new biotypes occur. It is therefore critical to determine the diversity and extent of distribution of biotypes in South Africa to successfully deploy Russian wheat aphid resistance in wheat. Screening of 96 Russian wheat aphid clones resulted in identification of three Russian wheat aphid biotypes. Infestations of RWASA1 caused susceptible damage symptoms only in wheat entries containing the Dn3 gene. Infestations of RWASA2 caused susceptible damage symptoms in wheat entries containing Dn1, Dn2, Dn3, and Dn9 resistant genes. Based on the damage-rating scores for the seven resistance sources, a new biotype, which caused damage rating scores different from those for RWASA1 and RWASA2, was evident among the Russian wheat aphid populations tested. This new biotype is virulent to the same resistance sources as RWASA2 (Dn1, Dn2, Dn3, and Dn9), but it also has added virulence to Dn4, whereas RWASA2 is avirulent to this resistance source.     

Distribution and diversity of russian wheat aphid (Hemiptera: Aphididae) biotypes in North America

Wheat, Triticum aestivum L., with Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae) resistance based on the Dn4 gene has been important in managing Russian wheat aphid since 1994. Recently, five biotypes (RWA1-RWA5) of this aphid have been described based on their ability to differentially damage RWA resistance genes in wheat. RWA2, RWA4, and RWA5 are of great concern because they can kill wheat with Dn4 resistance. In 2005, 365 Russian wheat aphid clone colonies were made from collections taken from 98 fields of wheat or barley, Hordeum vulgare L., in Oklahoma, Texas, New Mexico, Colorado, Kansas, Nebraska, and Wyoming to determine their biotypic status. The biotype of each clone was determined through its ability to differentially damage two resistant and two susceptible wheat entries in two phases of screening. The first phase determined the damage responses of Russian wheat aphid wheat entries with resistance genes Dn4, Dn7, and susceptible 'Custer' to infestations by each clone to identify RWA1 to RWA4. The second phase used the responses of Custer and 'Yuma' wheat to identify RWA1 and RWA5. Only two biotypes, RWA1 and RWA2, were identified in this study. The biotype composition across all collection sites was 27.2% RWA1 and 72.8% RWA2. RWA biotype frequency by state indicated that RWA2 was the predominant biotype and composed 73-95% of the biotype complex in Texas, Oklahoma, Colorado, and Wyoming. Our study indicated that RWA2 is widely distributed and that it has rapidly dominated the biotype complex in wheat and barley within its primary range from Texas to Wyoming. Wheat with the Dn4 resistance gene will have little value in managing RWA in the United States, based on the predominance of RWA2.     

Resistance evaluation of wheat germplasm containing Dn4 or Dny against Russian wheat aphid biotype RWASA3

Host plant resistance can effectively manage Russian wheat aphid (Diuraphis noxia) Kurdjumov (Homoptera: Aphididae) in areas where it is an economically important pest of wheat. However, biotypes of D. noxia virulent on wheat containing resistance gene Dn4 have been reported in both the United States and South Africa. Thirty wheat genotypes, including susceptible Yuma, resistant CItr2401, as well as 25 genotypes containing Dn4 and three genotypes containing Dny were planted under greenhouse conditions in Bethlehem, South Africa, and screened with D. noxia biotype RWASA3. RWASA3 caused susceptible damage symptoms in MTRWA92‐145, Ankor, Halt, Bond CL, 18FAWWON‐SA 262, Prowers99, 18FAWWON‐SA 264, Hatcher, Yumar, Corwa and Thunder CL all reported to contain the Dn4 resistance gene. Genotypes PI586956, Stanton and 18FAWWON‐SA 257, containing the Dny‐resistance gene were susceptible to RWASA3. Similarly, coinciding development of virulence to resistance genes Dn4 and Dny was reported in the United States. However, in this study, 13 Dn4‐containing genotypes showed moderate resistance when screened with RWASA3 alluding to a more complex biotype‐gene‐interaction. These findings could indicate that Dn4 and Dny may be related and possibly share a similar or common resistance factor. Further studies will be aimed at explaining these results investigating the possibility of an allelic cluster or series for Dn4, possibly including Dny.     

Genetic mapping of Dn7, a rye gene conferring resistance to the Russian wheat aphid in wheat

The Russian wheat aphid is a significant pest problem in wheat and barley in North America. Genetic resistance in wheat is the most effective and economical means to control the damage caused by the aphid. Dn7 is a rye gene located on chromosome 1RS that confers resistance to the Russian wheat aphid. The gene was previously transferred from rye into a wheat background via a 1RS/1BL translocation. This study was conducted to genetically map Dn7 and to characterize the type of resistance the gene confers. The resistant line '94M370' was crossed with a susceptible wheat cultivar that also contains a pair of 1RS/1BL translocation chromosomes. The F₂ progeny from this cross segregated for resistance in a ratio of 3 resistant: 1 susceptible, indicating a single dominant gene. One-hundred and eleven RFLP markers previously mapped on wheat chromosomes 1A, 1B and 1D, barley chromosome 1H and rye chromosome 1R, were used to screen the parents for polymorphism. A genetic map containing six markers linked to Dn7, encompassing 28.2 cM, was constructed. The markers flanking Dn7 were Xbcd1434 and XksuD14, which mapped 1.4 cM and 7.4 cM from Dn7, respectively. Dn7 confers antixenosis, and provides a higher level of resistance than that provided by Dn4. The applications of Dn7 and the linked markers in wheat breeding are discussed.Communicated by J. Dvorak     

Identification of Russian wheat aphid (Homoptera: Aphididae) populations virulent to the Dn4 resistance gene

The Russian wheat aphid, Diuraphis noxia (Mordvilko), is a serious worldwide pest of wheat and barley. Russian wheat aphid populations from Hungary, Russia, and Syria have previously been identified as virulent to D. noxia (Dn) 4, the gene in all Russian wheat aphid-resistant cultivars produced in Colorado. However, the virulence of Russian wheat aphid populations from central Europe, North Africa, and South America to existing Dn genes has not been assessed. Experiments with plants containing several different Dn genes demonstrated that populations from Chile, the Czech Republic, and Ethiopia are also virulent to Dn4. The Czech population was also virulent to plants containing the Dnx gene in wheat plant introduction PI220127. The Ethiopian population was also virulent to plants containing the Dny gene in the Russian wheat aphid-resistant 'Stanton' produced in Kansas. The Chilean and Ethiopian populations were unaffected by the antibiosis resistance in Dn4 plants. There were significantly more nymphs of the Chilean population on plants of Dn4 than on Dn6 plants at both 18 and 23 d postinfestation, and the Ethiopian population attained a significantly greater weight on Dn4 plants than on plants containing Dn5 or Dn6. These newly characterized virulent Russian wheat aphid populations pose a distinct threat to existing or proposed wheat cultivars possessing Dn4.     

Variation to cause host injury between Russian wheat aphid (Homoptera: Aphididae) clones virulent to Dn4 wheat

Biotypes are infraspecific classifications based on biological rather than morphological characteristics. Cereal aphids are managed primarily by host plant resistance, and they often develop biotypes that injure or kill previously resistant plants. Although molecular genetic variation within aphid biotypes has been well documented, little is known about phenotypic variation, especially virulence or the biotype's ability to cause injury to cultivars with specific resistance genes. Five clones (single maternal lineages) of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), determined to be injurious to wheat, Triticum aestivum L., with the Dn4 gene, were evaluated on resistant and susceptible wheat and barley, Hordeum vulgare L., for their ability to cause chlorosis, reduction in plant height, and reduction in shoot dry weight. Variation to cause injury on resistant 'Halt' wheat, susceptible 'Jagger' wheat, and resistant 'STARS-9301B' barley was found among the Dn4 virulent clones. One clone caused up to 30.0 and 59.5% more reduction in plant height and shoot dry weight, respectively, on resistant Halt than other clones. It also caused up to 29.9 and 55.5% more reduction in plant height and shoot dry weight, respectively, on susceptible Jagger wheat. Although STARS-9301B barley exhibited an equal resistant response to feeding by all five clones based on chlorosis, two clones caused approximately 20% more reduction in plant height and shoot dry weight than three other clones. The most injurious clones on wheat were not the most injurious clones on barley. This is the first report of variation to cause varying degrees of plant damage within an aphid biotype virulent to a single host resistance gene. A single aphid clone may not accurately represent the true virulent nature of a biotype population in the field.     

Influence of three resistance sources in winter wheat derived from TAM 107 on yield response to Russian wheat aphid

A study to determine yield response to the Russian wheat aphid, Diuraphis noxia (Mordvilko), was conducted during the 1997-1998 and 1998-1999 growing seasons at three eastern Colorado locations, Akron, Fort Collins, and Lamar, with three wheat lines containing either Russian wheat aphid-resistant Dn4 gene, Dn6 gene, or resistance derived from PI 222668, and TAM 107 as the susceptible control. Russian wheat aphids per tiller were greater on TAM 107 than the resistant wheat lines at the 10x infestation level at Fort Collins and Akron in 1999. Yield, seed weight, and number of seeds per spike for each wheat line were somewhat affected by Russian wheat aphid per tiller mainly at Fort Collins. The infested resistant wheat lines harbored fewer Russian wheat aphids and yielded more than the infested susceptible wheat lines. Wheat lines containing the Dn4, Dn6, and PI 222668 genes contain different levels of antibiosis or antixenosis and tolerance. Although differences existed among sites and resistance, there is a benefit to planting resistant wheat when there is a potential for Russian wheat aphid infestations.     

Plant damage and yield response to the Russian wheat aphid (Homoptera: Aphididae) on susceptible and resistant winter wheats in Colorado

Plant damage and yield response to the Russian wheat aphid, Diuraphis noxia (Mordvilko), were evaluated on a susceptible (TAM 107) and a resistant (RWA E1) winter wheat, Triticum aestivum L., in three Colorado locations in the 1993 and 1994 crop years. Russian wheat aphid was more abundant on TAM 107 than on RWA E1. Russian wheat aphid days per tiller were greater at the higher infestation levels. Yield losses as a result of Russian wheat aphid infestation occurred most of the time with TAM 107 but rarely with RWA E1. Seed densities were reduced at higher infestation levels in TAM 107 at two locations. Russian wheat aphids per tiller had a negative relationship to yield in TAM 107 but not in RWA E1. In TAM 107 yield decreased as aphid densities increased, but yield remained constant regardless of initial aphid abundance on RWA E1 in all environments. Seed densities were reduced at higher infestation levels in TAM 107 at two locations. The resistance conferred by the Dn4 gene seems to be an effective management approach across a range of field conditions.     

Yield response and categories of resistance to Russian wheat aphid in four Dn4 hard red winter wheat cultivars

A field experiment was conducted to determine whether resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), conferred by the Dn4 gene is affected by genetic background. This was done by comparing the yield responses to Russian wheat aphid-resistant wheat containing Dn4, derived through the backcross method, to those of the corresponding recurrent parents. Infested resistant cultivars had fewer Russian wheat aphids per tiller than infested susceptible cultivars at the Lamar and Fort Collins, CO sites but not at the Akron, CO site. At the Lamar site, resistant cultivars yielded more than the susceptible cultivars. 'Prairie Red' and 'Yumar' were more resistant than 'Prowers', especially at the higher infestation level. Resistance in these cultivars was categorized in a laboratory experiment to confirm this differential expression of resistance. Resistance in Prairie Red, 'Halt', 'Prowers 99', and Yumar was categorized at three plant growth stages. Antibiosis was expressed as reductions in maximum number of nymphs produced per 24 h and intrinsic rate of increase. The maximum number of nymphs produced per 24 h was reduced in Halt and 'Lamar'. Averaged over cultivars, the intrinsic rate of increase was less at jointing than at the seedling or tillering growth stages. Tolerance was expressed in the resistant cultivars as reduced chlorosis and leaf rolling. Growth reductions in infested Prowers 99 plants was less than the other cultivars. This study confirms that some cultivars containing Dn4 may express antibiosis and tolerance, whereas others may not show the same categories. Thus, expression is affected by genetic background.     

Fractionated extracts of Russian wheat aphid eliciting defense responses in wheat

It is hypothesized that the interaction between aphids and plants follows a gene-for-gene model. The recent appearance of several new Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), biotypes in the United States and the differential response of wheat, Triticum aestivum L., genotypes containing different resistance genes also suggest a gene-for-gene interaction. However, aphid elicitors remain unknown. This study was conducted to identify fractionated Russian wheat aphid extracts capable of eliciting differential responses between resistant and susceptible wheat genotypes. We extracted whole soluble compounds and separated proteins and metabolites from two Russian wheat aphid biotypes (1 and 2), injected these extracts into seedlings of susceptible wheat Gamtoos (dn7) and resistant 94M370 (Dn7), and determined phenotypic and biochemical plant responses. Injections of whole extract or protein extract from both biotypes induced the typical susceptible symptom, leaf rolling, in the susceptible cultivar, but not in the resistant cultivar. Furthermore, multiple injections with protein extract from biotype 2 induced the development of chlorosis, head trapping, and stunting in susceptible wheat. Injection with metabolite, buffer, or chitin, did not produce any susceptible symptoms in either genotype. The protein extract from the two biotypes also induced significantly higher activities of three defense-response enzymes (catalase, peroxidase, and beta-glucanase) in 94M370 than in Gamtoos. These results indicate that a protein elicitor from the Russian wheat aphid is recognized by a plant receptor, and the recognition is mediated by the Dn7-gene product. The increased activities of defense-response enzymes in resistant plants after injection with the protein fraction suggest that defense response genes are induced after recognition of aphid elicitors by the plant.     

Identification of microsatellite markers linked to Russian wheat aphid resistance genes Dn4 and Dn6

The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious economic pest of wheat worldwide. Host plant resistance is the preferred method to control RWA infestations. The identification and mapping of RWA-resistant genes and the development of resistant wheat cultivars can be facilitated through the use of molecular markers. In the present study, microsatellite (SSR) markers linked to the RWA-resistant genes Dn4 and Dn6 were identified using several F(2) mapping populations derived from crosses of susceptible wheat cultivars and resistant sources. Two flanking microsatellite markers Xgwm106 and Xgwm337 are linked in coupling phase with Dn4 on the short arm of wheat chromosome 1D at 7.4 cM and 12.9 cM, respectively. Two other microsatellite markers Xgwm44 and Xgwm111 are linked to Dn6 in coupling phase near the centromere on the short arm of chromosome 7D at 14.6 cM and 3.0 cM, respectively. This is the first report on the chromosomal location of Dn6, which proved to be either allelic or tightly linked to Dn1, Dn2 and Dn5. This result of Dn6 location contradicts previous reports that Dn6 was independent of Dn1, Dn2 and Dn5. The linked markers can be conveniently used for marker-assisted selection in wheat breeding programs for the identification and/or pyramiding of Dn4 and Dn6 genes.     

Categories and inheritance of resistance to Russian wheat aphid (Homoptera: Aphididae) biotype 2 in a selection from wheat cereal introduction 2401

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), is one of the most devastating insect pests of wheat (Triticum spp.) and barley (Hordeum spp.) in the world. Yield losses and control costs are valued at several hundred million dollars each year. The use of D. noxia-resistant cultivars is an ecologically, economically, and biologically sound method of managing this pest. Several D. noxia resistance (Dn) genes from wheat have been used to develop cultivars resistant to D. noxia. However, a new U.S. D. noxia biotype (biotype 2) in Colorado is virulent to all known Dn genes except the Dn7 gene from rye (Secale spp.). Hence, there is an immediate need to identify and characterize unique sources of D. noxia resistance to biotypes. In this article, we report resistance to D. noxia biotype 2, identified in a selection from wheat cereal introduction (CItr) 2401, that is controlled by two dominant genes. CItr2401 has a strong antibiosis effect that is exhibited as a reduced intrinsic rate of increase of D. noxia biotype 2. CItr2401 plants also exhibit tolerance to leaf rolling and chlorosis. No antixenosis was detected in CItr2401.     

Biotypic variation among north American Russian wheat aphid (Homoptera: Aphididae) populations

The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), has been a major economic pest of small grains in the western United States since its introduction in 1986. Recently, a new Russian wheat aphid biotype was discovered in southeastern Colorado that damaged previously resistant wheat, Triticum aestivum L. Biotype development jeopardizes the durability of plant resistance, which has been a cornerstone for Russian wheat aphid management. Our objective was to assess the relative amount of biotypic diversity among Russian wheat aphid populations collected from cultivated wheat and barley, Hordeum vulgare L. We conducted field surveys from May through June 2002 and August 2003 from seven counties within Texas, Kansas, Nebraska, and Wyoming. Based upon a foliar chlorosis damage rating, three new Russian wheat aphid biotypes were identified, one of which was virulent to all characterized sources of Russian wheat aphid resistance. The future success of Russian wheat aphid resistance breeding programs will depend upon the continual monitoring of extant biotypic diversity and determination of the ecological and genetic factors underlying the development of Russian wheat aphid biotypes.     

Development of RAPD and SCAR markers linked to the Russian wheat aphid resistance gene Dn2 in wheat

 RAPD (random amplified polymorphic DNA) analysis was used to identify molecular markers linked to the Dn2 gene conferring resistance to the Russian wheat aphid (Diuraphis noxia Mordvilko). A set of near-isogenic lines (NILs) was screened with 300 RAPD primers for polymorphisms linked to the Dn2 gene. A total of 2700 RAPD loci were screened for linkage to the resistance locus. Four polymorphic RAPD fragments, two in coupling phase and two in repulsion phase, were identified as putative RAPD markers for the Dn2 gene. Segregation analysis of these markers in an F₂ population segregating for the resistance gene revealed that all four markers were closely linked to the Dn2 locus. Linkage distances ranged from 3.3 cM to 4.4 cM. Southern analysis of the RAPD products using the cloned RAPD markers as probes confirmed the homology of the RAPD amplification products. The coupling-phase marker OPB10_880c and the repulsion-phase marker OPN1_400r were converted to sequence characterized amplified region (SCAR) markers. SCAR analysis of the F₂ population and other resistant and susceptible South African wheat cultivars corroborated the observed linkage of the RAPD markers to the Dn2 resistance locus. These markers will be useful for marker-assisted selection of the Dn2 gene for resistance breeding and gene pyramiding. Received: 1 July 1997 / Accepted: 20 October 1997     

Field evaluation of emmer wheat-derived synthetic hexaploid wheat for resistance to Russian wheat aphid (Homoptera: Aphididae)

Broadening the genetic base for resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), in bread wheat, Triticum aestivum L., is desirable. To date, identified Russian wheat aphid resistance genes are either located to the D chromosomes or to rye translocation of wheat, and resistance derived from the A or B genomes of tetraploid Triticum spp. would therefore be highly beneficial. Fifty-eight synthetic hexaploid wheat, derived from interspecific crosses of Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal. and their parents were evaluated for resistance to Russian wheat aphid under field conditions. Plots infested with aphids were compared with plots protected with insecticides. The T. dicoccum parents were highly resistant to Russian wheat aphids, whereas the Ae. tauschii parents were susceptible. Resistance levels observed in the synthetic hexaploids were slightly below the levels of their T. dicoccum parents when a visual damage scale was used. but no major resistance suppression was observed among the synthetics. Russian wheat aphid infestation on average reduced plant height and kernel weight at harvest in the synthetic hexaploids and the T. dicoccum parents by 3-4%, whereas the susceptible control 'Seri M82' suffered losses of above 20%. Because resistance in the synthetic hexaploid wheat is derived from their T. dicoccum parent, resistance gene(s) must be located on the A and/or B genomes. They must therefore be different from previously identified Russian wheat aphid resistance genes, which have all been located on the D genome of wheat or on translocated segments.     

Reproductive rates of Russian wheat aphid (Hemiptera: Aphididae) biotypes 1 and 2 on a susceptible and a resistant wheat at three temperature regimes

The reproductive rates of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), Biotype 1 (RWA 1) and Biotype 2 (RWA 2) were compared in the laboratory at three temperature regimes on a Russian wheat aphid resistant cultivar ('Prairie Red') and a susceptible cultivar ('TAM 107'). The objective of this study was to expose RWA 1 and RWA 2 to three temperature regimes and two levels of resistance to find whether there were reproductive differences that may occur within each biotype as well as differences in reproduction between biotypes. In addition, temperature effects of the Dn4 gene on biotype reproduction were noted. Differences in reproductive rates between the two biotypes seem to be driven by temperature. For both biotypes, longevity and reproductive rate parameters, except for intrinsic rate of increase, were lower at the 24-29 degree C temperature regime than the 13-18 degree C and 18-24 degree C temperature regimes. The intrinsic rate of increase was higher for both biotypes at the 18-24 degree C and 24-29 degree C temperature regimes than at the 13-18 degree C temperature regime. Reproductive rates between biotypes were similar at the two higher temperature regimes, but the fecundity for RWA 1 was less than RWA 2 at the 13-18 degree C temperature. The change in fecundity rates between RWA 1 and RWA 2 at lower temperatures could have ecological and geographical implications for RWA 2.     

Microsatellite markers linked to six Russian wheat aphid resistance genes in wheat

The Russian wheat aphid (RWA), Diuraphis noxia Mordvilko, is a serious economic pest of wheat and barley in North America, South America, and South Africa. Using aphid-resistant cultivars has proven to be a viable tactic for RWA management. Several dominant resistance genes have been identified in wheat, Triticum aestivum, including Dn1 in PI 137739, Dn2 in PI 262660, and at least three resistance genes (Dn5+) in PI 294994. The identification of RWA-resistant genes and the development of resistant cultivars may be accelerated through the use of molecular markers. DNA of wheat from near-isogenic lines and segregating F₂ populations was amplified with microsatellite primers via PCR. Results revealed that the locus for wheat microsatellite GWM111 (Xgwm111), located on wheat chromosome 7DS (short arm), is tightly linked to Dn1, Dn2 and Dn5, as well as Dnx in PI 220127. Segregation data indicate RWA resistance in wheat PI 220127 is also conferred by a single dominant resistance gene (Dnx). These results confirm that Dn1, Dn2 and Dn5 are tightly linked to each other, and provide new information about their location, being 7DS, near the centromere, instead of as previously reported on 7DL. Xgwm635 (near the distal end of 7DS) clearly marked the location of the previously suggested resistance gene in PI 294994, here designated as Dn8. Xgwm642 (located on 1DL) marked and identified another new gene Dn9, which is located in a defense gene-rich region of wheat chromosome 1DL. The locations of markers and the linked genes were confirmed by di-telosomic and nulli-tetrasomic analyses. Genetic linkage maps of the above RWA resistance genes and markers have been constructed for wheat chromosomes 1D and 7D. These markers will be useful in marker-assisted breeding for RWA-resistant wheat. Received: 17 May 2000 / Accepted: 13 June 2000     

Population growth rate and relative virulence of the two South African biotypes of Russian wheat aphid,Diuraphis noxia,and bird cherry‐oat aphid,Rhopalosiphum padi,on resistant and non‐resistant barley

In South Africa a new biotype of the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), RWASA2, has appeared which exhibits an improved performance compared to the original biotype (RWASA1) on wheat containing the Dn1 resistance gene. We examined population growth rates as well as damage caused by RWASA1 and RWASA2, in addition to a different aphid species, the bird cherry‐oat aphid (BCA), Rhopalosiphum padi L. (Hemiptera: Aphididae), on three RWA‐resistant barley [Hordeum vulgare L. (Poaceae)] lines (STARS‐9577B, STARS‐0502B, and STARS‐9301B) and one susceptible control (PUMA). RWASA2 had a higher reproductive rate than RWASA1 on all barley lines tested, which is consistent with previous results on wheat. Two of the RWA‐resistant lines (STARS‐0502B and STARS‐9301B) also exhibited a similar resistance phenotype against BCA. In our experiments, severe chlorosis and leaf roll appeared earlier on the control PUMA barley variety as a result of RWASA2 feeding than was the case with RWASA1, probably due to the differences in reproductive rate. Although chlorosis appeared earlier on resistant plants after RWASA2 feeding, this symptom developed much faster during RWASA1 feeding on all three resistant lines tested. As chlorosis did not correlate well with aphid population numbers, we surmise that the differential chlorosis effects may be related to differences in the amount of saliva introduced by the two aphid clones during feeding. Our results indicate that the difference between RWASA2 and RWASA1 are broader than a ‘gene for gene’ interaction with the Dn1 resistance (R) gene in wheat, and that these biotypes also differ in important aspects of their biology.