Cyclical parthenogenetic reproduction in the Russian wheat aphid (Hemiptera: Aphididae) in the United States: sexual reproduction and its outcome on biotypic diversity

In 1986, the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), became an invasive species of United States. Nearly 20 yr later, new biotypes appeared that were capable of overcoming most sources of resistance and became a renewed threat to wheat, Triticum aestivum L., production. Cyclical (CP) and obligate (OP) parthenogenesis enables aphids to both adapt to changing environments and exploit host resources. We documented these forms of reproduction for Russian wheat aphid in wheat and wild grasses in the Central Great Plains and Rocky Mountain regions during falls 2004-2009. Colonies from sample sites also were held under unheated greenhouse conditions and observed for the presence of sexual morphs and eggs through the winter. Russian wheat aphid populations were mainly OP and attempted to overwinter as adults, regardless of region sampled. A few populations contained oviparae but no males (gynocyclic) and were not specific to any particular region. Observation of the Russian wheat aphid colonies under greenhouse conditions failed to produce males or eggs. In spring 2007, CP was confirmed in a small population of Russian wheat aphid that eclosed from eggs (fundatricies) on wild grasses and wheat near Dove Creek, CO, in the Colorado Plateau region where other aphid species undergo CP. Lineages from ninety-three fundatricies were screened against 16 resistant and susceptible cereal entries to determine their biotypic classification. A high degree of biotypic diversity (41.4%) was detected in this population. Although CP was a rare in Russian wheat aphid populations, genetic recombination during the sexual cycle creates new biotypes and can have significant effects on population genetics.

Host associations and incidence of Diuraphis spp. in the Rocky Mountain region of the United States, and pictorial key for their identification

The Russian wheat aphid, Diuraphis noxia Kurdjumov, is an introduced species first identified in 1986 into the United States. It has since become a major pest of wheat, Triticum aestivum L., and other small grains in the western United States. Three other Diuraphis species, Diuraphis frequens (Walker), Diuraphis mexicana (McVicar Baker), and Diuraphis tritici (Gillette), were already endemic to the United States before the introduction of D. noxia. The objective of this study was to determine the occurrence and host associations of these four Diuraphis spp. in the Rocky Mountain region that borders the western Great Plains to better understand their distribution and ecological interactions. In addition, a key to these species with photographs of live or fresh preparations of specimens is presented to aid in their identification. D. noxia was the most widely distributed species in the study area spanning the Rocky Mountain areas of Wyoming, New Mexico, Utah, and Colorado. This species was most common in the cereal-producing areas of the Colorado Plateau ecoregion. D. frequens was found to be the predominant species in the Alpine/Aspen Mountain areas of the South Central Rockies and Colorado Rockies ecoregions. The other Diuraphis species were rarely encountered even though their plant hosts occurred in the ecoregions sampled. D. noxia shared common hosts and was found co-infesting grasses with other Diuraphis species. Therefore, the potential exists for D. noxia to impact the other native Diuraphis species.

Biotypic diversity in greenbug (Hemiptera: Aphididae): microsatellite-based regional divergence and host-adapted differentiation

Nineteen isolates of the cereal aphid pest greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), were collected from wheat, Triticum aestivum L.; barley, Hordeum vulgare L.; or noncultivated grass hosts in five locations from Colorado and Wyoming. Parthenogenetic colonies were established. Biotypic profiles of the 19 isolates were determined based on their abilities to damage a set of host plant differentials, and 13 new biotypes were identified. Genetic diversity among the 19 isolates and five previously designated greenbug biotypes (E, G, H, I, and K) was examined with 31 cross-species transferable microsatellite (simple sequence repeat) markers. Neighbor-joining clustering analysis of marker data revealed host-adapted genetic divergence as well as regional differentiation of greenbug populations. Host associated biotypic variation seems to be more obvious in "agricultural biotypes," whereas isolates collected from noncultivated grasses tend to show more geographic divergence. It seems that the biotype sharing the most similar biotypic profiles and the same geographic region with current prevailing one may have the greatest potential to become the new prevailing biotype. Close monitoring of greenbug population dynamics especially biotypic variation on both crop plants and noncultivated grasses in small grain production areas may be a useful strategy for detecting potentially new prevailing virulent biotypes of the greenbug.

Biology and biotype determination of greenbug, Schizaphis graminum (Hemiptera: Aphididae), on seashore paspalum turfgrass (Paspalum vaginatum)

Greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), was first discovered damaging seashore paspalum (Paspalum vaginatum Swartz) turfgrass in November 2003 at Belle Glade, FL. Inquiries to several golf courses with seashore paspalum turf across southern Florida indicated infestation was wide spread by April 2004. Damage symptoms progress from water soaked lesions surrounding feeding sites within 24 h to chlorosis and necrosis of leaf tips within 96 h. Problems caused by greenbug feeding were initially misdiagnosed as fertilizer, disease, other insects, or water management problems because aphids were not previously found on warm season turfgrasses. Greenbug development and fecundity studies were conducted on six seashore paspalum varieties: 'Aloha,' 'SeaDwarf,' 'SeaGreen,' 'SeaIsle,' 'SeaWay,' and 'SeaWolf.' Greenbug did not survive on 'SeaWolf.' Development rates (mean +/- SEM) ranged from 7.6 +/- 0.2 to 8.2 +/- 0.2 d on the remaining varieties. Greenbug longevity and fecundity on 'Aloha' were significantly less than on the other varieties. The estimated intrinsic rate of natural increase (r(m)) for greenbug ranged from 0.24 to 0.26 across tested varieties. Values for net reproductive rate (R(o)) ranged from 12.3 on 'Aloha' to 40.4 on 'SeaWay.' In feeding trials on indicator plants, the Florida isolate of greenbug exhibited a unique biotypic profile most commonly found on noncultivated grass hosts. It was virulent on the wheat variety GRS1201 that is resistant to the principal agricultural biotypes attacking small grains and to all currently available resistant sorghum varieties.

Physiological and biochemical responses of resistant and susceptible wheat to injury by Russian wheat aphid

We examined the physiological and biochemical responses of resistant ('Halt' and 'Prairie Red') and susceptible ('TAM 107') wheat, Triticum aestivum L., to injury by the Russian wheat aphid, Diuraphis noxia (Mordvilko). Photosynthetic capacity was evaluated by measuring assimilation/internal CO2 (A/Ci) curves, chlorophyll fluorescence, chlorophyll, and nonstructural carbohydrate content. Total protein and peroxidase specific activity also were determined. No significant differences were detected in chlorophyll concentration between aphid-infested and control TAM 107 plants. The aphid-infested resistant cultivars had similar or significantly higher chlorophyll concentrations compared with their respective control plants. Measurements over time showed that infested Halt plants had delays in photosynthetic senescence, Prairie Red plants had photosynthetic rate changes that were similar to control plants, and TAM 107 plants displayed accelerated photosynthetic senescence patterns. The photochemical and nonphotochemical quenching coefficients were significantly higher in infested Halt plants compared with their respective control plants on day 3. Infested TAM 107 plants had significantly higher photochemical quenching compared with control plants at all times evaluated, and they had significantly higher nonphotochemical quenching on day 3. Throughout the experiment, infested Prairie Red plants exhibited photochemical and nonphotochemical quenching coefficient values that were not significantly different from control plants. Total protein content was not significantly different between aphid-infested and control plants for all cultivars. Differences between physiological responses of infested susceptible and resistant cultivars, particularly temporal changes in photosynthetic activity, imply that resistant Halt and Prairie Red wheat tolerate some impacts of aphid injury on photosynthetic integrity.

Physiological and biochemical responses of resistant and susceptible wheat to injury by Russian wheat aphid

We examined the physiological and biochemical responses of resistant ('Halt' and 'Prairie Red') and susceptible ('TAM 107') wheat, Triticum aestivum L., to injury by the Russian wheat aphid, Diuraphis noxia (Mordvilko). Photosynthetic capacity was evaluated by measuring assimilation/internal CO2 (A/Ci) curves, chlorophyll fluorescence, chlorophyll, and nonstructural carbohydrate content. Total protein and peroxidase specific activity also were determined. No significant differences were detected in chlorophyll concentration between aphid-infested and control TAM 107 plants. The aphid-infested resistant cultivars had similar or significantly higher chlorophyll concentrations compared with their respective control plants. Measurements over time showed that infested Halt plants had delays in photosynthetic senescence, Prairie Red plants had photosynthetic rate changes that were similar to control plants, and TAM 107 plants displayed accelerated photosynthetic senescence patterns. The photochemical and nonphotochemical quenching coefficients were significantly higher in infested Halt plants compared with their respective control plants on day 3. Infested TAM 107 plants had significantly higher photochemical quenching compared with control plants at all times evaluated, and they had significantly higher nonphotochemical quenching on day 3. Throughout the experiment, infested Prairie Red plants exhibited photochemical and nonphotochemical quenching coefficient values that were not significantly different from control plants. Total protein content was not significantly different between aphid-infested and control plants for all cultivars. Differences between physiological responses of infested susceptible and resistant cultivars, particularly temporal changes in photosynthetic activity, imply that resistant Halt and Prairie Red wheat tolerate some impacts of aphid injury on photosynthetic integrity.

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.

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.

Response of resistant and susceptible barley to infestations of five Diuraphis noxia (Homoptera: Aphididae) biotypes

Since 2003, four new biotypes of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), RWA2-RWA5, have been discovered that have the ability to damage most of the wheat germplasm resistant to the original Russian wheat aphid population (RWA1). Barley germplasm lines with resistance to RWA1 have not yet been evaluated against the newest biotypes. Our study compared how biotypes RWA1-RWA5 affected the growth and leaf damage of RWA1-resistant germplasm (STARS 9301B, STARS 9577B), moderately resistant germplasm (MR-015), and susceptible varieties (Schuyler, Harrington, and Morex) under greenhouse conditions. Russian wheat aphid population levels also were determined 14 d after plant infestation. STARS 9301B exhibited strong resistance by showing only small differences in leaf damage and growth parameters from the feeding by the biotypes. STARS 9577B showed greater differences in damage by the Russian wheat aphid biotypes than STARS 9301B, yet, the ratings were still within the resistant category (e.g., chlorosis rating 2.3-4.9). Leaf chlorosis ratings for MR-015 ranged from 5.0 to 6.9 and fell within the moderately resistant to susceptible categories for all the biotypes. The greatest difference in leaf chlorosis occurred in Morex where RWA2 showed less virulence than the other biotypes. Feeding by the Russian wheat aphid biotypes produced only small differences in leaf rolling and plant growth within plant entries. Population levels of the Russian wheat aphid biotypes did not differ within barley entries (n = 610-971) at the termination of the study (14 d). From our research, we conclude that the new Russian wheat aphid biotypes pose no serious threat to the key sources of resistance in barley (STARS 9301B and 9577B).

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.

Biotypic diversity in greenbug (Hemiptera: Aphididae): characterizing new virulence and host associations

Biotypic diversity of the greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), was assessed among populations collected from cultivated wheat, Triticum aestivum L., and sorghum, Sorghum bicolor (L.) Moench, and their associated noncultivated grass hosts. Greenbugs were collected during May through August 2002 from 30 counties of Kansas, Nebraska, Oklahoma, and Texas. Discounting the presumptive biotype A, five of the remaining nine letter-designated greenbug biotypes were collected; however, biotypes C, F, J, and K were not detected. Biotypes E and I exhibited the greatest host range and were the only biotypes collected in all four states. Sixteen greenbug clones, collected from eight plant species, exhibited unique biotype profiles. Eleven were collected from noncultivated grasses, three from wheat, and two from sorghum. The most virulent biotypes were collected from noncultivated hosts. The great degree of biotypic diversity among noncultivated grasses supports the contention that the greenbug species complex is composed of host-adapted races that diverged on grass species independently of, and well before, the advent of modern agriculture.

Biotypic diversity in greenbug (Hemiptera: Aphididae): characterizing new virulence and host associations

Biotypic diversity of the greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), was assessed among populations collected from cultivated wheat, Triticum aestivum L., and sorghum, Sorghum bicolor (L.) Moench, and their associated noncultivated grass hosts. Greenbugs were collected during May through August 2002 from 30 counties of Kansas, Nebraska, Oklahoma, and Texas. Discounting the presumptive biotype A, five of the remaining nine letter-designated greenbug biotypes were collected; however, biotypes C, F, J, and K were not detected. Biotypes E and I exhibited the greatest host range and were the only biotypes collected in all four states. Sixteen greenbug clones, collected from eight plant species, exhibited unique biotype profiles. Eleven were collected from noncultivated grasses, three from wheat, and two from sorghum. The most virulent biotypes were collected from noncultivated hosts. The great degree of biotypic diversity among noncultivated grasses supports the contention that the greenbug species complex is composed of host-adapted races that diverged on grass species independently of, and well before, the advent of modern agriculture.

Physiological modification of the host feeding site by cereal aphids (Homoptera: Aphididae)

Indole-3-acetic acid-l-14C and 14C-sucrose labels were used to study the effects of greenbugs, Schizaphis graminum (Rondani), and Russian wheat aphids, Diuraphis noxia (Mordvilko), on phloem function of wheat (Triticum aesticum L.). Greenbug feeding significantly reduced translocation from the immediate feeding site; however, phloem integrity was not impeded. In contrast, Russian wheat aphids had little effect on vein loading or phloem translocation at the feeding site. Similar results were obtained when resistant and susceptible wheats were infested with three different greenbug biotypes. Greenbugs fed artificial diets containing 14C-sucrose injected salivary material that was translocated to both root and shoot systems. The accumulation of salivary constituents in the roots of wheat seedlings fed upon by greenbugs may account for the significant reductions in root biomass that have previously been reported.

Mitochondrial DNA sequence divergence among Schizaphis graminum (Hemiptera: Aphididae) clones from cultivated and non-cultivated hosts: haplotype and host associations

A 1.0 kb region of the mitochondrial cytochrome oxidase subunit I gene from the greenbug aphid, Schizaphis graminum (Rondani), was sequenced for 24 field collected clones from non-cultivated and cultivated hosts. Maximum likelihood, maximum parsimony and neighbour-joining phylogenies were estimated for these clones, plus 12 previously sequenced clones. All three tests produced trees with identical topologies and confirmed the presence of three clades within S. graminum. Clones showed no relationship between biotype and mtDNA haplotype. At least one biotype was found in all three clades, suggesting exchange among clades of genetic material conditioning for crop virulence, or the sharing of a common ancestor. However, there was a relationship between host and haplotype. Clade 1 was the most homogeneous and contained 12 of 16 clones collected from cultivated hosts and five of the six collected from johnsongrass, Sorghum halepense, a congener of cultivated sorghum, S. bicolor. Four of the six clones collected from Agropyron spp. were found in clade 2. Clade 3 contained two clones from wheat, Triticum aestivum, and four from non-cultivated hosts other than Agropyron spp. A partitioning of populations by mtDNA haplotype and host suggests the occurrence of host adapted races in Schizaphis graminum.

Efficacy of pyramiding greenbug (Homoptera: Aphididae) resistance genes in wheat

Durable resistance to greenbug, Schizaphis graminum (Rondani), in wheat is a goal of wheat improvement teams, and one that has been complicated by the regular occurrence of damaging biotypes. Simulation modeling studies suggest that pyramiding resistance genes, i.e., combining more than one resistance gene in a single cultivar or hybrid, may provide more durable resistance than sequential releases of single genes. We examined this theory by pyramiding resistance genes in wheat and testing a series of greenbug biotypes. Resistance genes Gb2, Gb3, and Gb6, and pyramided genes Gb2/Gb3, Gb2/Gb6, and Gb3/Gb6 were tested for effectiveness against biotypes E, F, G, H, and I. By comparing reactions of plants with pyramided genes to those with single resistance genes, we found that pyramiding provided no additional protection over that conferred by the single resistance genes. Based on the results of this test, we concluded that the sequential release of single resistance genes, combined with careful monitoring of greenbug population biotypes, is the most effective gene deployment strategy for greenbug resistance in wheat.

Mitochondrial DNA sequence divergence among greenbug (Homoptera: aphididae) biotypes: evidence for host-adapted races

The full complement of known greenbug, Schizaphis graminum (Rondani), biotypes found in the USA were subjected to a molecular phylogenetic analysis based on a 1.2-kb portion of the cytochrome oxidase I mitochondrial gene. In addition to these nine biotypes (B, C, E, F, G, H, I, J and K), a probable isolate of the enigmatic biotype A (NY), a 'new biotype' collected from Elymus canadensis (L.) (CWR), and an isolate from Germany (EUR) were included. Schizaphis rotundiventris (Signoret) was included as an outgroup. Genetic distances among S. graminum biotypes ranged from 0.08% to 6.17% difference in nucleotide substitutions. Neighbour-joining, maximum parsimony and maximum likelihood analyses all produced dendrograms revealing three clades within S. graminum. Clade 1 contained the 'agricultural' biotypes commonly found on sorghum and wheat (C, E, K, I, plus J) and there were few substitutions among these biotypes. Clade 2 contained F, G and NY, and Clade 3 contained B, CWR and EUR, all of which are rarely found on crops. The rarest biotype, H, fell outside the above clades and may represent another Schizaphis species. S. graminum biotypes are a mixture of genotypes belonging to three clades and may have diverged as host-adapted races on wild grasses.

Relative suitability of crested wheatgrass and other perennial grass hosts for the Russian wheat aphid (Homoptera: Aphididae)

The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), reproduces parthenogenetically in North America and must survive year-round on host plants, including in late summer when small grains are not in cultivation. During this time, cool-season perennial wheatgrasses (Poaceae: Triticeae) contribute substantially to aphid survival, crested wheatgrass (Agropyron spp.) particularly. In greenhouse studies, the number of aphids per plant was measured after four infestation periods on unvernalized and vernalized wheatgrasses. Before placement on these test plant species, aphids were reared either on winter wheat or on the grass host species on which aphid progeny were counted. On vernalized plants, aphids reared on wheat resulted in more aphids per test plant than when the aphids were reared on wheatgrasses, but on unvernalized plants the number of aphids per test plant did not differ significantly regardless of rearing host. Aphids on crested wheatgrass were similar in number to the other grasses when plants were unvernalized. However, when plants were vernalized, crested wheatgrass supported significantly more aphids than some of the other hosts. Aphid numbers increased on all test species as infestation period lengthened, and plant growth was largely unaffected by aphid feeding. These results suggest if sufficient moisture is available during summer when small grains are not in cultivation, all host species observed are capable of sustaining aphids. Crested wheatgrass is an abundant and important host of the Russian wheat aphid in its northern range of the western United States, but other less prevalent wheatgrasses also may contribute to aphid survival during late summer when small grains are not in cultivation.