Diuraphis noxia and Rhopalosiphum padi (Hemiptera: Aphididae) interactions and their injury on resistant and susceptible cereal seedlings

Interspecific interactions between the symptomatic (chlorosis-eliciting) Russian wheat aphid, Diuraphis noxia (Mordvilko), and the asymptomatic (nonchlorosis-eliciting) bird cherry-oat aphid, Rhopalosiphum padi (L.), on four cereal genotypes were examined by simultaneous infestations. Four cereals (i.e., Diuraphis noxia-susceptible 'Arapahoe' wheat and 'Morex' barley, and D. noxia-resistant 'Halt' wheat and 'Border' oat) and four infestations (i.e., control, D. noxia, R. padi, and D. noxia/R. padi) were used in the research. Whereas D. noxia biomass confirmed D. noxia resistance among the cereals, R. padi biomass indicated that the D. noxia-resistant cereals did not confer R. padi resistance. D. noxia biomass was significantly lower in D. noxia/R. padi infestation than that in D. noxia infestation on all cereals, except Border oat, which indicated an antagonistic effect of R. padi on D. noxia. All aphid infestations caused a significant plant biomass reduction in comparison with the control. In comparison with D. noxia infestation, D. noxia/R. padi caused a significant plant biomass reduction on all cereals, except Morex barley. Although D. noxia biomass in D. noxia/R. padi infestation was significantly less than that in D. noxia infestation, leaf chlorophyll reduction was the same between D. noxia/R. padi and D. noxia infestations, which suggested that the asymptomatic R. padi enhanced the D. noxia-elicited leaf chlorophyll loss. The regression between chlorophyll content and aphid biomass indicated that the asymptomatic R. padi in the D. noxia/R. padi infestation enhanced chlorophyll loss, but interspecific aphid interaction on plant biomass varied among the cereals.     

Possible roles of esterase, glutathione S-transferase, and superoxide dismutase activities in understanding aphid–cereal interactions

Activities of the detoxification enzymes esterase, glutathione S‐transferase, and of superoxide dismutase in aphids and aphid‐infested cereal leaves were assayed using polyacrylamide gel electrophoresis and a spectrophotometer to elucidate the enzymatic mechanisms of aphid resistance in cereal plants. A chlorosis‐eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and non‐chlorosis‐eliciting bird cherry‐oat aphid, Rhopalosiphum padi (L.), and four cereals were used in this study. The four cereal genotypes were ‘Arapahoe’ (susceptible) and ‘Halt’ (resistant) wheat (Triticum aestivum L.), ‘Morex’ (susceptible) barley (Hordeum vulgare L.), and ‘Border’ (resistant) oat (Avena sativa L.). Esterase isozymes differed between the two aphid species, although glutathione S‐transferase and superoxide dismutase did not. Esterase, glutathione S‐transferase, and superoxide dismutase activities in either aphid species were not affected by the level of resistance of a cereal to D. noxia. The assays of cereal leaf samples showed that D. noxia feeding elicited an increase in esterase activity in all four cereal genotypes, although R. padi feeding did not. The increase of esterase activity in cereals, however, was not correlated to aphid resistance in the cereals. The time‐series assays of aphid‐infested cereal leaves showed that D. noxia‐infested Morex barley had a significant increase in esterase activity on all sampling dates (3, 6, and 9 days) in comparison with either uninfested or R. padi‐infested barley. No difference in glutathione S‐transferase activity was detected among either aphid infestations or sampling dates. The electrophoretic assays, however, revealed that aphid feeding elicited a significant increase in superoxide dismutase activity, which served as the control of glutathione S‐transferase activity assays. The increase in esterase and superoxide dismutase activities suggested that D. noxia feeding imposes not only toxic, but also oxidative stresses on the cereals. The ramification of using these enzyme activity data to understand the etiology of D. noxia‐elicited chlorosis is discussed.     

Seasonal dynamics of cereal aphids on Russian wheat aphid (Homoptera: Aphididae) susceptible and resistant wheats

Field experiments were conducted in 1997 and 1998 to evaluate the impact of resistance to Russian wheat aphid, Diuraphis noxia (Mordvilko), on the cereal aphid complex in wheat. Two spring wheats were planted: the variety "Centennial" (Russian wheat aphid susceptible) and the advanced line IDO488 (Russian wheat aphid resistant). IDO488 incorporates the resistance found in PI 294994 into a Centennial background. Field plots were artificially infested with adult D. noxia and sampled weekly. The most abundant aphid species in 1997 were Metopolophium dirhodum (Walker), Sitobion avenae (F.), D. noxia, and Rhopalosiphum padi (L.). In 1998, the order of abundance was M. dirhodum, R. padi, S. avenae, and D. noxia. The resistant genotype had significantly fewer D. noxia than the susceptible one during both years. However, plant genotype had no significant effect on the other aphid species in either year. Both the initial density of D. noxia and plant growth stage, had a significant effect on D. noxia population development, but had no effect on the other aphid species. There was no interaction between D. noxia resistance and the population density of the other aphid species observed.     

Feeding damage by Diuraphis noxia results in a nutritionally enhanced phloem diet

The feeding behavior of Diuraphis noxia Mordvilko (Homoptera: Aphididae) on susceptible hosts causes both ultrastructural and tissue level damage which may affect phloem composition. Genetic evidence suggests that endosymbiotic bacteria in most aphids overproduce limiting amino acids to benefit hosts but that D. noxia depends less on endosymbionts for these nutrients, possibly due to an enriched diet. To determine whether D. noxia feeding damage results in higher concentrations of essential amino acids, stylet exudates were analyzed from wheat (Triticum aestivum) damaged to different degrees. Comparison of samples from undamaged and damaged susceptible wheat revealed changes in amino acid composition and an increase in levels of essential amino acids, indicating a nutritionally enhanced ingesta. The changes in stylet exudates paralleled changes in leaf exudates, indicating that the effects are systemic. Feeding damage is not observed on a resistant wheat host, var. Halt, and leaf exudates from infested Halt did not show changes in amino acid composition. Mean relative growth of nymphs was significantly lower on Halt than on susceptible Arapahoe, indicating that Halt is a less suitable host. Both varieties show similar amino acid levels in non-infested samples, suggesting that D. noxia infestation does not enhance the phloem environment in Halt. This study provides evidence that aphid feeding can generate a nutritionally enhanced phloem diet.     

Nutritional enhancement of host plants by aphids - a comparison of three aphid species on grasses

Three aphid species were compared with respect to ability of enhancing the nutritional quality of their host plants. Rhopalosiphum padi, which does not induce macroscopic changes in its host plants, was compared with Schizaphis graminum and Diuraphis noxia, both of which induce distinctive types of chlorotic lesions. Phloem sap samples were collected from severed stylets of feeding aphids and from exudates of cut leaves of plants uninfested or infested with each aphid species. Samples were analyzed for concentrations of individual amino acids.Compared to R. padi, S. graminum ingested phloem sap with a two-fold higher concentration of amino acids and a much higher proportion of essential amino acids. Similar differences between these two aphid species were observed on both wheat and barley. For each aphid species, the absolute concentrations of amino acids and the relative proportions of essential amino acids were similar between the two host plants. Effects of D. noxia were similar to those of S. graminum, though less dramatic. Exudates from leaves infested with each aphid species showed relative concentrations of individual amino acids that were similar to those in the corresponding stylet exudates. Based on comparison of stylet exudates and cut leaf exudates from infested and uninfested plants, R. padi seems to have little effect on amino acid composition of phloem. Changes in the phloem induced by both S. graminum and D. noxia appear to be systemic, affecting at least the whole leaf they are feeding on. The changes observed for D. noxia and for S. graminum are likely to be nutritionally advantageous for the aphids and are expected to affect the aphids' dependence on nutritional supplementation by intracellular symbionts (Buchnera).     

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.     

Effect of a co-occurring aphid on the susceptibility of the Russian wheat aphid to lacewing predators

Previous field experiments indicated that the presence of the bird cherry-oat aphid, Rhopalosiphum padi (L.), on perennial grasses can decrease the effectiveness of predatory lacewings, Chrysoperla plorabunda (Fitch), in reducing populations of the Russian wheat aphid, Diuraphis noxia (Mordvilko). We tested the hypothesis that R. padi deflects predation away from D. noxia because it feeds in sites that are more accessible to predators. We quantified the behavior of lacewing larvae on crested wheatgrass plants bearing either D. noxia alone or an equal mixture of D. noxia and R. padi. On non-flowering plants, R. padi typically occurred on leaf sheaths or open blades, and was encountered and captured more often than D. noxia, which usually fed within immature, rolled leaves. Overall time-budgets of lacewings did not differ between the pure-D. noxia and mixed-species treatments, but >75% of the time spent consuming aphids in the mixed-species treatment was devoted to R. padi. On flowering plants, D. noxia usually aggregated on the flag leaf below the inflorescence, whereas R. padi occurred mostly on leaf sheaths. Predators again captured R. padi more often than D. noxia, and spent more time consuming aphids in the mixed-species treatment than in the pure-D. noxia treatment. These behavioral observations support the hypothesis that non-target prey can hamper the short-term effectiveness of biological-control agents against D. noxia.     

Influence of cereal leaf epicuticular wax on Diuraphis noxia probing behavior and nymphoposition

The effect of cereal leaf surface wax on Diuraphis noxia (Mordvilko), the Russian wheat aphid, probing behavior and nymphoposition was evaluated. Ultrastructure of leaf epicuticular wax from wheat (Triticum aestivum L.) c.v. Arapahoe and Halt was different from barley (Hordeum vulgare L.) c.v. Morex, and oat (Avena sativa L.) c.v. Border. Both wheat cultivars had similar rod-shaped epicuticular wax, while barley and oat plants had flakes. The chemical composition comparison of gas chromatograms also indicated that the extract of the two wheat cultivars had similar pattern of peaks, while the barley and oat leaves had similar peaks. Cereal variety significantly affected aphid probing behavior (P < 0.05), but wax removal using ethyl ether swab did not (P < 0.05). Aphids initiated significantly more probes on Border oat leaves than on Morex barley irrespective of wax removal, although total probing duration per aphid was not significantly different among the four cereals examined. Accumulative salivation duration per aphid on oat leaves with wax was significantly longer than other cereal leaves with wax, while accumulative ingestion duration per aphid on Arapahoe wheat and Morex barley was significantly longer than on oat. Nymphoposition of D. noxia on cereal leaves maintained on the benzimidazole-agar medium showed that aphids produced a greater number of nymphs on Morex barley and less on Border oat leaves, although wax removal did not affect aphid nymphoposition. Removal of leaf epicuticular waxes from the 4 cereal genotypes using ethyl ether swab indicated that the influence of wax on plant resistance to D. noxia probing and reproduction was limited. Morex barley was the most favorable, while Border oat was the least favorable cereal host of D. noxia.     

Light activation of Russian wheat aphid-elicited physiological responses in susceptible wheat

The impact of light and its role in Russian wheat aphid, Diuraphis noxia (Mordvilko), damage symptom formation, and photosynthetic capacity in 'Arapahoe' wheat (Triticum aestivum L.) were examined. After 72 h under continuous dark or continuous light regimes, the number of aphids (nymphs), leaf rolling and chlorosis ratings, fresh leaf weight, and chlorophyll contents were recorded. Photosynthetic rates, chlorophyll a, kinetics and chlorophyll extractions also were determined. Aphid infestation caused significant reductions in plant height, fresh weight, gas exchange, and chlorophyll fluorescence only under continuous light. Under the 72 h continuous dark regime, aphid infestation did not cause either damage symptom formation or reduction in plant growth or metabolism (photosynthesis). Furthermore, significantly more D. noxia nymphs were produced under continuous light condition than continuous dark. Our results demonstrate that the development of D. noxia feeding damage symptoms (i.e., leaf rolling and chlorotic streaks) on susceptible wheat seedlings is a light-activated process, even though the elicitor of the plant damage symptoms is aphid feeding.     

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).     

Compatibility of insect management strategies: Diuraphis noxia abundance on susceptible and resistant barley in the presence of parasitoids

Russian wheat aphid, Diuraphis noxia, and parasitoid abundance was monitored on field-grown barley, Hordeum vulgare L., varying in D. noxia susceptibility, to address the applicability of previous laboratory assessments of barley seedling resistance and parasitoid compatibility. Study sites were representative of the barley production region of the High Plains in the western USA, where D. noxia and its parasitoids occur. D. noxia abundance on resistant barley lines, characterized as partially tolerant and antibiotic to the aphid, was lower than on more susceptible lines. Parasitism by Diaeretiella rapae, Aphelinus albipodus, and A. asychis differed in seasonal occurrence and abundance. D. rapae mummies occurred sooner than aphelinid mummies, and there were larger increases in aphelinid mummies than in D. rapae mummies during seed head development. But in regard to plant resistance, parasitoid abundance, relative to D. noxia abundance, was similar on resistant and susceptible barley lines. Based on the susceptibility of commercial barley to D. noxia, the seasonal abundance of D. noxia and its parasitoids, and the compatibility of resistant barley and D. noxia parasitoids, the use of resistant barley in areas of parasitoid establishment is justified.     

Comparison of structural damage caused by Russian wheat aphid (Diuraphis noxia) and Bird cherry-oat aphid (Rhopalosiphum padi) in a susceptible barley cultivar, Hordeum vulgare cv. Clipper

The Russian wheat aphid (RWA, ( Diuraphis noxia ) and the Bird cherry-oat aphid (BCA, ( Rhopalosiphum padi L.) cause severe damage to grain crops, including barley. An investigation of the effects of these aphids on a susceptible cultivar revealed that BCA-infested barley plants remained healthy looking for 2 weeks after feeding commenced. In contrast, signs of stress and damage, including chlorosis and leaf necrosis were evident in RWA-infested plants. Our study suggests that damage to the vascular tissue because of sustained feeding by BCA was not as extensive as that caused by RWA. In addition, there is a marked difference in the salivary secretion pattern within xylem elements punctured by aphids tapping the xylem for water. RWA deposit electron-dense, amorphous to smooth saliva, which completely encases the inner walls of affected elements, and saliva encases pit membranes between xylem elements, and between xylem vessels and xylem parenchyma. Xylem tapped by BCA contained more granular saliva, which apparently does not occlude vessel wall apertures or the pit membranes to the same extent, as was observed with RWA. Damage to phloem tissue, including phloem parenchyma elements, sieve tube–companion cell (CC–ST) complexes as well as thick-walled ST, was extensive. Plasmodesmata between phloem parenchyma elements as well as pore plasmodesmata between the CC and ST were occluded by callose. We conclude that severe, perhaps permanent damage to conducting elements in RWA-infested leaves may be responsible for the detrimental chlorosis and necrosis symptoms. These symptoms are absent in BCA-infested plants.     

Enzymatic chlorophyll degradation in wheat near-isogenic lines elicited by cereal aphid (Homoptera: Aphididae) feeding

Chlorophyll degradation enzyme (i.e., chlorophyllase, Mg-dechelatase, and chlorophyll oxidase) activities of aphid-infested and uninfested 'Tugela' and Tugela near-isogenic wheat lines (i.e., Tugela-Dn1, Tugela-Dn2, and Tugela-Dn5) were assayed. Chlorophyllase activity was higher in bird cherry-oat aphid, Rhopalosiphum padi (L.) (Homoptera: Aphididae),-infested wheat lines compared with Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae)]-infested and uninfested plants. Mg-dechelatase activity was higher in D. noxia-infested wheat lines than in R. padi-infested and uninfested plants. Also, Mg-dechelatase activity was lower in Tugela wheat infested with D. noxia than in Tugela near-isogenic lines with Dn genes. Based on the in vitro assays of chlorophyll degradation enzyme (i.e., chlorophyllase and Mg-dechelatase) activities, we proposed that the chlorotic symptoms observed on D. noxia-infested Tugela wheat were most likely to be elicited by unbalanced chlorophyll biosynthesis and degradation.     

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.     

Comparison of chlorophyll and carotenoid concentrations among Russian wheat aphid (Homoptera: Aphididae)-infested wheat isolines

Russian wheat aphid, Diuraphis noxia (Mordvilko), feeding injury on 'Betta' wheat isolines with the Dn1 and Dn2 genes was compared by assessing chlorophyll and carotenoid concentrations, and aphid fecundity. The resistant Betta isolines (i.e., Betta-Dn1 and Betta-Dn2) supported similar numbers of aphids, but had significantly fewer than the susceptible Betta wheat, indicating these lines are resistant to aphid feeding. Diuraphis noxia feeding resulted in different responses in total chlorophyll and carotenoid concentrations among the Betta wheat isolines. The infested Betta-Dn2 plants had higher levels of chlorophylls and carotenoids in comparison with uninfested plants. In contrast, infested Betta-Dn1 plants had the same level of chlorophyll and carotenoid in comparison with uninfested plants. Our data provide essential information on the effect of D. noxia feeding on chlorophyll and carotenoid concentrations for Betta wheat and its isolines with D. noxia-resistant Dn1 and Dn2 genes.     

Aphid (Hemiptera: Aphididae) resistance in wheat near-isogenic lines

Plant and aphid biomass, photosynthetic pigment (chlorophylls a and b and carotenoids) concentrations, and chlorophyll a/b and chlorophyll/carotenoid ratios were quantified in aphid-infested 'Tugela' near-isogenic lines (Tugela, Tugela-Dn1, Tugela-Dn2, and Tugela-Dn5). The objectives were to quantify changes of photosynthetic pigments (chlorophylls a and b, and carotenoids) caused by aphid feeding and assess resistance of wheat isolines through aphid and plant biomass analysis. Biomass of bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae)-infested plants was lower than Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae),- infested plants. When infested by D. noxia, all lines showed increased biomass over time, except Tugela where biomass decreased on day 12. No difference in plant biomass was detected among R. padi-infested and uninfested wheat lines. Biomass of D. noxia from Tugela (D. noxia-susceptible) was significantly higher than from plants with Diuraphis noxia-resistant Dn genes. Diuraphis noxia biomass from Tugela-Dn1 and Dn2 lines was not different from each other, but they were lower than from Tugela-Dn5. In contrast, there was no difference in R. padi biomass among wheat lines. Concentrations of chlorophylls a and b and carotenoids were significantly lower in D. noxia-infested plants compared with R. padi-infested and uninfested plants. When infested by D. noxia, chlorophyll a and b concentrations were not different among wheat lines on day 3, but they were lower in Tugela and Tugela-Dn1 than in Tugela-Dn2 and -Dn5 plants on days 6 and 12. However, no difference was detected in chlorophyll a/b or chlorophyll/carotenoid ratio among Tugela lines. The study demonstrated that Dn genes in the Tugela isolines conferred resistance to D. noxia but not to R. padi. Tugela-Dn1 was antibiotic, Tugela-Dn2 was tolerant and antibiotic, and Tugela-Dn5 was moderately antibiotic.     

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.     

In vitro enzymatic chlorophyll catabolism in wheat elicited by cereal aphid feeding

Chlorophyll degradation is a complex phenomenon that often accompanies insect feeding damage to plants. Loss of chlorophyll can be initiated by several reactions, including oxidative bleaching, chlorophyllase activity, and Mg-dechelatase activity. Extracts from the Russian wheat aphid [Diuraphis noxia (Mordvilko)], the bird cherry-oat aphid [Rhopalosiphum padi (L.)], and aphid-infested and uninfested wheat plants were assayed in vitro for activities involved in chlorophyll degradation. Although the initial infestation was the same (10 apterous adults) for both aphid species, D. noxia weight was significantly higher than R. padi after feeding for 12 days. Consequently, D. noxia feeding caused greater fresh leaf weight reduction than R. padi feeding. Chlorophyll degradation assays showed no activity from either D. noxia or R. padi extracts. Plant extract assays showed a significant difference in Mg-dechelatase activity, while no difference was detected in either the chlorophyllase or oxidative bleaching pathways among the aphid-infested or uninfested plant extracts. Diuraphis noxia-infested leaf extracts showed a greater increase of Mg-dechelatase activity than either R. padi-infested or the uninfested plants. The findings suggest that leaf chlorosis elicited by D. noxia feeding is different from the chlorophyll degradation that occurs in natural plant senescence. Aphid-elicited chlorosis might be the result of a Mg-dechelatase-driven catabolism of chlorophyll in challenged wheat seedlings, however, the factor(s) from D. noxia that elicited the increase of Mg-dechelatase activity still remain to be determined.     

Seasonal abundance of aphids (Homoptera: Aphididae) in wheat and their role as barley yellow dwarf virus vectors in the South Carolina coastal plain

Aphid (Homoptera: Aphididae) seasonal flight activity and abundance in wheat, Triticum aestivum L., and the significance of aphid species as vectors of barley yellow dwarf virus were studied over a nine-year period in the South Carolina coastal plain. Four aphid species colonized wheat in a consistent seasonal pattern. Greenbug, Schizaphis graminum (Rondani), and rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), colonized seedlingwheat immediately after crop emergence, with apterous colonies usually peaking in December or January and then declining for the remainder of the season. These two aphid species are unlikely to cause economic loss on wheat in South Carolina, thus crop managers should not have to sample for the subterranean R. rufiabdominalis colonies. Bird cherry-oat aphid, Rhopalosiphum padi (L.), was the second most abundant species and the most economically important. Rhopalosiphum padi colonies usually remained below 10/row-meter until peaking in February or March. Barley yellow dwarf incidence and wheat yield loss were significantly correlated with R. padi peak abundance and aphid-day accumulation on the crop. Based on transmission assays, R. padi was primarily responsible for vectoring the predominant virus serotype (PAV) we found in wheat. Pest management efforts should focus on sampling for and suppressing this aphid species. December planting reduced aphid-day accumulation and barley yellow dwarf incidence, but delayed planting is not a practical management option. English grain aphid, Sitobion avenae (F.), was the last species to colonize wheat each season, and the most abundant. Sitobion avenae was responsible for late-season virus transmission and caused direct yield loss by feeding on heads and flag leaves during an outbreak year.