Oxidative responses of resistant and susceptible cereal leaves to symptomatic and nonsymptomatic cereal aphid (Hemiptera: Aphididae) feeding.

The impact of the leaf-chlorosis-eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and the nonchlorosis-eliciting bird cherry-oat aphid, Rhopalosiphum padi (L.), feeding on D. noxia-susceptible and -resistant cereals was examined during the period (i.e., 3, 6, and 9 d after aphid infestation) that leaf chlorosis developed. After aphid number, leaf rolling and chlorosis ratings, and fresh leaf weight were recorded on each sampling date, total protein content, peroxidase, catalase, and polyphenol oxidase activities of each plant sample were determined spectrophotometrically. Although R. padi and D. noxia feeding caused significant increase of total protein content in comparison with the control cereal leaves, the difference in total protein content between R. padi and D. noxia-infested leaves was not significant. Although R. padi-feeding did not elicit any changes of peroxidase specific activity in any of the four cereals in comparison with the control leaves, D. noxia feeding elicited greater increases of peroxidase specific activity only on resistant 'Halt' wheat (Triticum aestivum L.) and susceptible 'Morex' barley (Hordeum vulgare L.), but not on susceptible 'Arapahoe' and resistant 'Border' oat (Avena sativa L.). D. noxia-feeding elicited a ninefold increase in peroxidase specific activity on Morex barley and a threefold on Halt wheat 9 d after the initial infestation in comparison with control leaves. Furthermore, D. noxia feeding did not elicit any differential changes of catalase and polyphenol oxidase activities in comparison with either R. padi feeding or control leaves. The findings suggest that D. noxia feeding probably results in oxidative stress in plants. Moderate increase of peroxidase activity (approximately threefold) in resistant Halt compared with susceptible Arapahoe wheat might have contributed to its resistance to D. noxia, whereas the ninefold peroxidase activity increase may have possibly contributed to barley's susceptibility. Different enzymatic responses in wheat, barley, and oat to D. noxia and R. padi feeding indicate the cereals have different mechanisms of aphid resistance.     

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.     

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.     

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.     

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.     

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.     

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.     

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).     

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.     

Test of semiochemicals and a resistant wheat variety for Russian wheat aphid management in South Africa

Abstract:  Aphid behaviour-modifying semiochemicals were tested against Russian wheat aphid Diuraphis noxia in South African wheat. Volatile substances from plant essential oils, methyl salicylate, 1,8-cineole and menthol were tested in the laboratory and field in combination with the D. noxia -susceptible wheat variety Betta and the resistant variety Elands. All three substances were repellent to D. noxia in olfactometric tests. Diuraphis noxia settled less on Elands plants that had been exposed to the volatiles, whereas the effect of the volatiles on D. noxia settling on Betta was less obvious. A slow-release pellet formulation was used to apply semiochemicals in wheat in replicated plot field trials in 2004 and 2005. In 2004, semiochemicals reduced aphid populations in Elands, but led to increased aphid populations in Betta. Further, the impact of the chemicals on aphid numbers and grain quality (thousand grain weight) varied according to plant variety, indicating an interaction between semiochemicals and plant resistance/variety.     

麦双尾蚜自然种群的特定时间生命表

根据新疆伊犁、塔城两地小麦田调查资料,组建了麦双尾蚜Diuraphis noxia (Mordvilko) 自然种群特定时间生命表。由生命表分析得知,小麦早期生长发育阶段,麦双尾蚜迁移造成的种群数量损失很小,反映了麦双尾蚜在早期种群密度较低,个体对食物和空间资源的竞争压力较小。寄生性天敌在小麦田出现较晚,在两地小麦早期生长阶段,由寄生造成的种群损失也极低。整个麦类作物生长阶段,捕食天敌对麦双尾蚜种群的控制作用非常重要,是麦双尾蚜种群损失的关键因子之一。迁移造成的种群损失比寄生性天敌的还要大,说明迁移是麦双尾蚜种群自我调节的重要手段。     

麦双尾蚜与麦田中生物和非生物因子的灰色关联分析

利用灰色系统理论,将麦双尾蚜Diuraphis noxia (Mordvilko) 种群消长动态与麦田生物群落中主要生物因子和非生物因子进行灰色关联分析。结果表明影响麦双尾蚜及其它麦蚜种群消长的关键因子主要为麦蚜间的竞争和天敌的捕食压力。非生物因子的作用较小,其中较重要的因子有平均相对湿度和降水量。在塔城小麦田中关键天敌类群为瓢虫类,寄生天敌的影响较小;在伊犁小麦田中,麦双尾蚜及其它麦蚜的关键天敌类群为蚜小蜂类,大麦田中斑腹蝇类较重要,燕麦田中瓢虫类占优势。     

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.     

Amino acid budgets in three aphid species using the same host plant

Abstract. Nutrient provisioning in aphids depends both on the composition of ingested phloem sap and on the biosynthetic capabilities of the aphid and its intracellular symbionts. Amino acid budgets for three aphid species, Rhopalosiphum padi (L.), Schizaphis graminum (Rondani) and Diuraphis noxia (Mordvilko), were compared on a single host plant species, wheat Triticum aestivum L. Ingestion of amino acids from phloem, elimination of amino acids in honeydew, and the content of amino acids in aphids tissue were measured. From these values, ingestion rates were estimated and compared to honeydew and to estimated composition of aphid proteins. Ingestion rate was lowest in D. noxia due to low growth rate and low honeydew production; intermediate in S. graminum due to higher growth rate and intermediate honeydew production; and highest in R. padi , which had the highest rates for both variables. Both D. noxia and S. graminum induced increases in the amino acid content of ingested phloem. These changes in phloem content, combined with differences in ingestion rates, resulted in large differences among aphids in estimated rates of ingestion of individual amino acids. In honeydew, most essential amino acids were found in low amounts compared with the amounts ingested, especially for methionine and lysine. A few amino acids (arginine, cystine, histidine and tryptophan) were more abundant in honeydew of some aphids, suggesting oversupply. Aphid species differed in the composition of free amino acids in tissue but showed very similar composition in protein, implying similar requirements among the aphids. In R. padi and D. noxia , most essential amino acids were ingested in amounts insufficient for growth, implying dependence on symbiont provisioning. In S. graminum , most amino acids were ingested in amounts apparently sufficient for growth.     

Resistance to Rhopalosiphum padi (Homoptera: Aphididae) in three triticale accessions

Experiments were conducted to identify and characterize host plant resistance to bird cherry-oat aphid, Rhopalosiphum padi (L.), in various wheat and wheat-grass hybrids. Initial tests screened for resistance to R. padi among 12 grass accessions (eight wheat [Triticum aestivum L.], three triticale [XTriticosecale Wittmack], and 1XElytricum [Elytrigia elongata [Host] Nevski x Triticum aestivum hybrid]). R. padi had less population growth on triticale accessions '8TA5L' (PI 611760) and 'Stniism 3' (PI 386156) than on other accessions, but nymphiposition by R. padi did not differ among the 12 accessions. Follow-up experiments were conducted to characterize antibiosis, antixenosis, and tolerance to R. padi in three wheat and three triticale accessions. In antibiosis experiments, Stniism 3 and triticale 'H7089-52' (PI 611811) prolonged time to reproduction by R. padi compared with that on wheat accessions 'Arapahoe' (PI 518591), 'KS92WGRC24' (PI 574479), and 'MV4' (PI 435095), whereas time to reproduction on 8TA5L was intermediate and did not differ from that on the other five accessions. Also, R. padi produced fewest progeny on Stniism 3, and fewer progeny on 8TA5L than on H7089-52, Arapahoe, KS92WGRC24, and MV4. Stniism 3 showed antixenosis, because fewer winged R. padi selected Stniism 3 than Arapahoe, H7089-52, or MV4 in choice tests. In tolerance experiments, a 300 aphid-day infestation of R. padi limited shoot length of Arapahoe and KS92WGRC24 plants. Shoot lengths did not differ between infested and noninfested seedlings of MV4, 8TA5L, H7089-52, and Stniism 3, indicating tolerance to R. padi in these accessions. Triticale accessions 8TA5L, H7089-52, and Stniism 3 and MV4 wheat may be meaningful sources of R. padi resistance for small-grain breeding programs, and Stniism 3 may be particularly valuable, given reports of its additional resistance to the Russian wheat aphid, Diuraphis noxia (Mordvilko).     

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.     

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.     

Sources of variation in the interaction between three cereal aphids (Hemiptera: Aphididae) and wheat (Poaceae)

The relative contributions of host plant, herbivore species and clone to variation in the interaction between cereal aphids and wheat were investigated using five clones each of three species, Rhopalosiphum padi (Linnaeus), Sitobion avenae (Fabricius) and Schizaphis graminum (Rondani), on seedlings of two cultivars of Triticum aestivum L. and one cultivar of Triticum durum Desf. More individuals and biomass of R. padi than of the other two species were produced on seedlings. The three wheat cultivars lost similar amounts of biomass as a result of infestation by aphids, with the amount lost depending on aphid species: S. avenae caused the lowest loss in biomass. Variation in aphid biomass production was due mostly to differences among aphid species (70%), less to the interaction between wheat type and aphid species (7%), and least to aphid clone (1%). The specific impact of the aphids on the plants ranged from 1.7 to 3.7 mg of plant biomass lost per mg of aphid biomass gained, being lowest for R. padi and highest for S. graminum. Variation in plant biomass lost to herbivory was due mostly to unknown sources of error (95%), probably phenotypic differences among individual seedlings, with 3% due to aphid species and none attributable to aphid clone. For these aphid-plant interactions, differences among aphid clones within species contributed little to variation in aphid and plant productivity; therefore, a small sample of clones was adequate for quantifying the interactions. Furthermore, one clone of each species maintained in the laboratory for about 200 parthenogenetic generations was indistinguishable from clones collected recently from the field.     

Spatial Simulation of Epizootics Caused by Beauveria bassiana in Russian Wheat Aphid Populations

Models of disease dynamics commonly make the assumption of spatial homogeneity in the underlying host population. However, insect behavior may result in spatially heterogeneous populations with which pathogens interact. We modified a simulation model of temporal and spatial population dynamics of the Russian wheat aphid, Diuraphis noxia, on preferred or nonpreferred host plants, by incorporating effects of the entomopathogenic fungus, Beauveria bassiana. Epizootic parameters included time from inoculation of aphids until death, duration of sporulation, and estimated exposure probability. Simulations first predicted results of previously described experiments in which D. noxia adults were inoculated with conidial suspensions or water and placed on wheat or oat seedlings in 81-plant grids in cages. Subsequently, large-scale simulations were run for hypothetical field situations on 50 × 50-plant grids of wheat or oat. With B. bassiana present for both cage and larger scale simulations, results indicated that, on oat, an expanding infection front lagged behind the expanding aphid population front. Continual aphid movement from hosts resulted in many escapes, and the aphid population persisted at slightly reduced levels. On the preferred wheat host, patterns developed with pockets of infected aphids and other pockets of healthy aphids. Localized aphid populations that escaped initial infestation were able to proliferate, whereas other local populations were greatly reduced or became extinct due to lack of movement from the hosts, resulting in increased exposure to pathogen inoculum. Thus, proliferation and fluctuation of the pathogen were strongly influenced by the plant hosts' effects on aphid movement behavior. Incorporating spatial dynamics into disease models should prove useful in other efforts to predict biological control efficacy by entomopathogenic fungi in heterogeneous habitats.