Prevalences of fungal pathogens and othernatural enemies of cereal aphids (Homoptera:Aphididae) in wheat under dryland and irrigatedconditions in South Africa

Surveys were conducted in the summer andwinter rainfall wheat producing regions of SouthAfrica in a first attempt to investigate theidentity and impact of entomopathogenic fungi withinthe cereal aphid complex. Wheat produced underdryland and irrigated conditions was surveyed duringthe 1996 and 1997 seasons. Six cereal aphid specieswere recorded of which the Russian wheat aphid, Diuraphis noxia, was the most abundantunder dryland conditions in the summer rainfallregion as opposed to the oat aphid, Rhopalosiphum padi, in the winter rainfallregion. Rose grain aphid, Metopolophiumdirhodum, was most prevalent underirrigated conditions in the summer rainfall region.Five species of entomopathogenic fungi were recordedincluding four entomophthorales and the hyphomycete,Beauveria bassiana. TheEntomophthorales included Pandora neoaphidis, Conidiobolus obscurus, C.thromboides, and Entomophthoraplanchoniana. Pandora neoaphidis wasthe most important etiological agent recorded fromD. noxia, with up to 50% mycosis recordedunder dryland conditions in the Bethlehem summerrainfall region. Similarly, P. neoaphidis wasthe most prevalent species within populations ofM. dirhodum. under irrigated conditions in theBergville/Winterton summer rainfall region (up to77% mycosis). However, mycoses of R. padi didnot exceed 1.7% in samples from these areas,suggesting that R. padi may be lesssusceptible to P. neoaphidis than M.dirhodum. Epizootics in populations of D.noxia under dryland conditions in both the winterand summer rainfall regions indicated a high levelof susceptibility to P. neoaphidis.Occurrences of hymenopterous parasitoids andpredators in populations of D. noxia were low,although a parasitism level of 25% was recorded inone small sample of R. padi collected from anirrigated field in the summer rainfall region.     

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.     

Spatial patterns and sampling plans for cereal aphids [Hom.: Aphididae] killed by entomophthoralean fungi and hymenopterous parasitoids in spring wheat

Cereal aphids infesting spring wheat in southwestern Idaho were surveyed during 1988 and 1989 for the presence of entomophthoralean fungi and hymenopterous parasitoids. Cereal aphids killed by the fungi (cadavers) and parasitoids (mummies) includedDiuraphis noxia (Mordvilko),Metopolophium dirhodum (Walker),Sitobion avenae (F.), andSchizaphis graminum (Rondani). Taylor's power law was used to describe the relationships between mean densities of cadavers or mummies (number per tiller) and associated variances. Except forS. graminum mummies, which were at low levels throughout the growing seasons and tended to be randomly distributed, Taylor's slopes exceeded 1.0 for all the aphid cadavers and mummies, indicating varying degrees of clumping in spatial pattern. The spatial patterns ofD. noxia andM. dirhodum cadavers were similar, simply reflecting those of their own populations, but more aggregated than were their respective mummies resulting from parasitoid attack. The intercepts and slopes from the power law analysis were used to generate functional relationships between the proportion of wheat tillers bearing cadavers or mummies of each aphid species and the mean density, and develop optimal numerical (direct counting) and binomial (presence or absence) sample size curves for both cadavers and mummies of each aphid species.      

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.     

Are primary woody hosts ‘island refuges’ for host‐alternating aphids and important for colonization of local cereals?

Only few studies are available dealing with the relation between winter host density and spatial distribution and spring colonization of winter cereals by the host‐alternating cereal aphid species Rhopalosiphum padi and Metopolophium dirhodum. Large‐scale studies in climatically different agroecosystems in Germany from 2004 to 2006 revealed for R. padi and M. dirhodum larger spring/summer populations in landscapes with higher densities of winter hosts. A small‐scale study was performed in winter wheat fields adjacent to a large hedge with several typical winter hosts plants, bird cherry (Prunus padus) and wild rose species (Rosa spp.) to indentify distance effects (0–8, 8–24 and 24–60 m). Weekly measurements of aphid density between May to July showed significantly higher densities of R. padi compared with those of other aphids. Statistical analysis (Tukey–Kramer test and regression analyses) revealed significant gradients from the hedge to the field centre for R. padi and M. dirhodum. In comparative studies, winged R. padi from winter and adjacent summer hosts were genotyped using four microsatellite markers. The results showed that individuals from a certain winter host were not genetically similar with individuals from neighbouring summer hosts; it, therefore, seems that winter host clones did not significantly contribute to population built‐up in cereal fields over short distances. It could be concluded that on a regional scale, the density of sources for early migrants of R. padi is important for colonization intensity of surrounding summer hosts, but that the high local movement intensity and the relative small proportion of aphids that could be analysed in such tracking studies are blurring close spatial relations within short time periods.     

Aphid flight and change in abundance of winter wheat pests

Composition, dominance and change in population density of species flying over and feeding on winter wheat fields was studied between 1982 and 1998 at Mosonmagyaróvár. There were 29 aphid species among flying insects. The ones imposing risk to winter wheat were as follows: Diuraphis noxia Kurdj., Metopolophium dirhodum Walk., Rho‐palosiphum padi L., Schizaphis graminum Rond., Sitobion avenae Fabr. Number of flying individuals changed year by year, depending principally on climatic factors (temperature, relative humidity, and rainfall). Flight was continuous from late April to harvest. In the average of 17 years flight peak was observed in June.

Establishment of the firsts alate individuals of aphids with holocyclic development can be expected from the second decade of April. As a result of the continuous reproduction, the peak in number of individuals was observed in the second decade of June. During the six years between 1993 and 1998 this value was 17.18 aphids/ plant. Every year there were deviations from the average data, e.g., in 1994 aphid number per plant was 33.61. These pests appeared on the ears in the first decade of June, their number was continuously increasing due to drying of leaves.

During the six years when detailed data were recorded, Rh. padi L. was the dominant species three times (1994: 70.4%; 1995: 82.6%; 1996: 87.9%), M. dirhodum Walk. took this role two times (1993: 67.1%; 1997: 84.8%) and S. avenae Fabr. only once (1998: 53%).

Considering seventeen years’ data, it is necessary to produce resistant varieties and/or seed dressing, or insecticide treatment to control the first alates.     

The value of three cereal aphid species as food for a generalist predator

The value of the cereal aphid species Metopolophium dirhodum (Wlk.), Sitobion avenae (F.) and Rhopalosiphum padi (L.) as prey for the linyphiid spider Erigone atra (Bl.) was assessed. Fecundity of females was determined for spiders fed on eight experimental diets: three single‐species aphid diets, a mixed diet of all three aphid species, three mixed diets with each aphid species in combination with fruit flies Drosophila melanogaster (Meig.), and pure D. melanogaster as a high quality comparison diet. The development and survival of first‐instar juveniles fed on three diets of single aphid species, and on a diet of Collembola were compared with those subjected to starvation. Prey value for adult females was assessed by egg production, hatching success and offspring size. In pure diets all three aphid species were of low value to the spiders, causing a rapid decline in egg production and supporting no growth of significance of first‐instar juveniles. No difference in value of aphid species of single‐species aphid diets was found in the fecundity experiment, while a ranking of aphid species of M. dirhodum > R. padi > S. avenae was revealed in the survivorship experiment. A mixed‐aphid diet was not found to be advantageous compared with single‐species aphid diets, and no advantage of including aphids in mixed diets with fruit flies was found. Metopolophium dirhodum and R. padi were neutral in mixed diets, while a diet of S. avenae and fruit flies caused reduced egg production compared with the pure diet of fruit flies, revealing a toxic effect of S. avenae on the spider. The value‐ranking of aphid species in mixed diets was similar to that of single‐species diets. A similar ranking of aphid species was found for different fitness parameters (fecundity of adult females and development of juveniles). A ranking of aphids by offspring size of mothers on aphid‐only diets was S. avenae > M. dirhodum > R. padi. All aphid‐fruit fly diets resulted in larger offspring than a diet of only D. melanogaster, with the overall largest offspring being produced on the diet of M. dirhodum and fruit flies.     

Fungal endosymbionts affect aphid population size by reduction of adult life span and fecundity

The symbiosis between grasses and endophytic fungi is a common phenomenon and can affect herbivore performance through acquired, chemical plant defence by fungal alkaloids. In laboratory experiments, two species of common grass aphids, Rhopalosiphum padi and Metopolophium dirhodum were tested, in a population experiment (on four plant cultivars) and individually (on one plant cultivar) for the effects of the endophyte, Neotyphodium lolii, that forms symbiotic associations with perennial ryegrass Lolium perenne. In the population experiment that lasted for four aphid generations both aphid species showed decreased population sizes when feeding on each of the four endophyte-infected cultivars. Individuals of R. padi tested individually showed reduced adult life span and fecundity when feeding on infected plants. Individuals of M. dirhodum showed no response in any of the traits measured. This suggests that R. padi individuals are more sensitive to endophyte infection than M. dirhodum individuals. However, all infected grass cultivars reduced population sizes of both aphid species over four generations. Therefore, fungal endophytes can reduce populations of aphid herbivores independent of plant cultivars.     

Transmission of barley yellow dwarf virus by field collected aphids (Homoptera: Aphididae) and their relative importance in barley yellow dwarf epidemiology in southwestern Idaho

Populations of cereal aphids were sampled from 1985–1988 and assayed for transmission of barley yellow dwarf virus (BYDV), Rhopalosiphum padi, Rho-palosiphum maidis, Sitobion avenae, Metopolophium dirhodum, Schizaphis graminum and Macrosiphum euphorbiae collected from host plants transmitted BYDV in bioassays. Of the 1028 Diuraphis noxia collected from plants, one may have transmitted BYDV. The isolate involved resembled SGV in serological and biological characteristics, but since it was not recoverable by any of more than 800 D. noxia subsequently tested, we suspect it may have been a contaminant. Among those aphids collected during the autumn from a suction trap adapted for live collection, R. padi transmitted BYDV most frequently. Other trapped species which transmitted BYDV included: R. maidis, Rhopalosiphum insertum, Macrosiphum euphorbiae, Metopolophium dirhodum and Ceruraphis eriophori. An adapted Infectivity Index indicated that R. padi is by far the most important vector of BYDV during the autumn sowing season in southwestern Idaho. Male R. padi consistently transmitted BYDV more frequently than did females collected during the same period.     

Phenological responses in a sycamore–aphid–parasitoid system and consequences for aphid population dynamics: A 20 year case study

Species interactions have a spatiotemporal component driven by environmental cues, which if altered by climate change can drive shifts in community dynamics. There is insufficient understanding of the precise time windows during which inter‐annual variation in weather drives phenological shifts and the consequences for mismatches between interacting species and resultant population dynamics—particularly for insects. We use a 20 year study on a tri‐trophic system: sycamore Acer pseudoplatanus, two associated aphid species Drepanosiphum platanoidis and Periphyllus testudinaceus and their hymenopteran parasitoids. Using a sliding window approach, we assess climatic drivers of phenology in all three trophic levels. We quantify the magnitude of resultant trophic mismatches between aphids and their plant hosts and parasitoids, and then model the impacts of these mismatches, direct weather effects and density dependence on local‐scale aphid population dynamics. Warmer temperatures in mid‐March to late‐April were associated with advanced sycamore budburst, parasitoid attack and (marginally) D. platanoidis emergence. The precise time window during which spring weather advances phenology varies considerably across each species. Crucially, warmer temperatures in late winter delayed the emergence of both aphid species. Seasonal variation in warming rates thus generates marked shifts in the relative timing of spring events across trophic levels and mismatches in the phenology of interacting species. Despite this, we found no evidence that aphid population growth rates were adversely impacted by the magnitude of mismatch with their host plants or parasitoids, or direct impacts of temperature and precipitation. Strong density dependence effects occurred in both aphid species and probably buffered populations, through density‐dependent compensation, from adverse impacts of the marked inter‐annual climatic variation that occurred during the study period. These findings explain the resilience of aphid populations to climate change and uncover a key mechanism, warmer winter temperatures delaying insect phenology, by which climate change drives asynchronous shifts between interacting species.     

Does the presence of grassy strips and landscape grain affect the spatial distribution of aphids and their carabid predators?

• 1 We investigated, over the course of 2 years, the spatial distribution and abundance of two species of aphid, Metopolophium dirhodum and Sitobion avenae, and predatory species of carabid. This was undertaken in 24 wheat fields in ‘coarse‐grain’ and ‘fine‐grain’ landscapes in western France. A greater percentage of the latter landscape was covered by hedgerows and grassland and the total area covered by fields and the average size of the fields were smaller.
• 2 The effects on aphid abundance of the distance from field margins, the presence of grassy strips and carabid abundance were determined in both landscapes.
• 3 Both aphid species were more abundant in the ‘fine‐grain’ landscape, which may have been a result of the higher density of semi‐natural elements. In both types of landscape, the total numbers of aphids were negatively correlated with the distance from the field margin. This may have been because aphids were dispersing from overwintering sites in field margins. The abundance of M. dirhodum was strongly negatively correlated with the presence of grassy strips in the ‘coarse‐grain’ landscape, although there were no such significant correlations for either of the aphid species in the ‘fine‐grain’ landscape.
• 4 Aphid and carabid abundances were negatively correlated in the ‘fine‐grain’ and positively in ‘coarse‐grain’ landscape.
• 5 The results obtained in the present study emphasize the importance of semi‐natural areas in agricultural landscapes in shaping the spatial distribution of aphids and carabid beetles, their natural enemies, at different spatial scales.

     

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.     

Some factors affecting the build-up of cereal aphid infestations in winter wheat

Aphid populations were measured at 14-day intervals during May to July 1974 in 17 fields of wheat at 10 sites along an 85 km transect running east from Hertfordshire to the Essex coast. Metopolophium dirhodum was the most numerous species initially, but was later superseded by Sitobion avenae. Rhopalosiphum padi was third in order of abundance. Several characteristics of each site and crop were examined in relation to the development of aphid infestations: two were associated with inter-site differences in aphid infestation. Infestations started earlier at the eastern than at the western sites due to the earlier arrival of alate migrants, and this resulted in higher peak populations densities of R. padi but not of M. dirhodum or S. avenae. Greater aphid densities were associated with crops having low nitrate concentrations in plant sap in the early stages of growth.     

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.     

Diversity,frequency, and geographic distribution of facultative bacterial endosymbionts in introduced aphid pests

Facultative bacterial endosymbionts in insects have been under intense study during the last years. Endosymbionts can modify the insect's phenotype, conferring adaptive advantages under environmental stress. This seems particularly relevant for a group of worldwide agricultural aphid pests, because endosymbionts modify key fitness‐related traits, including host plant use, protection against natural enemies and heat tolerance. Aimed to understand the role of facultative endosymbionts on the success of introduced aphid pests, the distribution and abundance of 5 facultative endosymbionts (Hamiltonella defensa, Regiella insecticola, Serratia symbiotica, Rickettsia and Spiroplasma) were studied and compared in 4 cereal aphids (Sitobion avenae, Diuraphis noxia, Metopolophium dirhodum and Schizaphis graminium) and in the pea aphid Acyrthosiphon pisum complex from 2 agroclimatic zones in Chile. Overall, infections with facultative endosymbionts exhibited a highly variable and characteristic pattern depending on the aphid species/host race and geographic zone, which could explain the success of aphid pest populations after their introduction. While S. symbiotica and H. defensa were the most frequent endosymbionts carried by the A. pisum pea‐race and A. pisum alfalfa‐race aphids, respectively, the most frequent facultative endosymbiont carried by all cereal aphids was R. insecticola. Interestingly, a highly variable composition of endosymbionts carried by S. avenae was also observed between agroclimatic zones, suggesting that endosymbionts are responding differentially to abiotic variables (temperature and precipitations). In addition, our findings constitute the first report of bacterial endosymbionts in cereal aphid species not screened before, and also the first report of aphid endosymbionts in Chile.     

Tolerance of antibiotic and susceptible cereal seedlings to the aphids Metopolophium dirhodum and Rhopalosiphum padi

Some cereal seedlings exhibit antibiotic and antixenotic resistance to the aphids Metopolophium dirhodum (Walker) and Rhopalosiphum padi (L.), because the seedlings contain hydroxamic acids or gramine. The association between tolerance to aphids and aphid antibiosis was investigated for three cereals, Dollarbird wheat Vulcan wheat and Yagan barley. The dry biomass gained by the aphids and the simultaneous reduction in the biomass of the plants (biomass conversion ratio) quantified tolerance. Biomass production and the density dependence of biomass production by the aphids quantified antibiosis more effectively than fecundity. Vulcan wheat, which has more hydroxamic acid than Dollarbird wheat showed the highest level of antibiosis, and the barley was not antibiotic for either aphid. The biomass conversion ratio was a constant; the biomass of an infested plant was reduced by 3 mg for each mg of aphid biomass gained, regardless of aphid species, plant cultivar, or aphid density. The three plants showed no differential tolerance to the aphids, and therefore tolerance is not associated with antibiosis in this case.     

Physiological responses of wheat and barley to Russian wheat aphid, Diuraphis noxia (Mordvilko) and bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphididae)

Although aphids are among the most injurious of all agronomic insect pests, much remains unknown about how their feeding alters plant physiology. Two experiments were conducted to examine the physiological responses of wheat, Triticum aestivum L. and barley, Hordeum vulgare L. to injury by Diuraphis noxia (Mordvilko) and Rhopalosiphum padi (L.) (Hemiptera: Aphididae). Gas-exchange parameters, chlorophyll fluorescence, and chlorophyll content were examined at 3, 6, and 9 days post-infestation on control and aphid (D. noxia and R. padi) infested treatments. In general, chlorophyll content and chlorophyll fluorescence parameters (non-variable minimal fluorescence, maximal fluorescence, and variable fluorescence) were not significantly affected by either aphid species. Photochemical and non-photochemical quenching coefficients were significantly impacted by both aphid species, suggesting that aphid feeding may influence the photoprotective xanthophyll cycle altering the thylakoid membrane pH gradient. Feeding by both aphid species resulted in an increase in electron transport rate, but at different time periods. Wheat plants infested with D. noxia had accelerated declines in photosynthetic capacity when compared to R. padi-infested and control plants. These plants exhibited decreased values for A_max, which was accompanied by decreased values for V_cmax and J_max Neither aphid species negatively affected the photosynthetic capacity of the barley plants until day 9. At this time, aphid-infested plants had decreased values for A_max which was accompanied by decreased values in J_max. Although R. padi feeding does not typically result in visual damage symptoms as previously demonstrated, clearly this aphid does have an impact on the gas-exchange and chlorophyll fluorescence of its host plants. Handling editor: Heikki Hokkanen     

Implications of Russian wheat aphid, <Emphasis Type="Italic">Diuraphis noxia</Emphasis>, falling rates for biological control in resistant and susceptible winter wheat

The restriction of aphid reestablishment onto plants by epigeal predators represents a critical component of integrated pest management. To further realize the potential that these predators might have in control programs, it is necessary to quantify such behavior as aphid falling rate to reveal the number of aphids that are available as potential prey. This study calculated the falling rate of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Sternorrhyncha: Aphididae), and tested whether this aphid more likely fell from wheat plants that differed between flat leaf architecture versus those with furled leaves. Specifically, the hypothesis was tested that a resistant wheat line (flat leaves) will have a higher aphid falling rate than a susceptible closely related line (furled leaves). The experiment was performed at Fort Collins and Akron, Colorado, USA, from May through July, 2008. Aphids were sampled from infested wheat rows to estimate aphid density, and sticky traps were used to capture falling aphids and to measure falling rate. Falling rates ranged from 0.7 to 69.5% in Fort Collins and from 1.4 to 59.5% in Akron. The falling rate of D. noxia was more influenced by plant growth stage than aphid densities, with the highest falling rate occurring after wheat senescence. Wheat plants with flat leaf architecture did not significantly increase aphid falling rate. Diuraphis noxia falls at a higher rate at lower aphid densities, which is when epigeal predators could have their greatest biological control impact.     

Evidence for regional diversity and host adaptation in Iranian populations of the Russian wheat aphid

Effective pest management is greatly facilitated by knowledge of the genetic structure and host adaptation of the pest species in question. The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko) (Homoptera: Aphididae: Macrosiphini), is an important economic pest in many cereal‐growing areas of the world, and in this study we investigated these aspects of its populations, using microsatellite markers and host plant response assays. Diuraphis noxia was sampled from 38 locations in Iran and genotyped at four polymorphic microsatellite loci that had been isolated from various Sitobion species. We identified 50 multilocus genotypes in 376 individuals. The overall observed heterozygosity was 0.134. F‐statistics showed a regional partitioning in D. noxia populations with overall F_ST = 0.231. In addition, there was a significant correlation between genetic and geographic distances. In order to test for the ecological consequences of genetic variability in D. noxia, biotypic variation amongst the isolates collected from wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) was evaluated on a number of resistant and susceptible wheat varieties. The plant variables we measured were damage rating (based on leaf chlorosis, leaf rolling, wilting, and death of the host plant), host plant dry weight, and root length. Damage rating was the best criterion for detecting biotypic variation in D. noxia. Discriminant analysis correctly classified the isolates in respective groups in 80–91.8% of the cases. The barley isolate showed no differences in performance on resistant and susceptible wheat, indicating a lack of gene‐by‐gene relationship with wheat plants. In contrast, wheat isolates differentially damaged the resistant and susceptible plants and showed moderate to severe virulence.     

Endosymbionts (Buchnera) of the Aphid Uroleucon sonchi Contain Plasmids with trpEG and Remnants of trpE Pseudogenes

The aphid Uroleucon sonchi contains a prokaryotic endosymbiont (Buchnera) with plasmids having trpEG as well as remnants of trpE pseudogenes. In this respect it resembles Buchnera from the aphid Diuraphis noxia. Phylogenetic trees based on trpE (plasmid gene) and trpB (chromosomal genes) from eight species of aphids are congruent, indicating a lack of exchange of plasmids among endosymbionts from different aphid species. Received: 16 December 1996 / Accepted: 26 December 1996