Barley yellow dwarf virus infectivity of alate aphid vectors in west Wales

Live trapping at 0.9 m of alate aphid vectors of barley yellow dwarf virus (BYDV) at Aberystwyth from 1970 to 1979 showed that ten species transmitted the virus to oat test plants. Conversion of percentage infective at 0.9 m to numbers infective based on continuous trapping at 1.2 m showed Rhopalosiphum padi and R. insertum to be the main vector species in most years, whilst Metopolophium dirhodum and Sitobion auenae were normally of minor importance. The data obtained suggest that epiphytotics of BYDV in autumn-sown cereals were caused by numerous infective vectors flying late in the year and transmitting severe strains of the virus. Evidence is presented that gynoparae and males of R. padi are involved in the autumn spread of BYDV and that three further aphid species, Anoecia corni, Metopolophium albidum and M. frisicum are BYDV vectors. The use of live and continuous trapping techniques in forecasting BYDV epiphytotics is discussed.     

Suitability of the biomass crop Miscanthus sinensis as a host for the aphids Rhopalosiphum padi (L.) and Rhopalosiphum maidis (F.), and its susceptibility to the plant luteovirus Barley Yellow Dwarf Virus

1 The reproductive performance of two aphid pest species, Rhopalosiphum padi and Rhopalosiphum maidis, was investigated on two seedling growth stages of Miscanthus sinensis, rhizomatous M. sinensis‘Giganteus’ and barley. Rhopalosiphum padi was unable to complete its development on Miscanthus. Rhopalosiphum maidis was most fecund on rhizomatous plants compared with seedling stages. 2 The ability of R. maidis to transmit the RPV isolate of Barley Yellow Dwarf Virus (BYDV) to M. sinensis seedlings was further investigated. Following successful transmission, host plant symptomology and the effect of infection on the yield of Miscanthus were investigated. Total above soil biomass was reduced by around 23% following infection. 3 The inability of R. padi to utilize Miscanthus is reviewed in light of this species’ origin and inability to utilize C₄ host plants. 4 The potential pest status of R. maidis on Miscanthus is discussed together with the impact that Miscanthus cultivation could have on the ecology of this aphid species and BYDV in the U.K.     

Winter wheat (Triticum aestivum) response to Barley yellow dwarf virus at various nitrogen application rates in the presence and absence of its aphid vector,Rhopalosiphum padi

Barley yellow dwarf (BYD) is one of the most common diseases of cereal crops, caused by the phloem‐limited, cereal aphid‐borne Barley yellow dwarf virus (BYDV) (Luteoviridae). Delayed planting and controlling aphid vector numbers with insecticides have been the primary approaches to manage BYD. There is limited research on nitrogen (N) application effects on plant growth, N status, and water use in the BYDV pathosystem in the absence of aphid control. Such information will be essential in developing a post‐infection management plan for BYDV‐infected cereals. Through a greenhouse study, we assessed whether manipulation of N supply to BYDV‐infected winter wheat, Triticum aestivum L. (Poaceae), in the presence or absence of the aphid vector Rhopalosiphum padi L. (Hemiptera: Aphididae), could improve N and/or water uptake, and subsequently promote plant growth. Similar responses of shoot biomass and of water and N use efficiencies to various N application rates were observed in both BYDV‐infected and non‐infected plants, suggesting that winter wheat plants with only BYDV infection may be capable of outgrowing infection by the virus. Plants, which simultaneously hosted aphids and BYDV, suffered more severe symptoms and possessed higher virus loads than those infected with BYDV only. Moreover, in plants hosting both BYDV and aphids, aphid pressure was positively associated with N concentration within plant tissue, suggesting that N application and N concentration within foliar tissue may alter BYDV replication indirectly through their influence on aphid reproduction. Even though shoot biomass, tissue N concentration, and water use efficiency increased in response to increased N application, decision‐making on N fertilization to plants hosting both BYDV and aphids should take into consideration the potential of aphid outbreak and/or the possibility of reduced plant resilience to environmental stresses due to decreased root growth.     

Spread of barley yellow dwarf virus and relative importance of local aphid vectors in central Washington

Data from bioassays of field collected aphids, barley indicator plants exposed to natural conditions, and various types of aphid traps were used to describe the spread of barley yellow dwarf virus (BYDV) in wheat and barley near Prosser, Washington. Bioassays were also used to assess the relative importance of local vector species. Of alate aphids collected from grain in the 1982 and 1983 fall migration seasons, 3.4–14–5% transmitted BYDV. Data from concurrent and post-migration assays of resident aphids (apterae and nymphs) reflected an increase in the proportion of infected plants in the field. Maximum increase in the percentage of viruliferous aphids occurred in late November and December of 1982 and November of 1983. The 1982 increase occurred after aphid flights had ceased for the year, suggesting active secondary spread. Collections in pitfall traps and infected trap plants from November to February confirmed aphid activity and virus spread. Rhopalosiphum padi was the most important vector in central Washington in 1982 and 1983 because of its abundance and relative BYDV transmission efficiency. Metopolophium dirhodum was more winter-hardy than R. padi and equal to R. padi in its efficiency as a vector; however, it was not as abundant as R. padi except during the mild winter of 1982–83, when it was a major contributor to secondary spread. Sitobion avenae may be important in years when it is abundant, but it was only a quarter as efficient as R. padi. Rhopalosiphum maidis was a much less efficient vector than R. padi and it only reached high populations in late autumn barley.     

Barley yellow dwarf virus, wheat, and Sitobion avenae: a case of trilateral interactions

We analysed interactions in the system of two Barley Yellow Dwarf Virus (BYDV) strains (MAV and PAV), and wheat (cv. Tinos) as host plant for the virus, and the cereal aphid Sitobion avenae (F.) as vector, in particular whether or not infection by the virus might alter host plant suitability in favour of vector development. By measuring the amino acid and sugar content in the phloem sap of infected and non‐infected wheat plants we found a significant reduction in the concentration of the total amount of amino acids on BYDV‐infected plants. Qualitative and quantitative analysis of honeydew and honeydew excretion indicated a lower efficiency of phloem sap utilisation by S. avenae on infected plants. In addition, S. avenae excreted less honeydew on infected plants. Both BYDV strains significantly affected aphid development by a reduction in the intrinsic rate of natural increase. Hence, infection by the virus reduced the host suitability in terms of aphid population growth potential on BYDV‐infected plants. However, more alate morphs developed on virus‐infected plants. These findings are discussed in relation to the population dynamics of S. avenae, and, as a consequence, the spread of BYDV.     

The effect of natural enemies on the spread of barley yellow dwarf virus (BYDV) by Rhopalosiphum padi (Hemiptera: Aphididae)

The effects of two natural aphid enemies, adult Coccinella septempunctata Linneaus, a predator, and Aphidius rhopalosiphi de Stefani Perez, a parasitoid, on spread of barley yellow dwarf virus (BYDV) transmitted by the bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus) were studied under laboratory conditions. Predators or parasitoids were introduced to trays of durum wheat seedlings and the patterns of virus infection were observed after two, seven and 14 days of exposure. More plants were infected with BYDV in control trays without A. rhopalosiphi than in trays with the parasitoid present, both seven and 14 days after the introduction of parasitoids. Patterns of virus infection were found to be similar over time in trays with a parasitoid present and in control trays. More plants were infected in trays with C. septempunctata present than in control trays, both two and seven days after the introduction of the coccinellid. The spread of virus infections progressed differently over time for the two treatments (predator and parasitoid), differences between treatments being most marked after two days and seven days, when more plants exposed to predators but fewer exposed to parasitoids were infected with BYDV compared to their respective controls. However, by the 14th day 88% of all plants were infected and there was no significant difference between the two treatments. The role of natural enemies in spread of BYDV is discussed.     

Hydroxamic acids affecting barley yellow dwarf virus transmission by the aphid Rhopalosiphum padi

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), a hydroxamic acid (Hx) occurring in wheat, was shown to deter feeding by the aphid Rhopalosiphum padi (L.), and to reduce BYDV transmission to the plant. Dual choice tests with wheat leaves showed the preferential settlement of aphids on leaves with lower levels of DIMBOA. Electric monitoring of aphid feeding behaviour showed that in seedlings with higher DIMBOA levels fewer aphids reached the phloem and they needed longer times to contact a phloem vessel than in those with lower levels. When aphids carrying BYDV were allowed to feed on wheat cultivars with different DIMBOA levels, fewer plants were infected with BYDV in the higher DIMBOA cultivars than in the lower ones. Preliminary field experiments showed a tendency for wheat cultivars with higher Hx levels to be more tolerant to infection by BYDV than lower Hx level ones.     

Reactions in Maize Infected with Swedish Isolates of Barley Yellow Dwarf (BYDV)

Nine cultivars of maize (Zea mays L.) were tested for susceptibility to BYDV under three temperature ranges in the greenhouse. Three Swedish isolates of BYDV were used, two specifically transmitted by Rhopalosiphum padi (L.) (39/78) and by Sitobion avenae (Fabr.) (27/77), respectively, and the third by both species (70). The virus isolates were transmitted successfully from different grasses to maize and from infected maize to the susceptible oat cultivar "Sol II" by the respective aphid species. S. avenae showed very high ability to transmit the S. avenae specific isolate to and from maize plants.
The main symptoms that developed on maize were fine chlorotic irregular spots, reddish purple discoloration and malformed leaves.
The relationship between maize cultivar, temperature and percent of infection is discussed. Enzyme-linked Immunosorbent Assay (ELISA) was used with success to detect the virus isolate 27/77 in susceptible and symptomless maize plants.
Electron microscopy of maize (cv. LG ll) infected with the 27/77 isolate of BYDV revealed virus-like particles, about 22 nm in diameter, in the nuclei of companion cells, in the plasmodesmata connecting companion cells with mature sieve tubes, in the lumen of mature sieve tubes and in xylem tracheal elements.     

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.     

Plant virus and parasitoid interactions in a shared insect vector/host

Interactions between barley yellow dwarf luteovirus (BYDV) and the aphid parasitoid, Aphidius ervi Haliday (Hymenoptera: Aphidiidae), were investigated while sharing the vector/host, Sitobion avenae (F.) (Homoptera: Aphididae). Aphids, which were parasitized during their second larval stadium, had access to virus-infected plants before, immediately after, or several days after parasitoid attack. The larval development of A. ervi in S. avenae was significantly delayed when virus acquisition took place before or shortly after the parasitoid had hatched, but not when the parasitoid was at the second larval stage during virus acquisition. Similarly, the presence of BYDV led to a significantly higher aphid mortality when they acquired virus up to and including the time that A. ervi was at the first larval stage. Adult female parasitoids deposited fewer eggs in viruliferous aphids. Virus transmission was not reduced by parasitization, and in some experiments aphids which were subjected to parasitoid attack transmitted BYDV more efficiently than unattacked insects.     

High Levels of Resistance in Agropyron Species to Barley Yellow Dwarf and Wheat Streak Mosaic Viruses

Several Agropyron species were tested for new sources of resistance to barley yellow dwarf virus (Bydv ) and wheat streak mosaic virus (WSMV). With BYDV strain PAV, 11 of the 17 Agropyron species showed no virus transmission when plants were given access feed by viruliferous Rhopalosiphum padi. Similar trials with BYDV strain RMV (vectored by R. maidis) indicated that all plants, except susceptible control plants, remained virus free. Virus status was confirmed by enzyme-linked immunosorbent assays. When plants were mechanically inoculated with WSMV, 11 Agropyron species failed to express symptoms, while five other species showed a segregating response or had some accessions segregating and some resistant. Test results suggest that resistance to BYDV and WSMV in Agropyron species does not appear to be correlated with any specific genome of Agropyron species although most of the Agropyron species containing S genome were resistant to BYDV and WSMV.     

Vectoring ability of aphid clones of Rhopalosiphum padi (L.) and Sitobion avenae (Fabr.) and their capacity to retain barley yellow dwarf virus

Vectoring ability of four aphid clones, Rp-M and Rp-R26 of Rhopalosiphum padi and Sa-R1 and Sa-V of Sitobion avenae, to transmit barley yellow dwarf (PAV, MAV and RPV) luteoviruses (BYDV) was compared in controlled conditions. Significant differences between highly efficient vectors (HEV), Rp-M and Sa-Rl, and poorly efficient vectors (PEV), Rp-R26 and Sa-V, were found in transmission of their specific viruses with acquisition and inoculation access periods (AAP, IAP) of 5 days. BYD-RPV was occasionally transmitted by both clones of S. avenae. None of 150 tested apterous adults of the Rp-R26 transmitted BYD-MAV, while 10% of transmission was observed from those of the Rp-M in a parallel test. An improved ELISA and immuno-PCR were adapted to test for viruses in aphids. The results obtained by the improved ELISA indicated there was a good correlation between virus detection in single aphids of HEV clones after a 5 day AAP and virus transmission by them. In contrast, the percentages of virus-carrying aphids of PEV clones were generally higher than those of their transmission rates. BYD-MAV and BYD-RPV were also detected by the improved ELISA in single aphids of their PEV clones, with the exception of BYD-RPV in those of Sa-V. However, after a 2-day IAP, the improved ELISA in most cases failed to detect these viruses in single aphids of PEV clones. Detection by immuno-PCR demonstrated that all three viruses could be acquired and retained by the aphids of both HEV and PEV clones. But, as visualised from electrophoretic bands, after the 2-day IAP the amplified products from aphid extracts of PEV clones were reduced. The detection in a batch of nine aphids by the improved ELISA revealed that virus content in PEV clones decreased more rapidly than that in HEV clones during transmission. Thus, the difference in transmission efficiency of the aphid clones within species was not caused by an inability to acquire virus, but was determined by variation in vectoring ability between them. This was due to differences in ability to prevent the passage of virions from haemocoel to salivary duct and/or different capacities for the retention of BYDV.     

Long-term changes of aphid vectors of Barley yellow dwarf viruses in north-eastern Italy (Friuli-Venezia Giulia)

Migrations of aphid vectors of Barley yellow dwarf viruses (BYDV) were monitored using a Rothamsted Insect Survey suction trap in Friuli-Venezia Giulia (north-eastern Italy). Catches from 1983 to 2002 were studied for trends, correlations of total catches of each year with those of previous years, correlations between the autumn and the spring + summer catches of the same year and between spring + summer catches of one year with catches of the previous autumn. Infectivity of autumn alates was studied using biological tests, and infectivity indexes were calculated for all vector species and for Rhopalosiphum padi alone. Colonisation of barley and proportion of infected plants were checked in a field close to the suction trap from 1992 to 2002 and related to trap catches. Catches were also correlated to acreage dedicated to cereal and fodder crops in the region. During the 20 years, 15 BYDV vector species were caught in the trap, but only five species were found consistently colonising barley plants during autumn. R. padi was the most numerous species in catches, while Sitobion avenae was the predominant colonising species in the barley field. Relatively to R. padi , S. avenae colonies were about six times more numerous than expected from catches. The yearly abundance of catches of most species did not change significantly during the 20 years, with a few exceptions, significantly correlated to changes in the acreage dedicated to cereal and fodder crops. There was a significant decrease of the autumn catches of both R. padi and the total of BYDV vectors.     

Host plant recognition in monophagous weevils: Specialization ofCeutorhynchus inaffectatus to glucosinolates from its host plantHesperis matronalis

Several aphid honeydews were incorporated into sucrose solutions and presented to hop aphids, Phorodon humuli (Schrank), as artificial diets in free-choice bioassays. Small additions of honeydew collected from two species of aphid feeding on hop, Humulus lupulus L., arrested the searching behavior of the hop aphid and appeared to stimulate prolonged periods of ingestion. This effect was more dependent on the host plant honeydew source than the species of aphid that produced the honeydew. Aphid honeydews collected from plants other than hop (non-hosts to P. humuli) contained hop aphid phagostimulants that were less effective. Our results indicate that analysis of aphid honeydew could help describe chemical cues involved in the recognition of appropriate host plants by aphid species.     

Insect infestations, incidence of viral plant diseases, and yield of winter wheat in relation to planting date in the northern Great Plains

Planting date effects on arthropod infestation and viral plant disease are undocumented for winter wheat, Triticum aestivum L., in South Dakota and the northern Great Plains. Winter wheat was planted over three dates (early, middle, and late; generally from late August to late September) to determine the effect on abundance of insect pests, incidence of plant damage, incidence of viral plant disease, and grain yield. The study was conducted simultaneously at two sites in South Dakota over three consecutive cropping seasons for a total of six site yr. Cereal aphids (Homoptera: Aphididae) were abundant in three site yr. Rhopalosiphum padi (L.), bird cherry-oat aphid, was the most abundant cereal aphid at the Brookings site, whereas Schizaphis graminum (Rondani), greenbug, predominated at Highmore. Aphid-days were greater in early versus late plantings. Aphid abundance in middle plantings depended on aphid species and site, but it usually did not differ from that in early plantings. Incidence of Barley yellow dwarf virus (family Luteoviridae, genus Luteovirus, BYDV) declined with later planting and was correlated with autumnal abundance of cereal aphids. Incidence of BYDV ranged from 24 to 81% among 1999 plantings and was < 8% in other years. Damage to seedling wheat by chewing insects varied for two site-years, with greater incidence in early and middle plantings. Wheat streak mosaic virus, spring infestations of cereal aphids, wheat stem maggot, and grasshoppers were insignificant. Yield at Brookings was negatively correlated with BYDV incidence but not cereal aphid abundance, whereas yield at Highmore was negatively correlated with aphid abundance but not BYDV incidence. Planting on 20 September or later reduced damage from chewing insects and reduced cereal aphid infestations and resulting BYDV incidence.     

Spatial and temporal spread of Citrus tristeza virus and its aphid vectors in the North western area of Morocco

First report of Citrus tristeza virus (CTV,Closterovirus) in Morocco datesback to 1961 in collections of citrus varieties. An exhaustive survey of citrus in the north of the country in 2009 revealed that CTV was spread all over the citrus production area. We attempted to evaluate the relative contribution of different aphid species in the spread of CTV disease in a Citrus reticulata orchard at the Loukkous region during 2 years (2012 and 2013). The overall CTV incidence estimated in the experimental site increased from 17.8% in 2012 to 31.15% in 2013. The most abundant aphid species colonising clementine trees was Aphis spiraecola and A. gossypii. Both aphid species reached their maximum peaks during the spring season. The rate of viruliferous aphids, estimated by real‐time RT‐PCR of single aphid, revealed that 35.4% of winged A. gossypii and 28.8% of winged A. spiraecola were viruliferous, confirming a high inoculum pressure in the area surrounding the experimental site. The aphid species Toxoptera citricida, which is able to transmit the aggressive isolates of CTV, was not found in the Loukkous region. The study of the spatial distribution of the CTV showed that in general, the disease was randomly distributed in the field. Overall, the results seem to indicate that A. spiraecola may be considered as the major aphid species contributing to CTV spread in our experimental conditions. The prevalence of mild strains in the region and the high level of aphid flight activity could explain the rapid evolution of CTV incidence in the experimental area.     

Vector specificity of barley yellow dwarf virus serotypes and variants in southwestern Idaho

Plants with symptoms of barley yellow dwarf virus (BYDV) obtained in infection feeding assays of aphids collected in the field in Idaho between 1986 and 1988 were tested for virus transmissibility by possible aphid vectors. Isolates obtained during 1987–1988 were also tested with a range of polyclonal antisera which distinguished PAV, MAV, SGV, RPV and RMV serotypes. In 1989 some Idaho (ID) BYDV isolates, maintained as standards for comparison, were serotyped and tested for aphid transmissibility, using 11 species of aphids. There was not always the expected correspondence between serotype and vector specificity for ID isolates. For isolates obtained from field-collected Rhopalosiphum padi, vector transmissibility and serotype corresponded with previous reports; however, 44% of isolates which were serotyped as RMV were also transmissible by species other than Rhopalosiphum maidis. Similarly, the transmissibility of the ID laboratory standards did not always conform to the reported vector specificity of serotypes. The laboratory ID-MAV culture was transmitted by Metopolophium dirhodum and Myzus persicae as well as by Sitobion avenae. The laboratory ID-SGV culture was transmitted by R. padi and 5. avenae as well as by Schizaphis graminum. The ID-RPV culture was transmitted by S. graminum and Rhopalosiphum insertum as well as R. padi. Both of two laboratory ID-RMV cultures were transmissible by R. insertum and R. padi transmitted one of them. The results indicate that, for isolates collected in Idaho, vector specificity cannot be assumed from their serotypes.     

Potential distributions of phlebotomine sandfly vectors of human visceral leishmaniasis caused by Leishmania infantum in Morocco

Zoonotic visceral leishmaniasis is a common vector-borne systemic disease caused by Leishmania infantum (Kinetoplastida: Trypanosomatidae). In Morocco the situation is complex: many sandfly species have been collected in areas in which the disease is endemic, but only Phlebotomus ariasi, Phlebotomus perniciosus and Phlebotomus longicuspis (Diptera: Psychodidae) have been confirmed to have vectorial roles. The objective of the present study was to ascertain the potential distribution of L. infantum and its vectors in Morocco, using ecological niche modelling. Vector records were obtained from field collections of the Laboratory team and from previously published entomological observations. Epidemiological data for L. infantum modelling were obtained from Moroccan Ministry of Health reports. The jackknife test indicated that the bioclimatic variables with the greatest influence on model development for all species were annual precipitation and precipitation in the driest quarter of the year. MaxEnt model representations for sandfly species that act as vectors of L. infantum showed the widespread geographic distribution of these species in Morocco, specifically in northern and central Morocco, where foci of visceral leishmaniasis are found. The ecological niche modelling points out areas in which the probability of occurrence of these species is higher. This information should be considered as a starting point for further research to fully elucidate the ecology and epidemiology of these species, as well as of the pathogens they transmit.     

Barley yellow dwarf virus transmission and feeding behaviour of Rhopalosiphum padi on Hordeum bulbosum clones

A Hordeum bulbosum L. (Poaceae) clone A17 was identified, which showed complete resistance to Barley yellow dwarf virus (BYDV) and Cereal yellow dwarf virus (CYDV). It was not possible to infect plants of A17 with BYDV‐PAV, ‐MAV, or with CYDV‐RPV by the aphid vectors Rhopalosiphum padi (L.) or Sitobion avenae (Fabricius) (both Hemiptera: Aphididae). Plants of the A17 clone and of the BYDV‐susceptible H. bulbosum clone A21 revealed some resistance to R. padi compared to the susceptible winter barley cultivar Rubina [Hordeum vulgare L. (Poaceae)]. The development time to the imago was longer and the number of nymphs was reduced on both clones compared with cv. Rubina. The probing and feeding behaviour of R. padi on plants of the H. bulbosum clones was studied over 12 h and compared with that on plants of the barley cv. Rubina. Principal component analysis of the results of the feeding behaviour revealed a clear separation of the H. bulbosum genotypes from Rubina. On H. bulbosum the number of penetrations was higher but total feeding time was shorter. Significant differences were mainly found in the phloem feeding parameters for plants of both clones in comparison to Rubina, with the virus resistant A17 clone having the strongest effect and the susceptible A21 clone being intermediate. Most significant differences were found in parameters of the phloem salivation phase. On A17, an average of less than one (0.9) E1 phase per plant was observed (3.3 on A21 and 5.7 on Rubina) and its duration was reduced to less than 1 min (0.9 min) in comparison to 2.4 min on A21 and 5.7 min on Rubina. Also, the phloem feeding (E2) phase was clearly reduced on A17 plants with 0.5 E2 phases per test and a mean duration of 1.1 min in contrast with 2.9 and 3.5 E2 phases per test and 34.1 and 421.3 min for A21 and Rubina, respectively. These results point towards a phloem‐localized factor for aphid resistance in H. bulbosum, i.e., on A17 plants the phloem salivation time is too short for a successful infection by BYDV leading to vector resistance.     

Assessing the risk of primary infection of cereals by barley yellow dwarf virus in autumn in the Rennes basin of France

In the Rennes basin, Rhopalosiphum padi is anholocyclic and represents more than 90% of suction trap catches of potential vectors of barley yellow dwarf virus (BYDV) during autumn. From 1983 to 1987 the possibility of predicting the risk of BYDV infection of batches of barley test seedlings (sampling units) exposed each week from September to December near a 12.2 m high suction trap was investigated. Three kinds of variables were checked as possible predictors: weekly mean or maximum temperatures; weekly catches of R. padi (including or excluding males); and percentage of sampling units infested by aphids. Three contrasting examples were observed: during the first three years (1983–1985), infection was high and its change with time followed temperature, aphid catches and plant infestation changes; in 1986, high numbers of aphids caught and a high proportion of plants infested resulted in only low infection and in 1987, both infestation and infection were very low. Simple linear regression analysis showed that the more reliable predictors of infection were the proportion of infested plants and to a lesser extent the numbers of trapped aphids. Multiple linear regressions including either of the three groups of ‘predicting’ variables did not result in any improvement in the prediction. At a practical level, the use of counts of aphid catches would seem a better compromise between accuracy and consistency of prediction and ease of gathering data than that of plant infestation but any significant improvement of the prediction should be sought in an early estimate of the amount of virus available to aphids before they colonise the plants.