The influence of primary infection date and establishment of vector populations on the spread of yellowing viruses in sugar beet

In three field experiments in 1985 and 1986, we studied the effect of the date of primary infection on the spread of beet yellows closterovirus (BYV) and beet mild yellowing luteovirus (BMW) from artificially inoculated sugar beet plants. Laboratory-reared vector aphids, Myzus persicae, were placed on these sources of virus. There was no substantial natural immigration of vectors or viruses. In two experiments, one with BMYV in 1985 and the other in BYV in 1986, populations of vector aphids remained low and there was little virus spread, i.e. c. 50 infected plants from one primarily infected source. The cause of this small amount of spread was the low number of vector aphids. In the third experiment, with BYV in 1986, large populations of M. persicae developed and there was substantial virus spread: c. 2000 infected plants in the plots which were inoculated before canopy closure. In later-inoculated plots in the same experiment, there was much less spread: c. 100 infected plants per virus source plant. Differences between fields in predator impact are implicated as the most probable factor causing differences in vector establishment and virus spread between these three experiments. Virus spread decreased with later inoculation in all three experiments. A mathematical model of virus spread incorporating results from our work has been used to calculate how the initial proportion of infected plants in a crop affects the final virus incidence. This model takes into account the effect of predation on the development of the aphid populations. The processes underlying the spread and its timing are discussed.     

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.     

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.     

Cucurbit aphid‐borne yellows virus (CABYV) modifies the alighting,settling and probing behaviour of its vector Aphis gossypii favouring its own spread

Plant pathogens are able to influence the behaviour and fitness of their vectors in such a way that changes in plant–pathogen–vector interactions can affect their transmission. Such influence can be direct or indirect, depending on whether it is mediated by the presence of the pathogen in the vector's body or by host changes as a consequence of pathogen infection. We report the effect that the persistently aphid‐transmitted Cucurbit aphid‐borne yellows virus (CABYV, Polerovirus) can induce on the alighting, settling and probing behaviour activities of its vector, the cotton aphid Aphis gossypii. Only minor direct changes on aphid feeding behaviour were observed when viruliferous aphids fed on non‐infected plants. However, the feeding behaviour of non‐viruliferous aphids was very different on CABYV‐infected than on non‐infected plants. Non‐viruliferous aphids spent longer time feeding from the phloem in CABYV‐infected plants compared to non‐infected plants, suggesting that CABYV indirectly manipulates aphid feeding behaviour through its shared host plant in order to favour viral acquisition. Viruliferous aphids showed a clear preference for non‐infected over CABYV‐infected plants at short and long time, while such behaviour was not observed for non‐viruliferous aphids. Overall, our results indicate that CABYV induces changes in its host plant that modifies aphid feeding behaviour in a way that virus acquisition from infected plants is enhanced. Once the aphids become viruliferous they prefer to settle on healthy plants, leading to optimise the transmission and spread of this phloem‐limited virus.     

Farm‐scale assessment of movement patterns and colonization dynamics of the grain aphid in arable crops and hedgerows

1 Integrated management of crop pests requires the identification of the appropriate spatial scale at which colonization processes occurs. We assessed, by coupling demographic and genetic methods, the relative contribution of local and transient migrants of the grain aphid Sitobion avenae to wheat field colonization in spring. 2 We examined, during two consecutive years, the daily colonization of wheat by aphid migrants and compared this with daily aphid flight monitored by a local 12.2‐m suction trap. The genetic profiles of aphids landing on crops were compared with those of both flying aphids caught by the suction trap and local populations from arable crops and hedgerows. 3 In the first year, we observed: (i) a strong correlation between aphids colonizing the crop and those moving within the crop and a close genetic similarity between aphids from these samples and (ii) a high level of genetic differentiation between these aphids and populations from local cereals and field margins. In the second year, the number of migrants recorded on the wheat was three‐fold higher than in the previous year, and less correlated with that recorded by the suction trap. This was associated with a lack of genetic differentiation between all samples. 4 This variation in the colonization processes resulted mainly in an abrupt increase in abundance of genotypes from local over‐wintering sites in 2004. This suggests that, despite the long range dispersal potential of the grain aphid, outbreak risks could be mainly determined at a local scale, encouraging the design of relatively small management units.     

Forecasting migration of cereal aphids (Hemiptera: Aphididae) in autumn and spring

The migration of cereal aphids and the time of their arrival on winter cereal crops in autumn and spring are of particular importance for plant disease (e.g. barley yellow dwarf virus infection) and related yield losses. In order to identify days with migration potentials in autumn and spring, suction trap data from 29 and 45 case studies (locations and years), respectively, were set‐off against meteorological parameters, focusing on the early immigration periods in autumn (22 September to 1 November) and spring (1 May to 9 June). The number of cereal aphids caught in a suction trap increased with increasing temperature, global radiation and duration of sunshine and decreased with increasing precipitation, relative humidity and wind speed. According to linear regression analyses, the temperature, global radiation and wind speed were most frequently and significantly associated with migration, suggesting that they have a major impact on flight activity. For subsequent model development, suction trap catches from different case studies were pooled and binarily classified as days with or without migration as defined by a certain number of migrating cereal aphids. Linear discriminant analyses of several predictor variables (assessed during light hours of a given day) were then performed based on the binary response variables. Three models were used to predict days with suction trap catches ≥1, ≥4 or ≥10 migrating cereal aphids in autumn. Due to the predominance of Rhopalosiphum padi individuals (99.3% of total cereal aphid catch), no distinction between species (R. padi and Sitobion avenae) was made in autumn. As the suction trap catches were lower and species dominance changed in spring, three further models were developed for analysis of all cereal aphid species, R. padi only, and Metopolophium dirhodum and S. avenae combined in spring. The empirical, cross‐classification and receiver operating characteristic analyses performed for model validation showed different levels of prediction accuracy. Additional datasets selected at random before model construction and parameterization showed that predictions by the six migration models were 33–81% correct. The models are useful for determining when to start field evaluations. Furthermore, they provide information on the size of the migrating aphid population and, thus, on the importance of immigration for early aphid population development in cereal crops in a given season.     

播期对大豆蚜及其天敌种群动态的影响

【目的】研究大豆播期对大豆蚜Aphis glycines Matsumura及其天敌的影响。【方法】试验在2012年、2013年进行,设置了3个大豆播期处理。每周调查播期处理田大豆蚜种群及天敌种类和数量,分析大豆蚜种群数量、种群增长率的时序动态、大豆蚜和天敌的关联度。【结果】不同播期条件下大豆蚜有翅蚜及无翅蚜的种群动态趋势基本一致,有翅蚜蚜量高峰期要早于无翅蚜1周。处理间的大豆蚜田间始见期与终见期随着播期推后而延迟,大豆蚜在田间扩散和消退的时期也随着大豆播期延后。晚播的两个处理高峰期蚜量多于或等于正常播期处理的蚜量。大豆蚜与天敌关联度随着播期的推后而变高。在调查的7种天敌中大豆蚜与异色瓢虫的关联度最高,草蛉、小花蝽和蚜茧蜂也表现较高的关联度。【结论】播期会显著影响大豆蚜的田间始见期和终见期,随着播期的推迟大豆蚜种群高峰期蚜量以及大豆蚜与天敌的关联度都会提高。     

Alatae production and population increase of aphid vectors on virus-infected host plants

Summary Zucchini yellow mosaic virus (ZYMV) and Aphis gossypii Glover are two components of a recently identified plant-parasite system that provides an excellent opportunity to study interrelations between a virus and a vector that share the same host, but have no direct physiological interaction. In a field experiment we documented numbers of alate and apterous A. gossypii on healthy Cucurbita pepo and on plants inoculated with virus 0, 7, 14, and 21 days before aphid infestation. When plants were inoculated and infested simultaneously, more than twice as many alatae were produced after 20 days of colony growth than on any other treatment. This indicates that properties unique to the early stages of viral infection somehow stimulated wing formation. Because it is spread by the activities of alatae, virus dispersal would be greater as a result of these properties. Developmental rate, total numbers of aphids, and numbers of alatae and apterae decreased as the time between virus inoculation and aphid colonization increased.     

Parasitoids aid dispersal of a nonpersistently transmitted plant virus by disturbing the aphid vector

• 1 Aphids are the major group of insects that vector plant viruses, and they often display a preference for foliage showing disease symptoms. Although this behaviour will increase the numbers of vectors acquiring the pathogen, it will not in itself result in a greater spread of the disease.
• 2 The present study examined how infection of Vicia faba by the nonpersistently transmitted virus bean yellow mosaic virus (BYMV) affected colonization by pea aphids Acyrthosiphon pisum. We then examined how foraging by the hymenopterous parasitoid Aphidius ervi affected aphid settling/movement behaviour and the consequences for dissemination of the virus.
• 3 In Petri dish arenas, aphids colonized discs from BYMV‐infected leaves more rapidly than discs from uninfected plants. Reflectance from infected foliage was approximately 20% higher than from uninfected leaves in the green–yellow wavelengths, indicating that aphids might be responding to visual cues from the brighter foliage. Settling was reduced by A. ervi, with the foraging wasps preventing the aphids reaching and/or remaining on the leaf tissue.
• 4 In multiple plant arenas, A. ervi caused a reduction in aphid numbers but also a nine‐fold increase in BYMV infection. It is hypothesized that disturbance by the parasitoids resulted in more aphid movement as well as more cases of aphids probing on a BYMV‐infected plant and then a new host within the critical time period for successful inoculation to occur. This effect of parasitoids on virus dispersal should be considered in epidemiological models of insect‐vectored plant diseases, and also when evaluating the use of natural enemies in biocontrol strategies of insect herbivore/vector pests.

     

Effects of cereal borders, admixture with cereals and plant density on the spread of bean yellow mosaic potyvirus into narrow-leafed lupins (Lupinus angustifolius)

In studies of virus control measures, field experiments in 1987–1991 investigated the effects of cereal and fallow borders, admixture with cereals and plant density on spread of bean yellow mosaic potyvirus (BYMV) from pastures dominated by subterranean clover (Trifolium subterraneum) into plots of narrow-leafed lupins (Lupinus angustifolius). Virus spread was mainly monocyclic because BYMV killed infected lupin plants and between systemic movement and death there was only a brief period for BYMV acquisition and transmission to other plants by vector aphids. In plots with cereal borders, the rate and extent of BYMV spread into the lupins was decreased; at final assessment the numbers of infected plants were 43–60% less than in plots with fallow borders. Admixture with cereals also decreased the rate and extent of BYMV spread into lupin plots, numbers of infected plants being decreased by 76–96% at the time of final assessment. When lupins were sown at different seeding rates to generate a range of plant densities and weeds were removed, high densities decreased BYMV infection. The higher incidences of BYMV infection in sparse stands were attributed partly to smaller plant numbers and partly to incoming viruliferous vector aphids being more attracted to plants with bare earth around them, than to a plant canopy. BYMV infection decreased grain yield of samples from infected lupin plants by 94–100%. In plots with 34% infection and sparse stands, grain yield was decreased by about one third. Plotted progress curves for the accumulated numbers of alate aphids of the BYMV vector species Acyrthosiphon kondoi and Myzus persicae resembled those for numbers of BYMV infected plants in 1990, but in 1991 only the curve plotted for M. persicae did so. There was a 2 week delay between the curves for aphid numbers and virus counts which reflected the time taken for obvious systemic necrotic symptoms to develop in lupins.     

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.     

Effects of Aphis fabae Scop, and of its attendant ants and insect predators on yields of field beans (Vicia faba L.)

Predators (mainly coccinellid adults and larvae and syrphid larvae), although few, were important in decreasing numbers of Aphis fabae on a small plot of field beans during the early stages of infestation in a year favourable to the aphid. At the same time, ants (Lasius niger L.), attending aphids on other plants on the same plot, effectively protected the aphids from predators for about 2 weeks, enabling the attended aphids to multiply faster than the unattended. When all aphid populations started to decline, predators became more numerous and accelerated the decline on both sets of plants. Bean plants without aphids yielded fifty-six seeds per plant; those with aphids but free from ants gave seventeen; and those with ant-attended aphids, eight seeds per plant. The damage and loss of yield was caused by the large aphid populations that developed when the pods were maturing, and not by the fewer aphids present when the plants were in flower. It appears that small, temporary infestations during flowering might increase the yield of field beans.     

Aerial flow of barley yellow dwarf viruses and of their vectors in western France

During the years 1989–1992 cereal aphids were caught alive in a low level (1.5 m high) suction trap operated in Le Rheu (Brittany, France) and tested for BYDV transmission. In most cases comparisons with data collected simultaneously by a 12.2 m suction trap operating in the same site resulted in good relationships between weekly catches at both heights. Results from transmission tests showed that: (i) the two main BYDV vectors were Rhopalosiphum padi and Metopolophium dirhodum during the years of experiment; (ii) PAV and MAV were the commonest viruses and RPV was relatively scarce; (iii) during spring M. dirhodum appeared to be the most important MAV vector and nearly as good a PAV vector as R. padi; (iv) during autumn R. padi was the only vector of the three viruses with mixed transmission allowing it to transmit also MAV probably by heteroencapsidation. To give an indication of the risk of infection, infectivity indices were calculated by multiplying the numbers of aphids caught by the 12.2 m suction trap by the proportion that were infective. These infectivity indices agreed with field records of primary infections.     

The effect of temperature and wind on the flight activity of cereal aphids

The lower temperature threshold for take-off in Sitobion avenae obtained from an analysis of daily 12·2 m suction trap catches was 16°C. In the laboratory, the take-off threshold for S. avenae was 17·5°C in increasing temperatures, but 19°C when aphids were kept at constant temperatures for a short period of time. The equivalent thresholds were both 20·5°C for Metopolophium dirhodum. Over a period of 16 h no S. avenae took-off from plants at 10°C, 70% at 15°C and all within 16 h at 20°C. It was concluded that suction trap catches can be used to compare the temperature thresholds for take-off of both different species and morphs of a species of aphid. Emigrants of Rhopalosiphum padi, but not of M. dirhodum, took-off at a higher temperature than the alate exules. High winds were found to delay but not inhibit take-off both in the field, and in the laboratory, using both artificial and plant substrates. All aphids eventually flew, even from favourable hosts. It was concluded that adverse weather conditions will delay but not prevent cereal aphid migration in early summer.     

Vector activity of three aphid species (Hemiptera: Aphididae) modulated by host plant selection behaviour on potato (Solanales: Solanaceae)

Viral diseases non-persistently transmitted by aphids are of great economic importance in several annual crops. Transmission efficiency of these non-persistent phytoviruses is dependant on vector efficiency (i.e. vector intrinsic ability to transmit the virus) but also on the vector activity that implies the early steps of aphid host plant selection process (i.e. brief intracellular stylet punctures after landing) and to their interplant movement ability. In Europe, Macrosiphum euphorbiae (Thomas 1878) is considered as one of the most serious virus vectors on potato (Solanum tuberosum L. 1753). Nevertheless, several alate aphid species that do not colonise potato plants are trapped in potato crops. Therefore, we investigated, through laboratory experiments, vector activity of one potato colonising aphid, M. euphorbiae, and two non-colonising potato aphids, the bird cherry-oat aphid Rhopalosiphum padi (L. 1758) and the pea aphid Acyrthosiphon pisum (Harris 1776). A settling experiment was used to evaluate dispersal activity, and the electrical penetration graph (EPG) technique was used to investigate probing activity on potato plants. Results showed that M. euphorbiae exhibited a better vector activity than other two aphid species in terms of landing and probing. By contrast, interplant movements were only recorded on non-colonising aphids, suggesting a better vector activity than M. euphorbiae in terms of locomotive behaviour. These data confirm the involvement of A. pisum and R. padi in the spread of non-persistent viruses.     

Raspberry viruses affect the behavior and performance of Amphorophora agathonica in single and mixed infections

Pathogens may alter their hosts, which consequently increases transmission efficiency by vectors. We examined the effects of Raspberry leaf mottle virus [RLMV; Closterovirus (Closteroviridae)] and Raspberry latent virus [RpLV; Reovirus (Reoviridae)], alone and in a co‐infection in raspberry, Rubus idaeus L. (Rosaceae) cv. Meeker, on the behavior and performance of its vector, Amphorophora agathonica Hottes (Hemiptera: Aphididae). Longevity was increased in aphids feeding on all infected‐plant treatments compared with healthy plants, but aphid fecundity only increased in the co‐infection treatment. In a two‐way choice study between infected and healthy plants, aphids showed no difference in preference between plants after 30 min of exposure. After 24 h, aphids significantly preferred to settle on plants infected with RLMV over healthy, but healthy plants over plants infected with RpLV. There were no differences in settling preferences between healthy and co‐infected plants. An electrical penetration graph study showed no differences in aphid feeding behavior on plants infected with RLMV and RLMV+RpLV when compared with healthy controls. Our results are consistent with past findings that infected plant's impact vector performance and behavior, but also highlight the need to further investigate greater virus diversity and effects of mixed infections.     

Infection of potato plants with potato leafroll virus changes attraction and feeding behaviour of Myzus persicae

Potato leafroll virus (PLRV; genus Polerovirus, family Luteoviridae) is a persistently transmitted circulative virus that depends on aphids for spreading. The primary vector of PLRV is the aphid Myzus persicae (Sulzer) (Homoptera: Aphididae). Solanum tuberosum L. potato cv. Kardal (Solanaceae) has a certain degree of resistance to M. persicae: young leaves seem to be resistant, whereas senescent leaves are susceptible. In this study, we investigated whether PLRV‐infection of potato plants affected aphid behaviour. We found that M. persicae's ability to differentiate headspace volatiles emitted from PLRV‐infected and non‐infected potato plants depends on the age of the leaf. In young apical leaves, no difference in aphid attraction was found between PLRV‐infected and non‐infected leaves. In fact, hardly any aphids were attracted. On the contrary, in mature leaves, headspace volatiles from virus infected leaves attracted the aphids. We also studied the effect of PLRV‐infection on probing and feeding behaviour (plant penetration) of M. persicae using the electrical penetration graph technique (DC system). Several differences were observed between plant penetration in PLRV‐infected and non‐infected plants, but only after infected plants showed visual symptoms of PLRV infection. The effects of PLRV‐infection in plants on the behaviour of M. persicae, the vector of the virus, and the implications of these effects on the transmission of the virus are thoroughly discussed.     

Effect of cucumber mosaic virus infection on morphology,yield and phenolic contents of tomato

Ten tomato genotypes were screened for their resistance against cucumber mosaic virus (CMV) and its vector Myzus persicae under natural infection in field, using aphids M. persicae under net-house and mechanical inoculation under greenhouse. Large differences were observed among genotypes for infection percentage (IP) and severity index (SI) among the testing methods used. All genotypes showing tolerance to CMV in the field or through aphid inoculation, however, become susceptible and highly susceptible after mechanical inoculation. All the test genotypes also showed susceptibility to the aphid M. persicae population. Plants inoculated with CMV showed substantial decrease in yield and yield-contributing parameters which varied with cultivars that probably depended upon its genetic make up. All the test genotypes exhibited 0.97–30.19% decrease in plant height, 11.47–52.65% decrease in root length, 46.56–95.56% decrease in fresh plant weight, 65.78–92.84% decrease in root fresh weight, 19.97–87.65% decrease in the dry weight of plants, 75.63–95.43% decrease in dry root weight, 69.51–95.65% reduction in the number of fruits and 89.04–99.89% decrease in yield per plants. After 15 days of inoculation, the quantitative analysis using double beam spectrophotometer showed an increase in total phenolics in CMV-inoculated plants as compared to un-inoculated plants among genotypes. Similarly the thin layer chromatography (TLC) on silica gel G indicated that the number of phenolic compounds was increased in most of the inoculated genotypes while in others they were either decreased or remained same.