Comparison of sampling methods for alate aphids and observations on epidemiology of soybean mosaic virus in Nanjing, China

Using standard diversity indices, samples of aphids collected on a vertical net were found to be more diverse than collections from green pan traps in experiments conducted in Nanjing, China in 1981. Some species, notably Myzuspersicae, Aphis citricola, and Eriosoma (Schizoneura) japonicum, were collected in greater relative numbers in the pan traps. Several species, including the Macrosiphoniella spp., were under-represented in the green pan traps presumably because they were not attracted to the green colour of the traps. Yellow pan traps, used concurrently to monitor landing rates of economic species, caught relatively more Aphis craccivora and M. persicae and relatively fewer Aphis gossypii than green ones. In the Nanjing area soybean mosaic virus (SMV) usually spreads in spring planted soybeans during the latter half of May, causing damage. The predominant aphid species trapped at that time were M. persicae, Lipaphis erysimi, A. gossypii, A. craccivora and E. japonicum, of which the first four are known vectors. Summer planted soybeans are usually affected during a second period of virus spread occurring in mid-August when various Aphis spp. are abundant. The predominant vector of SMV at that time was thought to be Aphis glycines which colonises the crop; in 1981, however, A. citricola and A. gossypii, which do not colonise soybean and A. craccivora. which rarely colonises soybean, were much more abundant in mid-August than A. glycines. A. citricola, A. gossypii and A. craccivora are all able to transmit SMV, and thus should be considered as potentially important vectors. In infectivity assays, only four of 1040 aphids transmitted SMV. These included an A. craccivora, two Toxoptera citricidus and one Aphis sp.     

Detection of potato virus Y in the aphid Myzus persicae by enzyme-linked immunosorbent assay (ELISA)

Potato virus Y was detected by enzyme linked immunosorbent assay (ELISA) in at least 50% of groups of five Myzus persicae. The mean A₄₀₅ value for groups of viruliferous aphids was 2–3 times greater than that for virus-free ones. PVY was not detected in Aphis craccivora, A. citricola or A. gossypii, three other species which transmitted the virus to peppers, and it was detected in only a small proportion of groups of Acyrthosiphon pisum. In a series of trials, success in detection of PVY by ELISA was not correlated with the ability of other aphids from the same source plant to transmit the virus to test plants. The limitation of ELISA for quantitative assay of PVY in aphids and for epidemiological work is discussed.     

Determination of aphid transmission efficiencies for N, NTN and Wilga strains of Potato virus Y

Potato virus Y (PVY, genus Potyvirus, family Potyviridae) causes high economic losses worldwide, especially in the production of seed potatoes (Solanum tuberosum). PVY control systems rely on measuring virus pressure and vector pressure in the field. Calculation of the vector pressure is based on the relative efficiency factors (REFs) of aphid species. These REFs express the transmission efficiency of aphid species in relation to the transmission efficiency of Myzus persicae, the most efficient vector of PVY. In this paper, we report on the determination of aphids' relative transmission efficiency factors (REFs) for isolates of the PVY strains PVY^N, PVY^NTN and PVY^N-Wi. Biotype Mp2 of M. persicae was tested for its transmission efficiency for six PVY isolates (one PVY^N, three PVY^NTN and two PVY^N-Wi isolates) and showed comparable average transmission efficiencies for all isolates. The transmission rate of this biotype for the six PVY isolates was set to 1 and Mp2 was used as an internal control in transmission experiments to determine the REFs of three other biotypes of M. persicae and 16 other aphid species (three biotypes per species when available) for the six PVY isolates. Comparing the calculated REFs for PVY^N with the REFs reported in the previous century for PVY^N, we observe overall comparable REFs, except for Aphis fabae, Aphis spp., Hyperomyzus lactucae, Macrosiphum euphorbiae and Rhopalosiphum padi, which have a lower REF in our experiments, and Aphis frangulae and Phorodon humuli, which have now a higher REF. Comparing the new REFs found for the PVY^NTN strains with the new REFs for PVY^N, we observe that they are overall comparable, except for A. frangulae (0.17 compared with 0.53) and Schizaphis graminum (0.05 compared with 0.00). Comparing the REFs calculated for PVY^N-Wi with those calculated for PVY^N, we can observe six aphid species with higher REFs (Acyrthosiphon pisum, A. fabae, Aphis nasturtii, Aphis spp., P. humuli and R. padi). Only the species A. frangulae shows a lower REF for PVY^N-Wi compared with the transmission efficiency of PVY^N. Three aphid species (Aulacorthum solani, Myzus ascalonicus and S. graminum) for which no REF was determined earlier were found to be capable to transmit PVY and their REFs were determined.     

Blockage of stylet tips as the mechanism of resistance to virus transmission by Aphis gossypii in melon lines bearing the Vat gene

Aphis gossypii is the main virus vector in muskmelon crops. The melon gene Vat confers resistance to non‐persistent virus transmission by this aphid. The mechanism of this resistance is not well understood, but no relationship has been detected between resistance and the probing behaviour of aphids on resistant plants. Results presented here suggest that temporary blockage of aphid stylet tips preventing virus particle release may explain the resistance conferred by Vat gene. We performed experiments in which viruliferous aphids were allowed to probe different sequences of resistant (Vat‐bearing) and/or susceptible melon plants. The results demonstrated that A. gossypii inoculates Cucumber mosaic virus (CMV) efficiently in susceptible plants having previously probed resistant plants, showing that the resistance mechanism is reversible. Furthermore, the infection rate obtained for susceptible plants was the same (25%) regardless of whether the transmitting aphid had come directly from the CMV source or had subsequently probed on resistant plants. This result suggests that virus is not lost from stylet to plant during probing of resistant plants, supporting the temporary blockage hypothesis. We also found that the ability of Myzus persicae to transmit CMV is noticeably reduced after probing on resistant plants, providing evidence that this aphid species also responds to the presence of the Vat gene. Finally, we also found that in probes immediately after virus acquisition M. persicae inoculates resistant plants with CMV more efficiently than susceptible plants, perhaps because the Vat gene product induces increased salivation by this aphid.     

Role of ants in structuring the aphid community on apple

1. The aphids Dysaphis plantaginea Passerini, Aphis spp. (Aphis pomi De Geer and Aphis spiraecola Patch), and Eriosoma lanigerum Hausmann are commonly found together in apple orchards. Ants establish a mutualistic relationship with the myrmecophilous aphids D. plantaginea and Aphis spp. but not with E. lanigerum. 2. Field surveys and one experiment manipulating the presence of ants and the aphid species were conducted to test the hypothesis that ants play a role in structuring the community of these aphids on apple. 3. Ants tended D. plantaginea and Aphis spp. but not E. lanigerum colonies. In the field, D. plantaginea performed better in the presence of ants while no effect was observed in Aphis spp. Contrarily, populations of Aphis spp. in the manipulative experiment performed better in the presence of ants while no differences were observed for D. plantaginea. Such differences between field and manipulative conditions could be related to thermal tolerance, phenology, and life cycles. In contrast, populations of E. lanigerum were reduced in the presence of ants. 4. Ants also had a significant negative effect on the abundance of natural enemies, which could partially explain the benefits to the tended aphids. However, while ants did not provide a benefit to Aphis spp. when it was reared alone, in the presence of other species ant attendance increased Aphis abundance by 256% and simultaneously reduced E. lanigerum abundance by 63%. Therefore, ants benefited Aphis by reducing competition with other aphid species, which involves a different mechanism, explaining the benefit of ant attendance. Considering all the aphid species together, ants had a net positive effect on aphid abundance, which was consequently considered harmful for the plant. 5. Our results highlighted the role that ants play in structuring apple aphid communities and give support to the observed pattern that ants can benefit tended aphids while simultaneously reducing the abundance of untended herbivores.     

Infection of Arabidopsis by cucumber mosaic virus triggers jasmonate-dependent resistance to aphids that relies partly on the pattern-triggered immunity factor BAK1

Many aphid-vectored viruses are transmitted nonpersistently via transient attachment of virus particles to aphid mouthparts and are most effectively acquired or transmitted during brief stylet punctures of epidermal cells. In Arabidopsis thaliana, the aphid-transmitted virus cucumber mosaic virus (CMV) induces feeding deterrence against the polyphagous aphid Myzus persicae. This form of resistance inhibits prolonged phloem feeding but promotes virus acquisition by aphids because it encourages probing of plant epidermal cells. When aphids are confined on CMV-infected plants, feeding deterrence reduces their growth and reproduction. We found that CMV-induced inhibition of growth as well as CMV-induced inhibition of reproduction of M. persicae are dependent upon jasmonate-mediated signalling. BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor enabling detection of microbe-associated molecular patterns and induction of pattern-triggered immunity (PTI). In plants carrying the mutant bak1-5 allele, CMV induced inhibition of M. persicae reproduction but not inhibition of aphid growth. We conclude that in wildtype plants CMV induces two mechanisms that diminish performance of M. persicae: a jasmonate-dependent and PTI-dependent mechanism that inhibits aphid growth, and a jasmonate-dependent, PTI-independent mechanism that inhibits reproduction. The growth of two crucifer specialist aphids, Lipaphis erysimi and Brevicoryne brassicae, was not affected when confined on CMV-infected A. thaliana. However, B. brassicae reproduction was inhibited on CMV-infected plants. This suggests that in A. thaliana CMV-induced resistance to aphids, which is thought to incentivize virus vectoring, has greater effects on polyphagous than on crucifer specialist aphids.     

Further studies on cucumber mosaic virus infection of narrow-leafed lupin (Lupinus angustifolius): seed-borne infection, aphid transmission, spread and effects on grain yield

Four field trials were done with narrow-leafed lupins (Lupinus angustifolius) in 1988 - 1989, to examine the effect of sowing seed with 5% and 0.5% cucumber mosaic virus (CMV) infection on subsequent virus spread, grain yield and percentage of infection in harvested seed. A proportion of the CM V-infected seed failed to produce established plants and thus, plots sown with 5% and 0.5% infected seed contained 1.5-2.9% and 0.2-0.3% of seed-infected plants respectively. The rate of virus spread by aphids was faster and resulted in more extensive infection at maturity in plots sown with 5% infected seed than with 0.5% infected seed. In three trials, sowing 5% infected seed resulted in yield losses of 34 - 53% and CMV infection in the seed harvested of 6 - 13%. The spread of CMV infection resulting from sowing 0.5% infected seed did not significantly decrease yield. However, late CMV spread in these plots caused > 1% seed infection. In the fourth trial, which was badly affected by drought, CMV spread only slowly, there was no significant effect of CMV on grain yield and the percentage of infected seed harvested was 3–5 times less than that in the seed sown. When CMV-infected seed was sown at different depths, target depths of 8 and 11 cm decreased the incidence of seed-infected plants by c. 15% and c. 50% respectively compared with sowing at 5 cm. However, in glasshouse tests, treatment with the pre-emergence herbicide simazine failed to selectively cull out seed-infected plants. The field trials were colonised by green peach (Myzus persicae), blue-green (Acyrthosiphon kondoi) and cowpea (Aphis craccivora) aphids. When the abilities of these aphid species and of the turnip aphid (Lipaphis erysimi) in transmitting CMV from lupins to lupins were examined in glasshouse tests, short acquisition access times favoured transmission. With 5–10 min acquisition access times, overall transmission efficiencies were 10.8%, 9.4%, 6.1% and 3.9% for the green peach, cowpea, blue-green and turnip aphids respectively.     

Cucumber mosaic virus epidemiology in pepper: Aphid dispersal,transmission efficiency and vector pressure

Cucumber mosaic virus (CMV) causes significant damage and yield losses in peppers. The objective of this study is to determine the efficiency of prevalent aphid species occurring in pepper fields to transmit this virus within pepper plants and to identify their vector pressure in order to target the critical species implicated in CMV epidemics spread. Alatae and apterae were monitored in an experimental pepper field in northern Tunisia for 3 years. Sixty-eight species were captured in winged form in yellow water traps. The most abundant species were Myzus persicae, Aphis gossypii, Aphis fabae, Aphis spiraecola, Acyrthosiphon pisum, Metopolophium dirhodum, Rhopalosiphum maidis, Aphis craccivora, Aphis nerii, Hyalopterus pruni, Sitobion avenae and Rhopalosiphum padi constituting 90% of aphid populations in the field. Their temporal dynamic showed a high period of flight activity from April to June and a second peak in September was registered. Two of these species, M. persicae and A. gossypii were also found in their wingless form on pepper leaves with a prevalence of 99.5% and 0.5%, respectively. The 12 most abundant aphid species were tested for their transmission efficiency of CMV (CMV-pepp2 isolate) with A. gossypii as a reference vector. All aphids tested, including colonizing and non-colonizing species on pepper, were verified to be vectors of this isolate. However, significant differences in the transmission efficiency were found between the aphid species (p < .001, SE = 7.29). M. persicae (60%) scored the highest transmission efficiency rate. Additionally, A. fabae solanella (50%) had higher transmission efficiency than the reference vector, A. gossypii (40%). H. pruni (16.67%) was documented as a new CMV vector to pepper. The single-aphid transmission probabilities ranged from 0.7% to 16.7%. The calculated mean Vector Pressure Index (VPI) for these 12 species showed a stronger relationship with the specific aphid population variance (R = 0.89, p < .01) than the variation of the specific single-aphid transmission probabilities (R = 0.62, p < .05). Indeed, for alate non-colonizer vectors, A. spiraecola has recorded the highest mean VPI (27.5), despite its moderate transmission efficiency (23.33%). Nevertheless, for colonizer vectors in both winged and wingless forms, M. persicae had the highest mean VPI (49.26) of all vector species and was mostly present in its apterous form. The 12 vector species contributed to a total mean VPI of 133.48 during the surveyed periods. This research determined key features of CMV epidemiology in pepper crops that might be helpful for CMV disease management at an early stage.     

Quantitative studies on the uptake and retention of potato leafroll virus by aphids in laboratory and field conditions

Enzyme-linked immunosorbent assay (ELISA) was adapted for the efficient detection and assay of potato leafroll virus (PLRV) in aphids. Best results were obtained when aphids were extracted in 0.05 M phosphate buffer, pH 7.0, and the extracts incubated at 37 °C for 1 h before starting the assay. Using batches of 20 green peach aphids (Myzus persicae), about 0.01 ng PLRV/aphid could be detected. The virus could also be detected in single aphids allowed a 1-day acquisition access period on infected potato leaves. The PLRV content of aphids depended on the age of potato source-plants and the position of source leaves on them. It increased with increase in acquisition access period up to 7 days but differed considerably between individual aphids. A maximum of 7 ng PLRV/aphid was recorded but aphids more usually accumulated about 0.2 ng PLRV per day. When aphids were allowed acquisition access periods of 1–3 days, and then caged singly on Physalis floridana seedlings for 3 days, the PLRV content of each aphid, measured subsequently, was not strongly correlated with the infection of P. floridana. The concentration of PLRV in leaf extracts differed only slightly when potato plants were kept at 15, 20, 25 or 30 °C for 1 or 2 wk, but the virus content of aphids kept on leaves at the different temperatures decreased with increase of temperature. PLRV was transmitted readily to P. floridana at all temperatures, but by a slightly smaller proportion of aphids, and after a longer latent period, at 15 °C than at 30 °C. The PLRV content of M. persicae fed on infected potato leaves decreased with increasing time after transfer to turnip (immune to PLRV). The decrease occurred in two phases, the first rapid and the second very slow. In the first phase the decrease was faster, briefer and greater at 25 and 30 °C than at 15 and 20 °C. No evidence was obtained that PLRV multiplies in M. persicae. These results are compatible with a model in which much of the PLRV in aphids during the second phase is in the haemocoele, and transmission is mainly limited by the rate of passage of virus particles from haemolymph to saliva. The potato aphid, Macrosiphum euphorbiae, transmitted PLRV much less efficiently than M. persicae. Its inefficiency as a vector could not be ascribed to failure to acquire or retain PLRV, or to the degradation of virus particles in the aphid. Probably only few PLRV particles pass from the haemolymph to saliva in this species. The virus content of M. euphorbiae collected from PLRV-infected potato plants in the field increased from early June to early July, and then decreased. PLRV was detected both in spring migrants collected from the plants and in summer migrants caught in yellow water-traps. PLRV was also detected in M. persicae collected from infected plants in July and August, and in trapped summer migrants, but their PLRV content was less than that of M. euphorbiae, and in some instances was too small for unequivocal detection.     

Factors affecting aphid acquisition and transmission of soybean mosaic virus

Myzus persicae transmitted soybean mosaic virus (SMV) most efficiently following 30 or 60 s acquisition probes on infected plants. There were no differences in susceptibility to SMV infection of soybean plants 1 to 12 wk old, but symptoms were more severe in plants inoculated when young than when old. Soybeans inoculated between developmental stages R3 and R6 only showed yellowish-brown blotching on one or more leaves. There were no observable differences in the time of appearance or type of symptoms shown by soybean seedlings inoculated either by sap or by aphids; infected plants became acquisition hosts for aphids 5–6 days after inoculation. There was no change in the efficiency with which M. persicae transmitted SMV from source plants up to 18 wk after inoculation. M. persicae transmitted SMV from leaves of field-grown soybeans when plants were inoculated at developmental stages V6, R2, and R3 and tested as sources 57–74 days after inoculation but not from plants inoculated at R5 and tested as sources 14 to 32 days after inoculation. M. persicae acquired SMV from soybean buds, flowers, green bean pods, and unifoliolate, trifoliolate, and senescent leaves. Middle-aged and deformed leaves were better sources of the virus than buds, unfolding and old symptomless leaves. The results are being incorporated into a computer model of SMV epidemiology.     

Aphid Transmission of Potato aucuba mosaic virus Strains Mediated by Different Strains of Potato virus Y

Three British strains of potato aucuba mosaic virus (PAMV) were tested for transmissibility by the aphid Myzus persicae. None was aphid transmissible on its own but all three were transmitted in the nonpersistent manner by aphids that had previously been fed on a source of the potyvirus potato virus Y (PVY). Different PVY strains mediated PAMV transmission from Nicotiana clevelandii to Capsicum annuum to different degrees, and different PAMV strains were transmitted at different frequencies when assisted by the same PVY strain. These results are compatible with the idea that subtle differences in the PAMV coat protein and in the PVY helper component are responsible for diffrences in frequencies of transmission of PAMV, without however, excluding the possibility of effects of other undefined factors. Transmission of PAMV was no less frequent when mediated by a PVY strain that was unable to infect C. annuum than when a C. annuum‐infecting PVY strain was used.     

Forecasting virus disease in seed potatoes using flight activity data of aphid vectors

We compiled data from the Swiss seed certification programme for the country‐wide incidence of viruses in seed potato crops for the years 1989–2012. Model selection techniques were used to regress year‐to‐year variation in the incidence of potato viruses – largely dominated by Potato virus Y (PVY) – in three susceptible varieties against the abundance of virus vectors (winged aphids), obtained in a suction trap, to identify the most important vector species. The ultimate aim of this study was to develop a decision‐support system capable of forecasting virus spread during the current season using trap data of aphid flights. The average virus incidence in the varieties Bintje, Sirtema and Charlotte varied considerably among years, ranging from 1.0% in 2009 to 13.6% in 1989 (N = 150–611 seed lots per year). A linear regression model including the cumulative sums (until mid‐June) of two aphid species (Brachycaudus helichrysi and Phorodon humuli) as predictor variables for virus disease was remarkably well supported by the data (R² = 0.86). Similarly, using counts of B. helichrysi alone resulted in a good model fit (R² = 0.81). Cross‐validation revealed high predictive accuracy of the model. Although prediction root mean squared errors (RMSE) calculated for different timings of forecasts were high for extremely early forecasts, they rapidly declined for forecasts conducted by the end of May (i.e. 2–4 weeks after potato emergence). Winter temperature (January–February) was positively correlated with the abundance of B. helichrysi in early summer as well as with post‐harvest virus incidence. Remarkably, the abundance of Myzus persicae, often considered the main vector of PVY, was not correlated with virus incidence. Taken together, our analysis suggests that the early migrating aphid B. helichrysi, rather than M. persicae, is the main vector of PVY in Switzerland, and that suction trap data are useful for the design of decision‐support systems aimed to optimise virus control in seed potato production.     

The Ability of Aphis craccivora, A. gossypii and A. citricola to Transmit Single and Mixed Viruses to Cowpeas

The ability of populations of adult apterous Aphis craccivora, A. gossypii and A. citricola to transmit the cowpea aphid-borne virus (CAMV) and cucumber mosaic virus (CMV) to cowpea was investigated. CMV was more readily transmitted than CAMV by all three aphid species, but was most readily transmitted by A. craccivora and A. citricola, with transmissions ranging from 64–71 %. With CAMV, the infection level with A. gossypii was higher than with A. craccivora and both were more efficient than A. citricola, although the differences were not significant. With mixed infections of CAMV and CMV, there was a higher level of CAMV transmission using A. craccivora from sequential feeding when CMV-infected plants were fed on first followed by CAMV-infected plants, suggesting that cowpea plants were more prone to infection by CAMV when already infected by CMV.     

Measuring and modelling the effects of inoculation date and aphid flights on the secondary spread of Beet mosaic virus in sugar beet

The effect of the inoculation date on the spread of Beet mosaic virus (BtMV) in sugar beet field plots was studied. Two plants in the centre of each plot were inoculated with BtMV using Myzus persicae. The spread of the infection around these sources was monitored by inspecting the plants on two diagonal transects through the centre of the plot. Early inoculations resulted in a greater spread than late inoculations, but any inoculation before the onset of the aphid migration resulted in a similar‐sized spread. The spread was concentrated in patches around the inoculated plants, and its rate was explained by vector pressure, as shown by regression analysis and a mechanistic simulation model. This vector pressure was quantified using data obtained by catching aphids in a green water trap in the crop, catching aphids in a 12 m high suction trap at a distant location, and infection of bait plants from adjacent virus source plants. The daily total aphid catches obtained by a suction trap provided the best statistical explanation for the spread of this virus. The parameter r, describing the relationship between vector pressure and the rate of disease progress, was remarkably robust. This parameter varied less than 10% between treatments (infection date) within a single experiment, and less than a factor two between four experiments performed at different sites in two years. The robustness of this parameter suggests that the spread of a potyvirus may be predicted on the basis of the initial infection date and vector abundance.     

Information‐theory‐based model selection for determining the main vector and period of transmission of Potato virus Y

Potato virus Y (PVY, genus Potyvirus, family Potyviridae) is transmitted non‐persistently by aphids. It causes major losses in potato production (Solanum tuberosum), especially following seed tuber‐borne infection of plants. To limit the risk of PVY infection, seed potato production is located preferably in regions where vector pressure is low. The northern‐most high‐grade seed potato production area (HG zone) of Europe is in Finland. The aim of this study was to determine the incidence of aphid species with documented ability to transmit PVY and to use a modelling approach to determine their relative importance as vectors of PVY in the HG zone of Finland. Winged aphids were caught from six to seven potato fields in each of three growing seasons (2007–09) using yellow pan traps that were examined twice a week. Identification of more than 30 000 individuals indicated that 37% of the aphids belonged to nine species reported to transmit PVY. Incidence of PVY in seed lots was low (0–5.6%) and the seasonal increase of PVY incidence was also low in the potato crops. No potato‐colonising aphids were found on the plants in any of the years. The seasonal increase in PVY incidence was modelled using aphid counts in traps, the relative vector efficiencies of the aphids, virus resistance of cultivars, and the initial infection rate of the seed tubers as explanatory variables in generalised linear mixed modelling. Akaike's information criterion was employed to find the best set of explanatory variables for PVY in harvested tubers. Results of this modelling approach showed that the incidence of seed‐borne PVY infection and the early‐season vector flights are the most important factors contributing to the incidence of PVY in the yield. Compared to models with data from all potential vector species, models containing data from Aphis fabae only showed a better model fit with regard to the incidence of PVY in the harvested tubers. The explanatory power of the models was lost when A. fabae was omitted from the vector data, suggesting that other species play a negligible role as vectors of PVY in the HG zone of Finland. Results can be used to devise appropriate strategies for enhanced control of PVY.     

The effect of mineral oil on stylet activities and potato virus Y transmission by aphids

Mineral oil sprayed onto potato virus Y (PVY) infected tobacco plants reduced acquisition of this potyvirus by Myzus persicae (Sulz.). Although the pre-penetration activities of aphids were longer on oil treated leaves, the inhibitory effect of the oil could not be attributed to differences in the duration of stylet penetration. Aphids were therefore made part of a DC circuit in order to investigate their stylet activities during penetration of PVY infected source plants and healthy test plants. Both acquisition and inoculation of the virus were reduced by the presence of oil on the plant surface, but these reductions could not be related to electrically recorded differences in plant penetration behaviour. In particular, stylet punctures of plant cell membranes were not reduced by mineral oil. Non-behavioural reasons are suggested to explain the mode of action of the oil.     

Induction of aphid resistance in tobacco by the cucumber mosaic virus CMV∆2b mutant is jasmonate-dependent

Cucumber mosaic virus (CMV) is vectored by aphids, including Myzus persicae. Tobacco (Nicotiana tabacum ‘Xanthi’) plants infected with a mutant of the Fny strain of CMV (Fny-CMVΔ2b, which cannot express the CMV 2b protein) exhibit strong resistance against M. persicae, which is manifested by decreased survival and reproduction of aphids confined on the plants. Previously, we found that the Fny-CMV 1a replication protein elicits aphid resistance in plants infected with Fny-CMVΔ2b, whereas in plants infected with wild-type Fny-CMV this is counteracted by the CMV 2b protein, a counterdefence protein that, among other things, inhibits jasmonic acid (JA)-dependent immune signalling. We noted that in nontransformed cv. Petit Havana SR1 tobacco plants aphid resistance was not induced by Fny-CMVΔ2b, suggesting that not all tobacco varieties possess the factor(s) with which the 1a protein interacts. To determine if 1a protein-induced aphid resistance is JA-dependent in Xanthi tobacco, transgenic plants were made that expressed an RNA silencing construct to diminish expression of the JA co-receptor CORONATINE-INSENSITIVE 1. Fny-CMVΔ2b did not induce resistance to M. persicae in these transgenic plants. Thus, aphid resistance induction by the 1a protein requires JA-dependent defensive signalling, which is countered by the CMV 2b protein.     

Control of Myzus persicae and Lipaphis erysimi (Hemiptera: Aphididae) by adults and larvae of a flightless strain of Harmonia axyridis (Coleoptera: Coccinellidae) on non-heading Brassica cultivars in the greenhouse

The green peach aphid [Myzus persicae (Sulzer)] and turnip aphid [Lipaphis erysimi (Kaltenbach)] are economically important pests with a worldwide distribution. We have evaluated the efficacy of releasing adults and larvae of a flightless strain of the multicolored Asian lady beetle (Harmonia axyridis Pallas) as a control measure against these aphids on plants of non-heading Brassica cultivars. Both adults and larvae of H. axyridis were observed to be effective biocontrol agents, markedly decreasing the numbers of aphids. The residence duration of adults was longer than that of larvae. The proportion of non-marketable plants damaged by aphids was lower in plots into which either adults or adults and larvae of H. axyridis had been released. These results suggest that both adults and larvae of this flightless strain of H. axyridis are effective in controlling aphids on plants of non-heading Brassica cultivars.     

Crop borders reduce potato virus Y incidence in seed potato

Crop borders of soybean (Glycine max), sorghum (Sorghum bicolor), winter wheat (Triticum aestivum) and potato (Solanum tuberosum) were tested as a means of reducing potato virus Y (PVY) incidence in seed potato. Borders of fallow cultivated ground served as controls. Aphid landing rates were monitored weekly in plots using green tile traps, and PVY incidence was assessed by serologically testing tuber progeny from selected rows in each plot. Average weekly aphid landing rates in fallow-bordered and crop-bordered plots were not significantly different in 1992 (29.4 and 25.2 aphids, respectively) or 1993 (7.3 and 6.6 aphids, respectively). However, crop borders significantly reduced PVY incidence. In 1992, fallow-bordered and soybean-bordered plots averaged 47.8% and 35.0% PVY infection, respectively. In 1993, PVY infection averaged across all crop (soybean, sorghum, and wheat) bordered plots was 2.7% compared to 6.8% in fallow-bordered plots. PVY incidence in the centre rows of fallow-bordered and crop-bordered plots was statistically equivalent, while outer rows of crop-bordered plots had significantly less PVY than outer rows of fallow-bordered plots. Crop borders apparently reduced the number of viruliferous aphids landing on the edge of the plot. The choice of crop species used as a border, or treating the border with a systemic insecticide, did not affect aphid landing rates or PVY incidence. In 1995, PVY incidence in the centre 10 row block of potatoes averaged 2.1% across all crop borders (potato and soybean). PVY infection in the four row potato border averaged 5.7%. Crop borders are readily adaptable to current production practices, although the greatest benefits in reducing PVY incidence would occur in average sized, generation 0 (< 0.2 ha), elite seed potato fields.     

Infective behaviour and aphid-transmissibility of Italian isolates of potato virus Y in tobacco and peppers

When mechanically inoculated to susceptible tobacco (Nicotiana tabacum L.) cultivars, nine isolates of PVY from Umbria (Central Italy) and two from Southern Latium gave rise to rapid systemic infection which developed within 6–8 days after inoculation. Systemic spread of the same isolates was slower, or much slower, in infected pepper (Capsicum annuum L.) cultivars, 8–14 days for Southern Latium isolates and 20 - 35 days for Umbrian ones. Aphid (Myzus persicae)-moculation of pepper and tobacco plants with two of the Umbrian and one of the Southern Latium isolates confirmed the results from sap-transmission and showed that fewer inoculated pepper plants become infected, especially with Umbrian isolates. In agreement with the data on systemic spread, aphid-acquisition trials indicated that tobacco plants became efficient PVY sources for vectors 6–8 days after inoculation with either group of isolates. Peppers became efficient acquisition hosts 8–15 days after inoculation with Southern Latium isolates but not until 22–45 days after inoculation with Umbrian ones. Southern Latium isolates induced more severe symptoms in pepper cultivars than Umbrian isolates did. One of the Southern Latium isolates was able to systemically infect the resistant pepper cv. Yolo Y, which was never infected by the Umbrian isolates. The Umbrian isolates tested seem to be better adapted to tobacco than peppers, while Southern Latium ones are well adapted to both.