Mi‐mediated aphid resistance in tomato: tissue localization and impact on the feeding behavior of two potato aphid clones with differing levels of virulence

The Mi‐1.2 gene in tomato, Solanum lycopersicum L. (Solanaceae), confers resistance against several herbivores, including the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera: Sternorrhyncha: Aphididae) and the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Sternorrhyncha: Aleyrodidae). Previous studies on the tissue localization of resistance have given varying results; whitefly resistance was attributed to factors localized in the mesophyll or epidermis, whereas aphid resistance was attributed to factors localized in the phloem. Our study utilizes the direct current electrical penetration graph (DC‐EPG) technique to compare aphid feeding behavior on resistant (Mi‐1.2+) and susceptible (Mi‐1.2?) tomato plants. This study also compares the impact of resistance on the feeding behavior of two aphid clones that vary in their virulence, or their ability to survive and reproduce on resistant plants. Previous work had shown that the avirulent WU11 clone is almost completely inhibited by resistance, whereas the semi‐virulent WU12 clone can colonize resistant hosts. Here, DC‐EPG analysis shows that both aphid clones take longer to initiate cell sampling and to establish a confirmed sieve element phase on resistant plants than on susceptible hosts, and have shorter ingestion periods on resistant plants. However, the magnitude of these deterrent effects is far less for the semi‐virulent clone than for the avirulent aphids. In particular, the WU12 clone is less sensitive to factors that limit sieve element ingestion, showing shorter non‐probe duration and rapidly establishing sustained phloem ingestion on resistant plants when compared to the WU11 clone. We conclude that, in addition to previously described factors in the phloem that inhibit ingestion, Mi‐mediated aphid resistance also involves factors (possibly in the mesophyll and/or epidermis) that delay initiation of phloem salivation, and that act in the intercellular spaces to deter the first cell sampling. Furthermore, the relative effectiveness of these components of resistance differs among insect populations.     

Phloem specific aphid resistance in Cucumis melo line AR 5: effects on feeding behaviour and performance of Aphis gossypii

The feeding behaviour, excretion rate, and life history traits of the cotton-melon aphid, Aphis gossypii (Glover) (Homoptera, Aphididae), were measured on a resistant melon, Cucumis melo L., breeding line, AR 5. The site of resistance detection by the aphids was determined using the electrical penetration graph (EPG) technique. EPG recordings showed that resistance is expressed within the host plant, rather than on its surface, because the time to first stylet penetration was not significantly different between AR 5 and the closely related susceptible breeding line, PMR 5. EPG patterns associated with stylet pathway activities of the aphids were not significantly different between the resistant and susceptible lines. Significant behavioural differences were observed only after stylets contacted phloem sieve elements. On AR 5, the duration of salivation after sieve element puncture (waveform E1) was significantly longer, and the number of aphids showing phloem sap ingestion (waveform E2) was significantly reduced. We conclude that the resistance mechanism producing the effects seen in this study acts within the phloem sieve elements. Monitoring of excretion rates on the two genotypes showed that aphid feeding was delayed and greatly reduced on the resistant genotype. Comparisons of aphid life history traits and population development between host plant genotypes showed that the effects of resistance act throughout aphid development and are highly effective at slowing down population increase.     

Ultrastructure of compatible and incompatible interactions in phloem sieve elements during the stylet penetration by cotton aphids in melon

Resistance of the melon line TGR‐1551 to the aphid Aphis gossypii is based on preventing aphids from ingesting phloem sap. In electrical penetration graphs (EPGs), this resistance has been characterized with A. gossypii showing unusually long phloem salivation periods (waveform E1) mostly followed by pathway activities (waveform C) or if followed by phloem ingestion (waveform E2), ingestion was not sustained for more than 10 min. Stylectomy with aphids on susceptible and resistant plants was performed during EPG recording while the stylet tips were phloem inserted. This was followed by dissection of the penetrated leaf section, plant tissue fixation, resin embedding, and ultrathin sectioning for transmission electron microscopic observation in order to study the resistance mechanism in the TGR. The most obvious aspect appeared to be the coagulation of phloem proteins inside the stylet canals and the punctured sieve elements. Stylets of 5 aphids per genotype were amputated during sieve element (SE) salivation (E1) and SE ingestion (E2). Cross‐sections of stylet bundles in susceptible melon plants showed that the contents of the stylet canals were totally clear and also, no coagulated phloem proteins occurred in their punctured sieve elements. In contrast, electron‐dense coagulations were found in both locations in the resistant plants. Due to calcium binding, aphid saliva has been hypothesized to play an essential role in preventing/suppressing such coagulations that cause occlusion of sieves plate and in the food canal of the aphid's stylets. Doubts about this role of E1 salivation are discussed on the basis of our results.     

Probing behaviour of Cacopsylla pyri on a resistant pear selection

European pear psylla Cacopsylla pyri L. (Hemiptera Psyllidae) is one of the worst pests of pear (Pyrus communis L.) in Europe. We investigated probing behaviour in adults and nymphs of C. pyri by full EPG on a psylla‐resistant pear selection, NY 10353. Concerning stylet probing behaviour on the plant surface, the results showed no significant differences between the resistant selection and the susceptible cultivar Bartlett, and no differences were also detected for epidermis and mesophyll resistance in the same conditions. For mesophyll/phloem, no differences were found in adults. However, in nymphs, weak resistance factors (longer stylet penetration and mesophyll salivation) were detected in the resistant selection. In phloem, EPG data indicate strong resistance factors in NY 10353, especially for nymphs and summer‐form adults (longer time before the first phloem ingestion and a lower duration of each phloem ingestion event). No prolonged (>10 min) phloem ingestion was performed by nymphs and adults in the resistant selection. The results support the hypothesis that NY 10353 resistance factors are located in the phloem sap and cause high C. pyri nymph mortality: this could be useful as a basis for further investigations of resistance mechanisms at the metabolic, chemical and genetic levels.     

An analysis of the feeding behavior of three stages of Toxoptera citricida by DC electrical penetration graph waveforms

With the purpose of studying the feeding behavior of the brown citrus aphid pest, Toxoptera citricida (Kirkaldy) (Hemiptera: Aphididae), we compared stylet probing behaviors of third and fourth instars and adults on Citrus unshiu Marc (Rutaceae) seedlings using the electrical penetration graph (EPG) technique. EPG waveforms exhibited the full suite of stylet behaviors – stylet pathway, intracellular stylet puncture, phloem salivation (E1), sieve ingestion (E2), and xylem sap ingestion activities, plus the non‐penetration (Np) waveform. Before the phloem phase, the number of probes was significantly higher for third‐instar nymphs than for adults. Overall duration of Np events by adults was significantly lower than the duration of third and fourth instars. The number of short probes of the fourth instars was significantly higher than that of the adults. In the phloem phase, adults made more frequent and longer E1 events than the third and fourth instars. Third instars made more frequent but shorter E2 events, whereas adults made fewer but longer events. These results showed adults gained nutrients by increasing feeding time during phloem ingestion. Thus, the probability of phloem‐associated virus acquisition and transmission of T. citricida was higher in adults than in nymphs.     

Induced resistance by Myzus persicae in the peach cultivar `Rubira'

The effect of a previous infestation by the green peach aphid Myzus persicae (Sulzer) on the settling behaviour and reproduction of the same aphid species was investigated in the resistant peach cultivar Rubira, and compared with that observed in the susceptible control cultivar GF305. A previous infestation of 48 h triggered induced resistance in Rubira. There were significantly fewer aphids settling on preinfested than on uninfested plants, indicating an increased rejection of Rubira as a host plant. The level of induced resistance in preinfested plants was positively related to the duration of the first infestation. In GF305, previous infestation had no detrimental effect on aphid settlement and even slightly enhanced larviposition by adult females. The aphid probing behaviour after a 48-h preinfestation was also monitored for 8 h with the electrical peneration graph (EPG) technique. On preinfested GF305, most EPG parameters indicated an enhanced host plant acceptance. On preinfested GF305, aphids produced less sieve element salivation and more continuous sap ingestion than on uninfested GF305, indicating that the previous aphids provoked changes in plant properties beneficial to the test aphids. In Rubira, a major induced factor of resistance was thought to be expressed in the sieve element as phloem sap ingestion was 4-fold shorter on preinfested than on uninfested plants. The time taken by the aphid stylets to reach a sieve element was also significantly increased on preinfested Rubira, suggesting the induction of resistance factors outside the phloem. The originality of the Rubira/M. persicae interaction is discussed in the perspective of a better understanding of plant induced responses to aphids.     

Feeding by sugarcane aphid,Melanaphis sacchari,on sugarcane cultivars with differential susceptibility and potential mechanism of resistance

Feeding behavior of Melanaphis sacchari Zehntner (Hemiptera: Aphididae) was studied on sugarcane, Saccharum spp. (Poaceae), cultivars HoCP 91‐555 (resistant), LCP 85‐384 (moderately resistant), and L 97‐128 (susceptible) using the electrical penetration graph (EPG) technique. Constitutive concentrations of total phenolics and available carbohydrates, water potential at the whole‐leaf tissue level, and free amino acids (FAAs) in phloem sap extracts, and in honeydew produced by aphids fed on L 97‐128 and HoCP 91‐555 were determined. Cultivar did not influence time for M. sacchari to access phloem sieve elements. Total time in sieve elements was ca. two‐fold greater on L 97‐128 than on HoCP 91‐555, whereas it did not differ from LCP 85‐384 in either cultivar. The mean duration of individual events associated with phloem sap ingestion was ca. 50% shorter on both HoCP 91‐555 and LCP 85‐384 than on L 97‐128. Although cultivar effects were not detected for levels of total phenolics, available carbohydrates, and water potential, two free essential amino acids, histidine and arginine, were absent from phloem sap in HoCP 91‐555. Two free essential amino acids, leucine and isoleucine, and two free non‐essential amino acids, tyrosine and proline, were absent from honeydew of aphids fed on HoCP 91‐555. These results suggest that despite apparent biosynthesis of some FAAs, the absence of important FAAs in the phloem sap of HoCP 91‐555 and the inability of M. sacchari and its endosymbionts (e.g., Buchnera) to derive specific free essential and non‐essential amino acids from other ingested molecules, possibly along with other unidentified factors, underlie the pest's decreased phloem sap ingestion and consequently reduced growth potential on HoCP 91‐555.     

Electrical penetration graphs of the nymphal stage of Bemisia argentifolii

Electrical penetration graph (EPG, DC system) waveforms were recorded from first, second, and third instar Bemisia argentifolii nymphs. Waveforms recorded were similar among the three instars. Four waveforms were recorded and were named C, J, L, and H. Waveform J is new, whereas waveforms C, L, and H of B. argentifolii nymphs were similar to those published previously from greenhouse whitefly nymphs. As in the previous study on greenhouse whitefly nymphs, there was variation in each of waveforms C, L, and H. Waveform C was recorded at an extracellular voltage level, and represents a pathway phase where the stylets penetrate the plant tissue in an intercellular pathway. At the end of waveform C, the voltage dropped to an intracellular level, indicating penetration of a living cell, and the stylet tips then remained in that cell for the rest of the EPG recording, which was sometimes as long as 16 h. Three waveforms (J, L, and H) were recorded during this intracellular phase, beginning with J, a brief (average = 31 s), low amplitude, irregular waveform. J appeared only at the beginning of the intracellular phase, and was followed by either L (five out of eight times) or H (three out of eight times). Waveforms L and H then alternated with one another for the remainder of the intracellular phase. The most conspicuous difference between L and H was the frequency of their voltage fluctuations; L had a lower frequency and H a higher frequency. Usually the shape of waveform L was dominated by voltage peaks in a positive direction, while waveform H was characterized by strong voltage peaks in a negative direction; although some variants of both L and H had distinct voltage peaks in both directions. The electrical origin of both the positive and negative voltage peaks was electromotive force (emf) fluctuation rather than resistance fluctuation. During waveform H, copious amounts of honeydew were produced, indicating that the penetrated cell was a sieve element. We conclude, therefore, that H represents phloem sap ingestion; and because J and L are produced in the same cell as H, then phloem phase is represented by waveforms J, L, and H. The biological correlations for J and L are not yet known.     

Sieve element occlusion provides resistance against Aphis gossypii in TGR-1551 melons

Feeding behavior and plant response to feeding were studied for the aphid Aphis gossypii Glover on susceptible and resistant melons(cv.Iroquois and TGR-1551,respectively).Average phloem phase bout duration on TGR-1551 was<7% of the duration on Iroquois.Sixty-seven percent of aphids on TGR-1551 never produced a phloem phase that attained ingestion(EPG waveform E2)in contrast to only 7% of aphids on Iroquois.Average bout duration of waveform E2(scored as zero if phloem phase did not attain E2)on TGR-1551 was<3% of the duration on Iroquois.Conversely,average bout duration of EPG waveform El(sieve element salivation)was 2.8 times greater on TGR-1551 than on Iroquois.In a second experiment,liquid nitrogen was used to rapidly cryofix leaves and aphids within a few minutes after the aphids penetrated a sieve element.Phloem near the penetration site was then examined by confocal laser scanning microscopy.Ninety-six percent of penetrated sieve elements were occluded by protein in TGR-1551 in contrast to only 28% in Iroquois.Usually in TGR-1551,occlusion was also observed in nearby nonpenetrated sieve elements.Next,a calcium channel blocker,trivalent lanthanum,was used to prevent phloem occlusion in TGR-1551,and A.gossypii feeding behavior and the plants phloem response were compared between lanthanum-treated and control TGR-1551.Lanthanum treatment eliminated the sieve element protein occlusion response and the aphids readily ingested phloem sap from treated plants.This study provides strong evidence that phloem occlusion is a mechanism for resistance against A.gossypii in TGR-1551.     

亚洲柑橘木虱成虫和5龄若虫在感染黄龙病的柑橘上的取食行为及获菌效率比较

【目的】亚洲柑橘木虱Diaphorina citri是柑橘的毁灭性病害——黄龙病亚洲种‘Candidatus Liberibacter asiaticus’(‘Clas’)的主要传播媒介。本研究的目的是明确木虱成虫和5龄若虫的取食行为、获菌效率是否有差异,以及寄主感染黄龙病是否对5龄若虫取食产生影响。【方法】利用直流型刺吸电位仪(DC-EPG Giga-4)记录柑橘木虱成虫和5龄若虫在携带黄龙病的酸橘Citrus reticulata cv. Sunki嫩梢上10 h的取食行为,用qPCR单头检测其获得黄龙病病原菌的效率,并比较5龄若虫在感病和健康植株嫩梢上的取食行为。【结果】柑橘木虱成虫与5龄若虫在感染黄龙病的酸橘上的取食行为有显著差异。5龄若虫比成虫更快地开始在韧皮部和木质部进行吸食,口针在韧皮部的总过程以及吸食时间显著长于成虫。此外,5龄若虫和成虫在EPG测定(同时饲菌)10 h后获菌率分别为37.5%和20.0%,若虫明显高于成虫。寄主植物感染黄龙病对5龄若虫的取食行为有一定的影响,表现在感病植株上的刺探次数、唾液分泌次数和韧皮部吸食次数都显著少于健康植株,而两者分泌唾液和韧皮...     

Characterization of alfalfa germplasm expressing resistance to silverleaf whitefly, Bemisia argentifolii

Abstract: Thirty‐eight plants were taken from a University of California alfalfa selection nursery for developing resistance to silverleaf whitefly, Bemisia argentifolii Bellows & Perring. Seventeen of the plants had low whitefly infestation and were categorized as ‘potentially resistant’; 21 of the plants had high whitefly infestation and were categorized as ‘presumed susceptible’. Plants were propagated vegetatively so that replicated measurements of whitefly performance could be made on each genotype. Two colonies of silverleaf whiteflies were used: one reared on alfalfa (alfalfa‐experienced whiteflies), and the other on cotton (alfalfa‐naive whiteflies). The effect of variation among alfalfa genotypes on whitefly performance was similar for both whitefly sources, although on all genotypes, the alfalfa‐experienced whiteflies generally performed better than their alfalfa‐naive counterparts. In greenhouse tests, fecundity of newly eclosed adults (over a 5‐day period) on the 17 potentially resistant genotypes was relatively consistent in being lower than fecundity on the presumed susceptible genotypes. However, in nymphal survival tests, the response on the 17 potentially resistant genotypes was not consistent. Nymphal survival (egg to adult) on some of these was very low, as expected, while nymphal survival on others was as high as on the presumed susceptible genotypes. Fecundity and nymphal survival data were not correlated for alfalfa‐naive whiteflies, and were only weakly correlated (r² = 0.13, d.f. = 32, P = 0.04) for alfalfa‐experienced whiteflies. Thirteen genotypes then were examined in the greenhouse in stage‐specific survival tests, where four genotypes demonstrated high resistance (<10% nymphal survival) and three demonstrated moderate resistance (11–34% survival) compared with the three presumed susceptible genotypes that were tested (51–73% survival). Most of the mortality on the resistant genotypes occurred in the first instar, while mortality was more evenly distributed across the life stages on the susceptible genotypes. Interestingly, if nymphs survived to second instar on the resistant genotypes, then their subsequent survival to adult eclosion was similar to survival of second instar to adult on susceptible genotypes. Six of the genotypes used in the greenhouse stage‐specific survival test also were evaluated in the field for nymphal survival, and these results were consistent with the greenhouse tests.     

Stylet penetration by larvae of the greenhouse whitefly on cucumber

Probing behaviour of Trialeurodes vaporariorum (Westwood) larvae was monitored using the DC electrical penetration graph (EPG) technique on the host plant cucumber. EPGs were recorded for 16 h, simultaneously with honeydew excretion using a honeydew clock. Three waveforms were distinguished: a pathway waveform (C), and two phloem waveforms, one with a high (H), and one with a low frequency (L) signal. The C waveform mainly occurred in the crawler stage of the 1st instar larvae. EPGs recorded from larvae during and after moulting indicated that the process involves stylet withdrawal; hence the stylets of each new instar need to penetrate again from the leaf surface to the phloem.All sessile stages, from L1 to pre-pupa, spent almost their entire time in waveforms H and L. These waveforms alternated more frequently in the early instars than during the later ones, in which the H waveform became predominant. The H waveform was highly correlated with honeydew excretion and thus phloem sap ingestion. The L waveform was not related to honeydew excretion but EPGs indicated that the stylet tips remain in a sieve element during both waveforms. Periods of honeydew production demonstrated a delay of 30–40 min in relation to the onset and end of H and L waveforms. This delay is presumably related to the time needed for food passing through, or emptying of, the insect's gut. From the 1st instar to the pre-pupa, the frequency of excreted honeydew droplets decreased but their size increased, causing a net increase of the excretion rate.     

Brown planthopper (N. lugens Stal) feeding behaviour on rice germplasm as an indicator of resistance

Background

The brown planthopper (BPH) Nilaparvata lugens (Stal) is a serious pest of rice in Asia. Development of novel control strategies can be facilitated by comparison of BPH feeding behaviour on varieties exhibiting natural genetic variation, and then elucidation of the underlying mechanisms of resistance.

Methodology/Principal Findings

BPH feeding behaviour was compared on 12 rice varieties over a 12 h period using the electrical penetration graph (EPG) and honeydew clocks. Seven feeding behaviours (waveforms) were identified and could be classified into two phases. The first phase involved patterns of sieve element location including non penetration (NP), pathway (N1+N2+N3), xylem (N5) [21] and two new feeding waveforms, derailed stylet mechanics (N6) and cell penetration (N7). The second feeding phase consisted of salivation into the sieve element (N4-a) and sieve element sap ingestion (N4-b). Production of honeydew drops correlated with N4-b waveform patterns providing independent confirmation of this feeding behaviour.

Conclusions/Significance

Overall variation in feeding behaviour was highly correlated with previously published field resistance or susceptibility of the different rice varieties: BPH produced lower numbers of honeydew drops and had a shorter period of phloem feeding on resistant rice varieties, but there was no significant difference in the time to the first salivation (N4-b). These qualitative differences in behaviour suggest that resistance is caused by differences in sustained phloem ingestion, not by phloem location. Cluster analysis of the feeding and honeydew data split the 12 rice varieties into three groups: susceptible, moderately resistant and highly resistant. The screening methods that we have described uncover novel aspects of the resistance mechanism (or mechanisms) of rice to BPH and will in combination with molecular approaches allow identification and development of new control strategies.     

Analysis of wheat resistance to the cereal aphidSitobion avenae using electrical penetration graphs and flow charts combined with correspondence analysis

The behaviour ofSitobion avenae (F.), was compared on resistant wheat lines ofTriticum monococcum (L.) and a susceptible variety ofTriticum aestivum (L.). Firstly, stylet penetration activities were monitored with the Electrical Penetration Graph (EPG) technique and subsequently analysed using flow charts combined with correspondence analysis. Plant resistance was shown to be associated with repeated penetrations without access to either the xylem or the phloem, and with numerous failures in starting a sustained sap ingestion (as represented by pattern E2). Access to sieve elements of the phloem did not seem to be much affected on resistant plants but it took the aphid three times as long to produce a sap ingestion pattern when maintained on the resistant lineT. monococcum n^o 44 (Tm44) as compared with aphids maintained on susceptible plants. As a result the total time spent in ingesting from sieve elements was reduced by 72% on Tm44. Secondly, direct observations of freely-moving apterous adults were performed. Aphids did not discriminate between resistant and susceptible wheat during the first 30 min of access to test leaves, but only 4 out of 25 aphids were still probing after eight hours on resistant Tm44. The relevance of these results to possible location of the resistance factor(s) are discussed. Although detection of plant resistance before sieve elements are reached can not be rigorously excluded, the factors involved inT. monococcum resistance toS. avenae undoubtedly occur within the phloem vessels.     

Electrical penetration graphic waveforms in relation to the actual positions of the stylet tips of Nilaparvata lugens in rice tissue

The stylet penetration behavior of Nilaparvata lugens (Hemiptera: Delphacidae) in rice plants (Oryza sativa) was evaluated through the use of an electrical penetration graph (EPG). To accomplish this, we classified the EPG signals into seven different waveforms, np, N1, N2, N3, N4-a, N4-b, and N5, according to their shapes, amplitudes, and frequencies. The N4-b waveform was always preceded by N3 and N4-a, in that order. Continuous honeydew excretion only occurred during the N4-b period, and the honeydew deposited on a filter paper containing ninhydrin reagent during the N4-b period was stained violet. The tips of the stylets that were severed in the N3, N4-a, and N4-b periods were in the phloem region of rice. Moreover, the flow of plant sap after stylectomy only occurred during the N4-b period. Finally, sucrose was the only carbohydrate component identified when HPLC analysis of the plant sap was conducted. On the other hand, honeydew excretion hardly occurred during the N5 period and the tips of the stylets that were severed during the N5 period were located in the xylem region of rice. Based on the location of the stylets and honeydew excretion, the EPG waveforms for the stylet penetration behaviors of N. lugens were assigned to the following groups; np: non-penetration of stylets, N1: penetration initiation, N2: salivation and stylet movement, N3: an extracellular activity near the phloem region, N4-a: an intracellular activity in phloem region, N4-b: phloem sap ingestion, and N5: activity in the xylem region.     

Penetration of faba bean sieve elements by pea aphid does not trigger forisome dispersal

Immediately after their stylets penetrate a phloem sieve element, aphids inject saliva into the sieve element for approximately 30–60 s before they begin to ingest phloem sap. This salivation period is recorded as waveform E1 in electrical penetration graph (EPG) monitoring of aphid feeding behavior. It has been hypothesized that the function of this initial period of phloem salivation is to reverse or prevent plugging of the sieve element by one of the plant's phloem defenses: formation of P‐protein plugs or callose synthesis in the sieve pores that connect adjacent sieve elements. This hypothesis was tested using the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), and faba bean, Vicia faba L. (Fabaceae), as a model system, and the results do not support the hypothesis. In legumes, such as faba bean, P‐protein plugs in sieve elements are formed by dispersal of proteinaceous bodies called forisomes. Contrary to the hypothesis, the great majority of sieve element penetrations by pea aphid stylets do not trigger forisome dispersal. Thirteen sieve elements were cryofixed early in phloem phase before the aphids could complete their salivation period and the forisomes were not dispersed in any of the 13 samples. However, in these samples, the aphids completed on average a little over half of their normal E1 salivation period before they were cryofixed. Thus, it is possible that sieve element penetration triggered forisome dispersal in these samples but the abbreviated period of salivation was still sufficient to reverse dispersal. To rule out this possibility, 17 sieve elements were cryofixed during R‐pds, which are an EPG waveform associated with sieve element penetration but without the characteristic E1 salivation that occurs during phloem phase. In 16 of the 17 samples, the forisomes were not dispersed. Thus, faba bean sieve elements usually do not form P‐protein plugs in response to penetration by pea aphid stylets. Consequently, the characteristic E1 salivation that occurs at the start of each phloem phase does not seem to be necessary to prevent a plugging response because penetration of sieve elements during R‐pds does not trigger forisome dispersal despite the absence of E1 salivation. Furthermore, as P‐protein plugs do not normally form in response to sieve element penetration, E1 salivation that occurs at the start of each phloem phase is not a response to development of a P‐protein plug. Thus, the E1 salivation period at the beginning of the phloem phase appears to have function(s) unrelated to phloem sealing.     

Location of resistance factors in the leaves of potato and wild tuber-bearing Solanum species to the aphid Myzus persicae

Analysis of electrically recorded feeding behaviour of aphids was combined with colony‐development tests to search for sources of resistance to Myzus persicae (Sulzer) (Homoptera: Aphididae) in tuber‐bearing Solanum species (Solanaceae), aiming at a reduction of potato leaf roll virus (PLRV) transmission. Twenty genotypes, originating from 14 gene bank accessions, representing 13 wild tuber‐bearing Solanum spp., three Solanum tuberosum L. (potato) cultivars, and one S. tuberosum breeding line, were selected. Colony‐development tests were carried out in no‐choice experiments by placing adult aphids on plants of each genotype and counting numbers of nymphs and adults on young plants after 8 and 15 days, and on flowering plants after 14 and 30 days. Large differences were observed among genotypes: some developed small colonies and others developed large ones. Also, in a few genotypes, resistance in mature plants was different for leaves of different ages; young leaves were resistant to aphids whereas old senescent leaves were susceptible. The electrical penetration graph (DC‐EPG system) technique was used to study aphid feeding behaviour on each Solanum genotype for 6 h. Electrical penetration graph (EPG) results also showed large differences among the genotypes, indicating resistance at the leaf surface and at three different levels of plant tissue (epidermis, mesophyll, and phloem). Therefore, it was concluded that different mechanisms of resistance to M. persicae exist among the genotypes analysed. EPGs recorded from aphids on Solanum berthaultii Hawkes and Solanum tarijense Hawkes with and without glandular trichomes showed that strong surface resistance can bias EPG parameters associated with resistance located in deeper tissues. Experimental evidence is presented that the resistance to aphids in the genotypes with glandular trichomes strongly depends on these morphological structures.     

Probing behaviour of the green peach aphid Myzus persicae on resistant Prunus genotypes

Electrical penetration graphs of Myzus persicae (Sulzer) (Homoptera: Aphididae) feeding behaviour on four resistant and two susceptible genotypes of peach (Prunus persica L. Batsch) and related species showed that resistance was mainly linked to (i) reduced duration of phloem sap uptake, (ii) reduced percentage of pattern E1 (salivary secretion into sieve elements) followed by pattern E2 (sap ingestion) and (iii) increased number of shifts from E1 to E2 and back. These results suggest the unsuitability of phloem sap, and thus repetitive failures to initiate sustained ingestion. Extensive comparisons of the EPGs also revealed more specific trends. Aphids on the most susceptible cultivar GF305 produced significantly longer potential drops than on other peach genotypes. On the resistant Rubira, aphids generated more penetrations before the first E occurred, indicating the possible presence of a resistance factor before the phloem was reached. The clone P1908 of the wild species Prunus davidiana displayed traits of both susceptibility (less but longer probes) and resistance. In particular, aphids produced more E1, suggesting difficulties in preparing sieve elements before feeding. The aphid probing process could be correlated with aphid settling behaviour and bionomics, as previously reported, and gave evidence for the existence of different mechanisms underlying resistance in the tested genotypes against M. persicae.     

Characterisation of bird cherry-oat aphid (Rhopalosiphum padi L.) behaviour and aphid host preference in relation to partially resistant and susceptible wheat landraces

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a major pest of wheat (Triticum aestivum L.) and can cause up to 30% yield losses. Heritable plant resistance to aphids is both an economically and ecologically sound method for managing aphids. Here we report how the behaviour and performance of R. padi differs on two resistant, one susceptible wheat landrace and a susceptible elite wheat variety. Feeding behaviour differed among the genotypes, with aphids on resistant lines spending longer in the pathway phase and less time phloem feeding. These behaviours suggest that both inter- and intracellular factors encountered during pathway and phloem feeding phases could be linked to the observed aphid resistance. Locomotion and antennal positioning choice tests also revealed a clear preference for susceptible lines. Although feeding studies revealed differences in the first probe indicating that the resistance factors might also be located in the peripheral layers of the plant tissue, scanning electron microscopy revealed no difference in trichrome length and density on the surface of leaves. Aphids are phloem feeders and limiting the nutrient uptake by the aphids may negatively affect their growth and development as shown here in lower weight and survival of nymphs on resistant genotypes and decreased reproductive potential, with lowest mean numbers of nymphs produced by aphids on W064 (54.8) compared to Solstice (71.9). The results indicate that resistant lines markedly alter the behaviour, reproduction and development potential of R. padi and possess both antixenosis and antibiosis type of resistance.