Evidence that elevated CO2 reduces resistance to the European large raspberry aphid in some raspberry cultivars

Global climate change, such as elevated atmospheric carbon dioxide (eCO₂), may accelerate the breakdown of crop resistance to insect pests by compromising expression of resistance genes. This study investigated how eCO₂ (700 μmol/mol) affected the susceptibility of red raspberry (Rubus idaeus) to the European large raspberry aphid (Amphorophora idaei) Börner (Homoptera: Aphididae), using a susceptible cultivar (Malling Jewel) and cultivars containing either the A₁ (Glen Lyon) or A₁₀ (Glen Rosa) resistance genes. Compared to plants grown at ambient CO₂ (aCO₂) (375 μmol/mol), growth rates were significantly increased (ranging from 42–300%) in all cultivars at eCO₂. There was some evidence that plants containing the A₁ gene were more susceptible to aphids at eCO₂, with aphid populations doubling in size compared to the same plants grown at aCO₂. Moreover, aphids grew 38% larger (1.36 mg compared with 0.98 mg) on plants with the A₁ resistance gene at eCO₂ compared with those at aCO₂. Aphid performance on plants containing the A₁ gene grown at eCO₂ was therefore similar to that of aphids reared on entirely susceptible plants under either CO₂ treatment. In contrast, aphids did not respond to eCO₂ when reared on plants with the A₁₀ resistance gene, suggesting that plants with this resistance gene remained resistant to aphids at eCO₂.     

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different resistance genes

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different A. idaei resistance genes were studied under infestation tunnel, glasshouse and laboratory conditions. Each test consistently ranked raspberry cultivars, with increasing levels of resistance in the order, non-resistant (cv. Mailing Jewel), resistant cultivars containing minor genes, major gene A₁, gene A₁₀ (red raspberry) and gene A₁₀ (black raspberry) respectively. Resistance was expressed in three different ways; decreased alatae settling and feeding, decreased apterae settling and decreased aphid fecundity and rate of nymphal development. Following exposure to a large population of alatae, significantly fewer aphids settled on A₁, A₁₀ and to a lesser extent, minor gene-containing cultivars, compared to cv. Mailing Jewel. More alatae settled on the top than the bottom zone of non-resistant, minor gene resistant and two of four A₁-containing cultivars; alatae settling was low on all zones of A₁₀-containing raspberries. Aphid fecundity and nymph development patterns on different cultivars and resistance classes were similar to those found for alatae settling. After 7 days reproduction more than 30% of the nymphs developed to third or fourth instar on cv. Mailing Jewel, whilst on minor gene and major gene-containing cultivars the total number of nymphs and the proportion of later instars decreased. On resistant cultivars the nymphs were found mainly on the middle and bottom leaf zones, compared to cv. Mailing Jewel. A rapid (48 h) screening test using floating leaflets was developed and, on the basis of apterae settling, ranked cultivars reliably; it was particularly effective in distinguishing moderate and strong resistance to strain 1 A. idaei. Gene A₁-containing cultivars bred in England were much more resistant to strain 1 A. idaei than those bred in Scotland. Possible reasons for this difference are discussed, in relation to modified screening procedures and to the control of the viruses transmitted by this aphid vector.     

Further evidence that resistance in raspberry to the virus vector aphid, Amphorophora idaei, is related to the chemical composition of the leaf surface

In an approach to understand the mechanism(s) of resistance in raspberry to infestation by the aphid Amphorophora idaei, progeny plants segregating for the A. idaei resistance gene, A₁₀, were bioassayed and dichloromethane extracts from the leaf surface were examined by capillary column gas chromatography (GC). No single GC peak was detected that was present in only the resistant progeny plants. Nevertheless, thirteen compounds present in all samples were quantified and identified by mass spectrometry. They were of four major classes; straight chain hydrocarbons, acetic acid esters of long chain alcohols, tocopherols and triterpenoid compounds, including α and β amyrin. Several of these compounds were not recorded previously in raspberry leaves. Linear discriminant analysis, applied to the standardised chromatographic data in an attempt to relate chemical composition to resistance, successfully partitioned 24 of the 26 plants into resistant and susceptible types as determined by bioassay. These data provide further evidence that resistance in raspberry to A. idaei is related to the chemical composition of the leaf surface.     

Infectibility and sensitivity of UK raspberry, blackberry and hybrid berry cultivars to Rubus viruses

Six blackberry or hybrid berry cultivars and 19 raspberry cultivars were assessed for their infectibility with, and sensitivity to, graft inoculation with 10 distinct viruses found infecting Rubus in the UK. Cultivars were grafted with each of, two isolates of the pollen borne raspberry bushy dwarf virus (RBDV), five aphid borne viruses: black raspberry necrosis, raspberry leaf mottle (RLMV), raspberry leaf spot (RLSV), rubus yellow net and raspberry vein chlorosis (RVCV); and isolates of the nematode transmitted nepoviruses, arabis mosaic, raspberry ringspot, strawberry latent ringspot and tomato black ring. All tested cultivars were infectible with a resistance breaking isolate of RBDV but only about half of that number with the Scottish type isolate of the virus. The raspberry cvs Autumn Bliss, and occasionally Glen Garry and Glen Prosen, developed leaf yellowing symptoms following infection with RBDV, but none of the other infected cultivars showed obvious leaf symptoms when kept in a heated glasshouse during the growing season. All tested cultivars were infectible with each of the four viruses transmitted in nature by the aphid, Amphorophora idaei. Most were infected symptomlessly, but seven cultivars developed severe leaf spotting symptoms due to infection with RLMV or RLSV. All but one of the raspberry cultivars were infectible with RVCV, which is transmitted in nature by the aphid Aphis idaei, and almost all infected plants developed leaf symptoms; only one of the hybrid berry or blackberry cultivars tested was infected with RVCV. In tests with the four nepoviruses, all tested cultivars, except Tummelberry, were infectible with at least one or more of these viruses. However, cultivars responded differently to challenge inoculation with different isolates of individual nepoviruses. Several cultivars developed chlorotic leaf mottling following infection with some nepovirus isolates. The implications of these results for virus control are discussed in the light of the changing pattern of virus and virus vector incidence in the UK.     

Mapping of A1 conferring resistance to the aphid Amphorophora idaei and dw (dwarfing habit) in red raspberry (Rubus idaeus L.) using AFLP and microsatellite markers

Background  

Raspberry breeding programmes worldwide aim to produce improved cultivars to satisfy market demands and within these programmes there are many targets, including increased fruit quality, yield and season, and improved pest and disease resistance and plant habit. The large raspberry aphid, Amphorophora idaei, transmits four viruses and vector resistance is an objective in raspberry breeding. The development of molecular tools that discriminate between aphid resistance genes from different sources will allow the pyramiding of such genes and the development of raspberry varieties with superior pest resistance. We have raised a red raspberry (Rubus idaeus) F₁ progeny from the cross 'Malling Jewel' × 'Malling Orion' (MJ × MO), which segregates for resistance to biotype 1 of the aphid Amphorophora idaei and for a second phenotypic trait, dwarf habit. These traits are controlled by single genes, denoted (A ₁) and (dw) respectively.     

The effect of resistance to Amphorophora rubi in raspberry (Rubus idaeus) on the spread of aphid-borne viruses

The effectiveness of resistance to the aphid Amphorophora rubi in restricting the spread of aphid-borne viruses was assessed in a field experiment using six genotypes of red raspberry. In one block of the experiment, the genotypes alternated with rows of virus-infected Mailing Jewel raspberry, and in the other they alternated with virus-free Mailing Jewel. During 4 years, the numbers of A. rubi and the amount of 52V virus spread in the two blocks were similar, suggesting that this virus was mostly introduced from outside the plots. Lloyd George and Mailing Jewel raspberry became heavily infested with A. rubi and were rapidly infected with raspberry leaf mottle, raspberry leaf spot and 52V viruses. Glen Clova and Norfolk Giant raspberry, which contain minor genes for resistance to A. rubi, were infested with fewer A. rubi and virus spread more slowly in these cultivars. A. rubi were rare on Mailing Orion and an East Mailing raspberry selection (888/49) which have genes A₁ and A₁₀ respectively for resistance to A.rubi, and these plants remained largely free of virus. The role of minor and major gene resistance to A. rubi in restricting virus spread is discussed. A few Macrosiphum euphorbiae and Myzus ornatus were recorded on several of the raspberry genotypes.     

Further studies on the effect of resistance to Amphorophora idaei in raspberry (Rubus idaeus) on the spread of aphid-borne viruses

The rate of spread of viruses transmitted by the aphid Amphorophora idaei into genotypes of raspberry differing in resistance to infestation by A. idaei was studied in a field experiment which exposed plants to large numbers of infective aphids. Under these conditions, genotypes that are readily colonised by A. idaei were totally infected with virus after two to three growing seasons, whereas genotypes with a high degree of resistance were substantially free of virus after four growing seasons but 56% of plants were infected after seven seasons. Genotypes with intermediate resistance were also substantially free of virus after three seasons but 76% of plants were virus infected after seven seasons. The effectiveness of resistance to A. idaei in raspberry in restricting spread of viruses transmitted by this aphid is discussed.     

Development of two spotted spider mite (Acari: Tetranychidae) populations on strawberry and raspberry cultivars

Five strawberry (Fragaria sp.) and five raspberry (Rubus ideaus L.) cultivars were evaluated for resistance to two spotted spider mite (Tetranychus urticae Koch.). Two methods of assessing the development of two spotted mite populations using detached leaves were compared. The number of eggs laid and mites which developed were compared. The strawberry cvs Hapil and Pegasus had significantly greater development of two spotted mite populations than the cvs Rhapsody, Symphony and Elsanta. The raspberry cv. Joan Squires had higher populations of two spotted mite whilst the raspberry cv. Leo the least, when compared with cvs Glen Clova, Glen Moy and Glen Prosen. Differences were observed in oviposition sites and mite distribution when comparing raspberries with strawberries. The method of assessing the populations development of two spotted mite which involved maintaining the cut leaf stem in water may be of potential use for studying population dynamics of both two spotted mite and possible predators over extended periods of time.     

Natural infection with raspberry bushy dwarf virus (RBDV) of the putatively RBDV-resistant red raspberry cultivar Glen Moy, and the demonstration that it does not contain the RBDV resistance gene, Bu

When released to commerce in 1981, the red raspberry cv. Glen Moy was reported to be immune to the Scottish type isolate of raspberry bushy dwarf virus (RBDV-D200). Field observations of this cultivar in localities where RBDV was prevalent tended to support this claim of its resistance, but in the past 6–10 yr, RBDV infection has been reported in this cultivar in Australasia, USA and in several commercial crops in England. Therefore, experiments were made to investigate the reason(s) for this apparent anomaly using RBDV-infected material, putatively of cv. Glen Moy, from two locations in southern England and one each from Australia, New Zealand (NZ) and the USA. Genetic fingerprinting of genomic DNA from samples of these five RBDV-infected raspberry sources confirmed their identity as cv. Glen Moy. Comparisons of some serological and genomic properties of the five Glen Moy RBDV isolates indicated that, whilst they shared many properties with previously well characterised isolates of this virus, they were distinguishable from them. Characterisation of the isolate from NZ maintained in raspberry showed that it did not have a Rubus host range characteristic of resistance-breaking (RB) isolates, indicating that for this location, and probably also for those of Australia and the USA, RB isolates were not the cause of infection in cv. Glen Moy. When virus-tested plants of cv. Glen Moy and 45 progeny seedlings from the cross between cv. Glen Moy and the RBDV-susceptible cv. Autumn Bliss were graft inoculated with RBDV-D200, all grafted plants became infected indicating that cv. Glen Moy does not contain the RBDV resistance gene, Bu. Possible reasons for the previously reported resistance of cv. Glen Moy to RBDV are discussed.     

Inheritance of resistance to yellow rust (Phragmidium rubi-idaei) in red raspberry

The inheritance of resistance in red raspberry (Rubus idaeus) to yellow rust (Phragmidium rubi-idaei) was studied in a diallel cross among the cultivars Boyne, Meeker, Mailing Jewel, Glen Prosen and Glen Clova. The progenies and clonally propagated parents were exposed initially in a rust-infected plantation where the incidence and severity of infection was assessed at the telial stage. The following spring leaves on new canes of the same plants were inoculated with urediniospores in a glasshouse at 18 ± 2 °C. The latent period and number of uredinia per cm² of abaxial leaf surface were determined 13 and 18 days after inoculation. Cv. Boyne developed only chlorotic flecking on inoculated leaves in the glasshouse and no sporulation was observed. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed incomplete resistance of the slow rusting type, and segregants of Boyne selfed that were susceptible also showed low levels of rust, which suggests that Boyne carries both major- and minor-gene resistance. Diallel analyses of both the field and glasshouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant non-additive effects also occurred: cv. Meeker was the most resistant parent and contributed the most resistance to the progenies. The Meeker progenies also segregated for gene H, which determines cane hairiness and is known to be associated with resistance to other diseases. Segregants without hairs (genotypes hh) were found to be the more resistant to rust.     

Protected raspberry production alters aphid–plant interactions but not aphid population size

• 1 Aphid population dynamics in crops are often driven by interactions with their host plants, which can be extensively influenced by environmental change. Protective environments (i.e. plastic tunnels) are now frequently used for soft fruit production, which may affect the localized climate and alter such interactions. This two year study on red raspberry (Rubus idaeus) addressed how protected environments affected two aphid species; the large raspberry aphid Amphorophora idaei (LRA) and the small raspberry aphid Aphis idaei (SRA).
• 2 Temperatures were higher (up to 7–10 °C) in tunnels compared with the field. Plants in tunnels grew approximately 1.4 cm/week faster and had lower (approximately 35%) foliar amino acid concentrations than plants in the field.
• 3 Aphids affected plant growth differently depending on growing environment; they promoted plant growth by 18–37% in tunnels, although they had no such effect in the field. Aphids reduced total and essential amino acid concentrations, with SRA causing greatest reductions (approximately 40% and 33%, respectively).
• 4 Aphid population sizes were similar in both environments, although individual LRA were smaller in tunnels (30% smaller in 2007) compared with those in the field. We suggest that faster aphid development rates inside warmer tunnels were not realized as a result of the variable effects of the growing environment on amino acid composition.
• 5 We conclude that the increasing use of protected environments in crop production will not necessarily cause predictable increases in aphid populations, although it may alter aphid–plant interactions in terms of aphid‐induced changes to plant growth.

     

Common resistance in red raspberry to Botrytis cinerea and Didymella applanata, two pathogens occupying the same ecological niche

Comparisons were made between Botrytis cinerea and Didymella applanata, which occupy the same ecological niche on canes of red raspberry. Isolates of B. cinerea from diverse localities within the British Isles were pathogenic to the SCRI selection M30 when internodal wounds on young canes were inoculated. A single inoculation frequently caused the buds at several nodes to fail the following spring. Differences in the lengths of the stem lesions that formed indicated differences in isolate pathogenicity, but these were not related to isolate origin. Bud suppression and lateral shoot failure also occurred when petioles of cvs Mailing Orion, Mailing Jewel and Glen Clova were wound inoculated with B. cinerea up to late August. The relative resistance of seven cultivars to three isolates of each pathogen was determined. Principal components analysis of data from five estimates of resistance to each pathogen showed that 40% of the variation described a common resistance to the two diseases. Further analysis showed that cvs Chilcotin and Meeker had the strongest common resistance and that cvs Glen Prosen and Mailing Jewel had the weakest. The remaining variation described cultivar differences in relative bud length after petiole inoculation with either pathogen, and differences in the relative importance of spring and autumn symptoms. Only 7% of the variation indicated independent resistance to the two pathogens and this was not influenced by cultivar differences.     

Epicuticular wax composition in relation to aphid infestation and resistance in red raspberry (Rubus idaeus L.)

Epicuticular waxes from the aphid-resistant red raspberry (Rubus idaeus) cultivar Autumn Bliss and the aphid-susceptible cultivar Malling Jewel were collected from the newly emerging crown leaves, and also from the group of four more mature leaves immediately below the crown. Resistance and susceptibility status of the leaves to infestation by the large raspberry aphid, Amphorophora idaei, were determined by bioassay with the insect just prior to collection of the wax. Analysis showed the waxes to consist of a complex mixture of free fatty acids; free primary alcohols and their acetates; secondary alcohols; ketones; terpenoids including squalene, phytosterols, tocopherol and amyrins; alkanes and long chain alkyl and terpenyl esters. Compositional differences which may relate to A. idaei-resistance status were noticeably higher levels of sterols, particularly cycloartenol, together with the presence of branched alkanes, and an absence of C₂₉ ketones and the symmetrical C₂₉ secondary alcohol in wax from the resistant cultivar Bliss. There were also differences between the cultivars in the distribution of individual amyrins and tocopherols and in the chain length distribution for homologues of fatty acids, primary alcohols and alkanes, and these may also be related to resistance to A. idaei. Emerging leaves had lower levels of primary alcohols and terpenes, but higher levels of long-chain alkyl esters, and in general, more compounds of shorter chain-length than the more mature leaves. During bioassay A. idaei displayed a preference to settle on the more mature leaves. This may be due to greater wax coverage and higher levels of the compounds of shorter chain length found in the newly emerged younger leaves at the crown of the plant.     

Identification of Rubus yellow net virus as a distinct badnavirus and its detection by PCR in Rubus species and in aphids

Rubus yellow net virus (RYNV) infects Rubus species and cultivars worldwide and is an essential component of raspberry veinbanding mosaic (RVBMD), a virus disease complex that causes serious decline in plant vigour and productivity. The virus is transmitted, probably in a semi‐persistent manner, by the large raspberry aphid, Amphorophora idaei in Europe, and A. agathonica in North America. The particles of RYNV are bacilliform in shape and measure 80–150 × 25–30 nm, similar to those of badnaviruses. A1.7 kb fragment of the viral DNA was amplified by PCR and then directly sequenced. Analysis of this sequence suggests that RYNV is possibly a distinct species in the genus Badnavirus and is most closely related to Gooseberry vein banding associated virus (GVBAV) and Spiraea yellow leaf spot virus, two other badnaviruses described recently. Using the sequence derived from the PCR‐amplified viral DNA fragment, RYNV‐specific primers were designed and used in PCR to assay for RYNV in a range of Rubus germplasm infected with RYNV, with other unrelated viruses and virus‐like diseases found in Rubus, and in healthy plants. RYNV was detected in all glasshouse cultures of RYNV‐infected plants, whether alone or in complex infections with other viruses, but not from healthy Rubus plants, nor from plants infected with other viruses. It was also detected in field‐grown raspberry plants with and without symptoms of RVBMD and in raspberry plants infected with RYNV by viruliferous A. idaei. RYNV was also detected by PCR in A. idaei following access feeds on RYNV‐infected plants of 1 h or more. PCR failed to amplify DNA from gooseberry infected with GVBAV confirming the specificity of the RYNV analysis. PCR detection of RYNV in dormant raspberry buds allows assays to be made outside the natural growing season, providing a useful application for plant introduction and quarantine programmes.     

Top‐down control by Harmonia axyridis mitigates the impact of elevated atmospheric CO2 on a plant–aphid interaction

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Propagation of black raspberry necrosis virus (BRNV) in mixed culture with solanum nodiflorum mottle virus, and the production and use of BRNV antiserum

The concentration of particles of black raspberry necrosis virus (BRNV), which is normally extremely low in herbaceous plants, increased about 1000-fold when Nicotiana clevelandii plants were inoculated with a mixture of BRNV and an unrelated virus, solanum nodiflorum mottle (SNMV). In sap from N. clevelandii infected with the mixed culture, BRNV infectivity survived dilution to 10?⁴ but not 10?⁵, and storage for 6 but not 8 days at 20 ^oC, for 6 but usually not 10 days at 4 ^oC and for more than 13 days at – 15 ^oC. When plants were inoculated with the mixed culture, BRNV induced typical symptoms in several Chenopodium species and infected several previously unreported hosts. Purified preparations of particles of the mixed culture contained only a small proportion of BRNV particles, which sedimented in sucrose density gradients as two components, one, probably non-infective, of c. 505, and the other, infective, of 120-130S. An antiserum prepared to purified particles of the mixed culture was cross-absorbed with SNMV particles and used in indirect ELISA to detect BRNV in herbaceous plants infected with the mixed culture, and also in a wide range of Rubus species, cultivars and hybrids infected naturally, by grafting or by inoculation with the aphid Amphorophora idaei. The reliability of ELISA for detecting BRNV in raspberry leaves depended on the cultivar and time of year. Some cultivars, such as Glen Clova, had low concentrations of BRNV, which was detected reliably only in late spring/early summer, whereas other cultivars, such as Lloyd George and Mailing Enterprise, had greater BRNV concentrations. In small-scale surveys in eastern Scotland, BRNV was detected by ELISA in many raspberry cvs, including some that contain major gene resistance to the vector, A. idaei; in five of nine raspberry stocks entered for the Standard grade certificate but in none of five stocks entered for the Stock Cane certificate; and in 40% of wild raspberry and 14% of wild bramble plants growing near commercial raspberry crops. The significance of these findings for the control of BRNV is discussed.     

Oviposition and feeding behaviour by the vine weevil Otiorhynchus sulcatus on red raspberry: effects of cultivars and plant nutritional status

• 1 The vine weevil Otiorhynchus sulcatus is a major pest of horticultural crops worldwide, with root‐feeding larvae causing most damage. Adult oviposition aboveground may therefore influence levels of damage as the larvae are relatively immobile after oviposition.
• 2 The present study investigated feeding and oviposition behaviour on red raspberry Rubus idaeus using intact plants, ensuring that choices reflected the realistic differences in cultivar appearance and chemical composition. Previous studies investigating vine weevil feeding and oviposition on other crops have used excised plant material, which may inadvertently influence behaviour.
• 3 Adult weevils significantly preferred to feed on particular cultivars in the choice experiment (e.g. Tulameen), although they consumed significantly more foliage (0.22–1.03 cm²/day) on different raspberry cultivars (e.g. Glen Moy, Glen Rosa and a wild accession) in no‐choice situations.
• 4 In choice experiments, weevils tended to avoid laying eggs on some cultivars (e.g. Glen Moy and the wild accession). The number of eggs laid (1.91–4.32 eggs per day) did not, however, differ significantly between the cultivars in a no‐choice situation. Foliar nitrogen and magnesium concentrations were positively, although weakly, correlated with the total number of eggs laid.
• 5 The present study highlights the importance of considering both choice and no‐choice tests when assessing crop susceptibility to attack because weevils may avoid feeding on certain cultivars (e.g. Glen Moy) when given a choice, although this would cause significant damage to such cultivars if they were grown in monoculture (i.e. when there is no alternative).

     

Ripening-related changes in raspberry cell wall composition and structure

Cell walls were prepared from the fruit of two cultivars of raspberry at three stages of ripening; green, white and red (ripe). The cultivars. Glen Clova and Glen Prosen, are subjectively classified, at harvest by growers, as soft and firm fruit, respectively. The cell walls were analysed for neutral sugar composition, uronic acid content, degree of methyl esterification, lignin and ferulic acid-derived dehydrodimers. Solid-state 31C NMR and diffuse reflectance infrared (DRIFT) spectra were acquired for the cell wall residues. For both cultivars the progression from green to white produced minimal changes, save for a reduction in pectin. NMR analyses indicated that the solubilized pectin was acetylated. Progression to the red (ripe) stage, in both cultivars, was accompanied by a reduction in the ordered cellulose and a dramatic reduction in pectin content and the degree of methyl-esterification. Significantly, the softer fruit (Glen Clova) exhibited greater reductions in both parameters, implicating increased pectin hydrolysis, as one of the main factors contributing to the difference in firmness between the cultivars. A relative increase in cell wall-associated protein was seen at the red stage. The nature and function of the protein(s) are, as yet unknown.     

Combining plant resistance and a natural enemy to control <Emphasis Type="Italic">Amphorophora idaei</Emphasis>

The European large raspberry aphid Amphorophora idaei Börner (Homoptera: Aphididae) is a virus vector of at least four plant virus complexes making it the most important aphid pest of raspberries in Northern Europe. An approach combining a bottom-up control (plant resistance) and a top-down control (an aphid parasitoid) using Aphidius ervi Haliday (Hymenoptera: Aphidiinae) was investigated in the laboratory. Aphid performance (pre-reproductive period, total reproductive output, lifespan and r _m) were compared when reared on both a susceptible cultivar and a resistant cultivar with significantly poorer performance on the resistant cultivar. Parasitoid attack behaviour increased with aphid density on both cultivars, but was significantly lower on resistant plants than susceptible plants. Aphids showed a greater tendency to drop from the plant when feeding on resistant plants compared with susceptible plants. The significance of the results is discussed in the context of possible control of the aphid using these combined methods.