Nasonovia Ribisnigri Research Articles

Susceptibility of Larvae and Pupae of the Aphid ParasitoidAphelinus abdominalis(Hymenoptera: Aphelinidae) to the Entomopathogenic FungusBeauveria bassiana

The lettuce aphid, Nasonovia ribisnigri (Mosley), is an economically important pest of lettuce worldwide. Recently, the entomopathogenic fungus, Beauveria bassiana strain GHA, and the aphelinid parasitoid, Aphelinus abdominalis Dalman, have been reported to be potential biological control candidates for use against N. ribisnigri . However, no information is available on the interaction between B. bassiana and A. abdominalis when both are applied. This study therefore examined the compatibility of B. bassiana and A. abdominalis in laboratory experiments. Specifically, we assessed the susceptibility of two A. abdominalis developmental stages (larvae and pupae) to two spore concentrations of B. bassiana (high: 1 × 109 and low: 1 × 104 conidia/ml) and a control of 0.01% Tween 80. We found parasitoid larvae to be highly susceptible to infection at the high spore concentration of B. bassiana , as measured by rates of mummy formation (mean ± SE: 14% ± 2.23) and adult emergence (mean ± SE: 10% ± 5.56) compared with the control treatment (mummification: mean ± SE: 79% ± 3.22; adult emergence: mean ± SE: 87% ± 4.40). In contrast, B. bassiana had no effects on parasitoid development when parasitoid larvae were treated with the lower spore concentration or parasitoid pupae were treated with either high or low spore concentrations. This study suggests that it might be possible to combine B. bassiana and A. abdominalis for integrated pest management of N. ribisnigri . As such, the application of B. bassiana should be timed to coincide with the presence of advanced developmental stages of A. abdominalis to protect the parasitoid. Another option would be to delay the release of A. abdominalis after B. bassiana application, when A. abdominalis is no longer susceptible to fungal infection.

Crystal Structures and Binding Dynamics of Odorant-Binding Protein 3 from two aphid species Megoura viciae and Nasonovia ribisnigri.

Aphids use chemical cues to locate hosts and find mates. The vetch aphid Megoura viciae feeds exclusively on the Fabaceae, whereas the currant-lettuce aphid Nasonovia ribisnigri alternates hosts between the Grossulariaceae and Asteraceae. Both species use alarm pheromones to warn of dangers. For N. ribisnigri this pheromone is a single component (E)-β-farnesene but M. viciae uses a mixture of (E)-β-farnesene, (−)-α-pinene, β-pinene, and limonene. Odorant-binding proteins (OBP) are believed to capture and transport such semiochemicals to their receptors. Here, we report the first aphid OBP crystal structures and examine their molecular interactions with the alarm pheromone components. Our study reveals some unique structural features: 1) the lack of an internal ligand binding site; 2) a striking groove in the surface of the proteins as a putative binding site; 3) the N-terminus rather than the C-terminus occupies the site closing off the conventional OBP pocket. The results from fluorescent binding assays, molecular docking and dynamics demonstrate that OBP3 from M. viciae can bind to all four alarm pheromone components and the differential ligand binding between these very similar OBP3s from the two aphid species is determined mainly by the direct π-π interactions between ligands and the aromatic residues of OBP3s in the binding pocket.

Feeding behavior and performance of Nasonovia ribisnigri on grafts, detached leaves, and leaf disks of resistant and susceptible lettuce

AbstractAphids are dependent on the phloem sap of plants as their only source of nutrients. Host‐plant resistance in lettuce, Lactuca sativa L. (Asteraceae), mediated by the Nr gene is used to control the lettuce aphid Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). The resistance is located in the phloem; however, the exact mechanism of resistance is unknown. In this study, we investigated whether the resistance factor (or factors) is synthesized in the root or in the shoot. The feeding behavior and performance of avirulent N. ribisnigri were studied on grafts of resistant and susceptible lettuce. In addition, the persistence of resistance in excised lettuce tissue was measured, by studying the feeding behavior and performance of N. ribisnigri on detached leaves and leaf disks of resistant lettuce. It appears that the resistance factor encoded by the Nr gene is produced in the shoots: aphid feeding was reduced on resistant shoots grafted on susceptible roots, whereas aphids were able to feed on grafts of susceptible shoots on resistant roots. Partial loss of resistance was observed after detachment of leaves and excision of leaf disks from resistant plants. Aphids fed longer on excised resistant plant tissue compared with intact resistant plants; however, compared with excised plant tissue of the susceptible cultivar, the time spent on feeding was shorter, indicating resistance was not completely lost. Our findings caution against the use of excised leaf material for aphid resistance bioassays.

Molecular tracking of arthropod predator–prey interactions in Mediterranean lettuce crops

The feeding habits of the generalist arthropod predators in agroecosystems are often difficult to determine, as they are small, mobile and live among the vegetation or in the soil. DNA-based gut-content analysis is a powerful tool that enables the study of arthropod predator–prey interactions. Predation on two of the main pests of Mediterranean lettuce crops, the lettuce aphid, Nasonovia ribisnigri, and the western flower thrips, Frankliniella occidentalis, as well as on Collembola, the most abundant non-pest prey, was studied. Generalist arthropods, like hoverflies, anthocorids (Orius spp.), coccinellids and spiders were collected in lettuce plots in two seasons (spring and summer) and analysed by conventional PCR using N. ribisnigri, F. occidentalis and Collembola-specific primers. Our results showed that in spring the main pest was N. ribisnigri, which was consumed by hoverfly larvae and coccinellids. In summer, the main pest was F. occidentalis, which was mainly predated by Orius spp. followed by hoverfly larvae. Spiders, which fed mainly on Collembola, did not seem to contribute to control of either target pest. This study offers a deeper knowledge of the trophic relationships present in Mediterranean lettuce crops, laying the groundwork for implementing biological control programmes based on the conservation of natural enemies.

Paecilomyces niveus Stolk & Samson, 1971 (Ascomycota: Thermoascaceae) as a pathogen of Nasonovia ribisnigri (Mosley, 1841) (Hemiptera, Aphididae) in Brazil.

Nasonovia ribisnigri is a key pest of lettuce (Lactuca sativa L.) in Brazil that requires alternative control methods to synthetic pesticides. We report, for the first time, the occurrence of Paecilomyces niveus as an entomopathogen of the aphid Nasonovia ribisnigri in Pinhais, Paraná, Brazil. Samples of mummified aphids were collected from lettuce crops. The fungus P. niveus (PaePR) was isolated from the insect bodies and identified by macro and micromorphology. The species was confirmed by sequencing Internal Transcribed Spacer (ITS) rDNA. We obtained a sequence of 528 bp (accession number HQ441751), which aligned with Byssochlamys nivea strains (100% identities). In a bioassay, 120 h after inoculation of N. ribisnigri with pathogenic P. niveus had an average mortality of 74%. The presence of P. niveus as a natural pathogen of N. ribisnigri in Brazil suggests that it may be possible to employ P. niveus to minimize the use of chemical insecticides.

Patch Experience Changes Host Acceptance of the Aphid Parasitoid Aphidius ervi

We investigated patch quality assessment by the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae) when exploiting colonies of the lettuce aphid Nasonovia ribisnigri Mosley (Hemiptera: Aphididae). Three host patches of different qualities were sequentially offered to a naive female parasitoid. High quality patches (HQ) consisted of 20 N. ribisnigri reared on susceptible lettuce; low quality patches (LQ) consisted of 20 N. ribisnigri reared on partially resistant lettuce and mixed quality patches (MQ) consisted of equal numbers (10) of aphids reared on both lettuce types. Parasitized aphids were reared until parasitoid emergence; number of mummies and sex ratio were noted. On the first host patch encountered, the mean number of A. ervi mummies produced was significantly higher for HQ host patches (X ± SD; 7.1 ± 5.0) than for LQ host patches (3.8 ± 4.5). This suggests that female A. ervi do not need prior experience to assess patch quality; females probably using an innate estimate of patch quality when encountering a first host patch. This initial patch quality assessment can be modified with experience. When females encountered a MQ patch, they kept their exploitation level constant on the following patch, whatever its quality. Females increased their level of host acceptance on the third patch when three LQ patches were offered successively; accepting low-quality hosts could be preferable when better hosts are not currently available in the habitat. These results suggest that A. ervi females behave in a manner consistent with a Bayesian updating process when foraging for hosts.

Laboratory and semi-field evaluation of Beauveria bassiana (Ascomycota: Hypocreales) against the lettuce aphid, Nasonovia ribisnigri (Hemiptera: Aphididae)

The lettuce aphid, Nasonovia ribisnigri (Mosley), is an economically important pest of lettuce worldwide. The entomopathogenic fungus Beauveria bassiana strain GHA has recently been reported as a potential biocontrol candidate for use against the lettuce aphid. This study provides information on the mortality inflicted by B. bassiana when applied against different life stages of the lettuce aphid under laboratory conditions and how fungus infection affects the aphid fecundity. In addition, temporal changes in persistence of fungus inoculum applied to foliage of young lettuce plants under semi-field conditions was analysed. Immature life stages were generally the least susceptible to fungal infection and the susceptibility of all stages was dose-dependent, with the highest mortality occurring at the highest dose. B. bassiana significantly affected the rate of nymph production by the lettuce aphid, with the highest effect seen when the alatoid fourth instar of N. ribisnigri was inoculated with B. bassiana. The persistence of B. bassiana conidia on lettuce foliage was not influenced by leaf position. Within 5days, the cumulative percentage decline in the conidial population was 38% which declined further to 92% and 99% on day 11 and 20 post-spraying, respectively. In accordance, the infectivity to second instar lettuce aphid nymphs of B. bassiana conidia deposited on leaves declined according to an exponential decay model predicting an intercept of 0.59±0.03 (S.E), a reduction in aphid mortality at a rate of 11% with each increasing day after fungal application and a fungus half-life of 6.34±0.69days.

Understanding trophic interactions of Orius spp. (Hemiptera: Anthocoridae) in lettuce crops by molecular methods.

The aphid Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae) and the thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) are common pests in Mediterranean lettuce crops, where Orius spp. are common generalist predators. Predation by Orius spp. was studied in a lettuce plot by conventional polymerase chain reaction (PCR) and real-time PCR analyses using specific primers of both main pests. Also, high-throughput sequencing was used to have a wider approach of the diet of these predators in natural field conditions. Molecular analyses indicated a higher predation on N. ribisnigri in spring and on F. occidentalis in summer. Predation on alternative prey, like Collembola, was also found in both seasons. Real-time PCR was more sensitive than conventional PCR in showing the target trophic links, whereas high-throughput sequencing revealed predation on other natural enemies - intraguild predation (IGP), showing other trophic interactions of Orius majusculus within the studied ecosystem. This study gives important information about the trophic relationships present in Mediterranean lettuce crops in different periods of the year. The detected predation by Orius spp. on alternative prey, as well as on other natural enemies, should be further investigated to clarify whether it adds or detracts to the biological control of N. ribisnigri and F. occidentalis.

Preference and life history traits of Aphelinus abdominalis (Hymenoptera: Aphelinidae) when offered different development stages of the lettuce aphid Nasonovia ribisnigri (Hemiptera: Aphididae)

The degree of parasitisation, host feeding, developmental time, adult emergence and female sex ratio of the parasitoid Aphelinus abdominalis were evaluated when different host stages of N. ribisnigri (1st, 2nd, 3rd, alatoid 4th instar or newly moulted apterous adult) were offered as hosts under no-choice conditions in the laboratory at 22 °C, 70 % RH and 16:8 L:D. In addition, the parasitoid preference between 2nd and alatoid 4th instar was examined. In the no-choice experiments, the highest degree of successful parasitisation (leading to mummy formation) (41–56 %) was observed when young and intermediate stages of the lettuce aphid were exposed for parasitism compared with older developmental stages. This pattern was supported in the choice experiment where significantly more 2nd instar lettuce aphids were parasitised than alatoid 4th instars, with Manly’s preference index (mean ± SE) for the former stage being 0.79 ± 0.03. Incomplete parasitisation was recorded highest in 1st instar (mean ± SE: 29.95 % ± 4.71), with much lower values (1–8 %) attained in the 2nd, 3rd, alatoid 4th instar and apterous adult with no significant differences. Host feeding was low (2–4 %) and only recorded in 1st and 2nd instars. The mean development time of A. abdominalis from egg to adult emergence was longest for 1st instar (19.03 ± 0.12 days) and the shortest for apterous adults (16.59 ± 0.29 days). A high percentage of adult emergence (>80 %) from mummified aphids as well as a strongly female-biased sex ratio (>78 %) were found across all host stages of the lettuce aphid.

High-throughput phenotyping of plant resistance to aphids by automated video tracking.

BackgroundPiercing-sucking insects are major vectors of plant viruses causing significant yield losses in crops. Functional genomics of plant resistance to these insects would greatly benefit from the availability of high-throughput, quantitative phenotyping methods.ResultsWe have developed an automated video tracking platform that quantifies aphid feeding behaviour on leaf discs to assess the level of plant resistance. Through the analysis of aphid movement, the start and duration of plant penetrations by aphids were estimated. As a case study, video tracking confirmed the near-complete resistance of lettuce cultivar ‘Corbana’ against Nasonovia ribisnigri (Mosely), biotype Nr:0, and revealed quantitative resistance in Arabidopsis accession Co-2 against Myzus persicae (Sulzer). The video tracking platform was benchmarked against Electrical Penetration Graph (EPG) recordings and aphid population development assays. The use of leaf discs instead of intact plants reduced the intensity of the resistance effect in video tracking, but sufficiently replicated experiments resulted in similar conclusions as EPG recordings and aphid population assays. One video tracking platform could screen 100 samples in parallel.ConclusionsAutomated video tracking can be used to screen large plant populations for resistance to aphids and other piercing-sucking insects.Electronic supplementary materialThe online version of this article (doi:10.1186/s13007-015-0044-z) contains supplementary material, which is available to authorized users.

Molecular assessment of predation by hoverflies (Diptera: Syrphidae) in Mediterranean lettuce crops.

Hoverflies (Diptera: Syrphidae) are generalist predators of a great variety of pests. Nasonovia ribisnigri (Hemiptera: Aphididae) and Frankliniella occidentalis (Thysanoptera: Thripidae) are two common pests in Mediterranean lettuce crops, where they occur alongside alternative prey (e.g., Collembola). A semi-field experiment was conducted in an experimental lettuce plot where hoverfly predation on N. ribisnigri, F. occidentalis and Collembola was studied by conventional PCR and qPCR using specific primers, as well as by next-generation sequencing (NGS) in order to reveal other potential trophic interactions. Trophic linkages between hoverflies and N. ribisnigri were the strongest both in spring and summer. F. occidentalis and Collembolans were also detected in both seasons, but with less frequency. qPCR detected a higher frequency of consumption than conventional PCR when both tests were run at optimal conditions. NGS analyses showed intraguild predation on other hoverfly species, as well as on anthocorids, spiders and even aphid parasitoids. Conventional PCR and qPCR provided important insights into Mediterranean hoverfly species predation on target pest and non-pest prey. NGS gave a complementary approach revealing a broader diet of these predators within the studied ecosystem.

Partial aphid resistance in lettuce negatively affects parasitoids.

This study investigated the effects of partial plant resistance on the lettuce aphid Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae), a major pest of cultivated lettuce (Lactuca sativa L.), and one of its parasitoids, Aphidius ervi Haliday (Hymenoptera: Braconidae). Aphids were reared on susceptible (L. sativa variety Estival; S) or partially resistant (Lactuca serriola L. PI 491093; PR) lettuce, and next parasitized by A. ervi females. Fitness proxies were measured for both aphids and parasitoids. Developmental time to adult stage took longer for alate and apterous aphids (an average of 3.5 and 1.5 additional days, respectively) on PR than on S lettuce, and fecundity of alate aphids reared on PR lettuce was reduced by 37.8% relative to those reared on S lettuce. Size (tibia length) and weight of aphids reared on PR lettuce were lower than for aphids reared on S lettuce from the third and second instar onward, respectively. Parasitism of aphids reared on PR plants resulted in lower parasitoid offspring emergence (-49.9%), lower adult female (-30.3%) and male (-27.5%) weight, smaller adult female (-17.5%) and male (-11.9%) size, and lower female fecundity (37.8% fewer eggs) than when parasitoids developed from aphids reared on S plants. Our results demonstrate that partial aphid resistance in lettuce negatively affects both the second and third trophic levels. Host plant resistance in cultivated lettuce may therefore create an ecological sink for aphid parasitoids.

Rearing history affects behaviour and performance of two virulent Nasonovia ribisnigri populations on two lettuce cultivars

AbstractMany aphid species have become virulent to host‐plant resistance, which limits the sustainability of insect resistance breeding. However, when this adaptation to resistant plants is associated with fitness costs for the aphids, virulence can be lost in the absence of resistant plants. For two populations of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae), we evaluated whether virulence to Nr‐gene‐based resistance was lost on a susceptible lettuce, Lactuca sativa L. (Asteraceae), and assessed possible costs of virulence. The feeding behaviour and performance of these aphids, reared and tested on susceptible and resistant lettuce, were investigated. The rearing plant affected feeding behaviour and performance of the aphids. Temporary reduction and long‐term loss of virulence were found. The total duration of phloem intake was shorter after being reared on susceptible lettuce and tested on resistant lettuce. In addition, one population had a lower survival on resistant lettuce after being reared on susceptible lettuce. There were also indications of fitness costs of the virulence in both populations.

Nitric Oxide as a Potent Fumigant for Postharvest Pest Control

There is a great demand for safe and effective alternative fumigants to replace methyl bromide and other toxic fumigants for postharvest pest control. Nitric oxide, a common signal molecule in biological systems, was found to be effective and safe to control insects under ultralow oxygen conditions. Four insect species including western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera Thripidae); aphid, Nasonovia ribisnigri (Mosley) (Homoptera: Aphididae); confused flour beetle, Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae); and rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), at various life stages were fumigated with 0.1-2.0% nitric oxide under ultralow oxygen levels of < or = 50 ppm in 1.9-liter glass jars at 2-25 degrees C depending on insect species. Fumigations were effective against all four insect species. Efficacy of nitric oxide fumigation increased with nitric oxide concentration, treatment time, and temperature. There were also considerable variations among insect species as well as life stages in susceptibility to nitric oxide fumigation. Complete control of thrips was achieved in 2 and 8 h with 2.0 and 0.2% nitric oxide, respectively, at 2 degrees C. At the same temperature, complete control of the aphid was achieved in 3, 9, and 12 h with 1.0, 0.5, and 0.2% nitric oxide, respectively. Larvae, pupae, and adults of confused flour beetle were effectively controlled in 24 h with 0.5% nitric oxide at 20 degrees C. Complete mortality of confused flour beetle eggs was achieved in 24 h with 2.0% nitric oxide at 10 degrees C. Rice weevil adults and eggs were effectively controlled with 1.0% nitric oxide in 24 and 48 h, respectively, at 25 degrees C. These results indicate that nitric oxide has potential as a fumigant for postharvest pest control.

Estimation of vector propensity for Lettuce mosaic virus based on viral detection in single aphids

Lettuce mosaic virus (LMV) is transmitted by aphids nonpersistently causing severe disease outbreaks in commercial lettuce crops. New strategies to control plant viruses have arisen based on molecular techniques, which analyze plantvirus- vector interactions. In this work, two PCR-based methods with a previous immunocapture phase, have been developed to detect LMV in single aphids. Detection rates using a RT-nested-PCR method in single aphids and transmission efficiency of Myzus persicae (vector species) and Nasonovia ribisnigri (nonvector species) were compared. Although the percentage of viruliferous aphids for N. ribisnigri (45.8 ± 2.3) was higher than for M. persicae (39.2 ± 3.5) after the same acquisition access period, N. ribisnigri was unable to transmit the virus while M. persicae proved to be an efficient vector (with a transmission rate per single aphid of 10.4 ± 0.8). A method was proposed to estimate vector propensity for nonpersistent viruses based on the relationship between the percentage of viruliferous aphids and their transmission ability. This methodology could be applied to decision-making and implementing control strategies to prevent virus spreading

The green lacewing, Chrysoperla carnea: preference between lettuce aphids, Nasonovia ribisnigri, and Western flower thrips, Frankliniella occidentalis.

This study investigated the prey preference of 3rd instar green lacewing, Chrysoperla carnea Stephens (Neuroptera: Chrysopidae), between western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), and lettuce aphids, Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae) in laboratory experiments at 25 ± 1° C and 70 ± 5% RH with five prey ratios (10 aphids:80 thrips, 25 aphids:65 thrips, 45 aphids:45 thrips, 65 aphids:25 thrips, and 80 aphids:10 thrips). Third instar C. carnea larvae readily preyed upon both thrips and aphids, with thrips mortality varying between 40 and 90%, and aphid mortality between 52 and 98%. Chrysoperla carnea had a significant preference for N. ribisnigri at two ratios (10 aphids:80 thrips, 65 aphids:25 thrips), but no preference for either prey at the other ratios. There was no significant linear relationship between preference index and prey ratio, but a significant intercept of the linear regression indicated an overall preference of C. carnea for aphids with a value of 0.651 ± 0.054. The possible implications of these findings for control of N. ribisnigri and F. occidentalis by C. carnea are discussed.

Feeding behaviour and performance of different populations of the black currant‐lettuce aphid, Nasonovia ribisnigri, on resistant and susceptible lettuce

AbstractWhen crops are bred for resistance to herbivores, these herbivores are under strong selection pressure to overcome this resistance, which may result in the emergence of virulent biotypes. This is a growing problem for crop species attacked by aphids. The Nr‐gene in lettuce confers near‐complete resistance against the black currant‐lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). Since 2007, populations of N. ribisnigri have been reported in several locations in Europe to infest resistant lettuce varieties that possess the Nr‐gene. The objective of this study was to analyse the behaviour and level of virulence of several N. ribisnigri populations observed to have colonized Nr‐locus‐containing lettuce lines. We analysed the stylet penetration and feeding behaviour, and the performance of these N. ribisnigri populations on resistant and susceptible lettuce lines. Large variation in the degree of virulence to the Nr‐locus‐containing lettuce lines was found among populations of the Nr:1 biotype. The German population was highly virulent on the Nr‐containing resistant lettuce lines, and showed similar feeding behaviour and performance on both the susceptible and resistant lettuces. The French population from Paris was the second most virulent, though reproduction on the resistant lines was reduced. The French population from Perpignan and a population from Belgium, however, showed reduced performance and feeding rate on the resistant compared to the susceptible lettuces. The lettuce background in which the Nr‐gene is expressed influences the level of resistance to the various Nr:1 aphid populations, because the performance and feeding behaviour differed between the aphids on the cultivars (romaine lettuce) compared to the near‐isogenic lines (butterhead/iceberg lettuce). This study also shows that being able to feed on a plant not automatically implies that a population can successfully develop on that plant, because aphids showed phloem ingestion during the 8‐h recording period on resistant lettuce, but were not able to survive and reproduce on the same lettuce line.

Resistance to a new biotype of the lettuce aphid Nasonovia ribisnigri in Lactuca virosa accession IVT280

Host plant resistance is an effective protection strategy to control aphids in many crops. However, the evolution of insensitive aphid biotypes necessitates the search for new resistance sources. Wild relatives of crop plants can be important sources for resistance genes to be introgressed into new cultivars. Nasonovia ribisnigri (Mosely) (Homoptera: Aphididae) is an important pest of cultivated lettuce, Lactuca sativa. Since 1982, resistance introduced into lettuce cultivars has relied on the Nr-gene, originating from a wild relative, Lactuca virosa. In 2007 first reports appeared that Nr-based resistance had become ineffective against certain populations of N. ribisnigri. The objective of this study was to establish if the original donor of the Nr-gene L. virosa accession, IVT 280, is resistant against recently emerged virulent (Nr:1) N. ribisnigri biotypes. To this end we investigated feeding and penetration behaviour of virulent and avirulent (Nr:0) aphids on the resistant L. virosa, IVT 280, and two susceptible L. virosa accessions, using the electrical penetration graph method. Additionally, aphid performance was analysed in terms of survival, development time and reproduction on these accessions. L. virosa accession IVT 280 was resistant against all populations of N. ribisnigri tested. The ingestion of phloem was strongly reduced on the resistant accession compared to the susceptible L. virosa accessions. Additionally, none of the aphids survived on the resistant accession that, therefore, constitutes a good source of resistance in lettuce against both biotypes of N. ribisnigri.

Agronomic aspects of strip intercropping lettuce with alyssum for biological control of aphids

Organic lettuce growers in California typically use insectary strips of alyssum (Lobularia maritima (L.) Desv.) to attract hoverflies (Syrphidae) that provide biological control of aphids. A two year study with transplanted organic romaine lettuce in Salinas, California investigated agronomic aspects of lettuce monoculture and lettuce-alyssum strip intercropping on beds in replacement intercropping treatments where alyssum transplants replaced 2 to 8% of the lettuce transplants, and in additive intercropping treatments where alyssum transplants were added to the standard lettuce density without displacing lettuce transplants. Alyssum and lettuce dry matter (DM) were determined at lettuce maturity. Alyssum transplants produced less shoot DM in the additive than in the replacement intercropping treatments. The number of open inflorescences of alyssum increased with alyssum DM, and among treatments ranged from 2 to15 inflorescences per lettuce head. Compared with monoculture lettuce, lettuce heads on intercropped beds were slightly smaller and had lower nitrogen concentrations in the both additive treatments and in some replacement treatments. This research provides the first information on a novel additive intercropping approach to provide alyssum floral resources for biological control of lettuce aphids, and suggests that this approach may be a more land-efficient particularly for producing smaller lettuce heads for romaine hearts or for markets with less strict size requirements. Additional research is needed to determine if the increased competition between alyssum and lettuce in additive intercropping would reduce lettuce yields for wholesale markets with larger head size requirements. Practical aspects of implementing the various intercropping arrangements and alternatives are discussed.

The development of an economic threshold for Nasonovia ribisnigri (Hemiptera: Aphididae) on lettuce in central Spain.

This study reports economic thresholds for the lettuce aphid Nasonovia ribisnigri (Mosley), based exclusively on cosmetic damage, that is, presence or absence of aphids at harvest time. Field trials were conducted in La Poveda Experimental Farm, Madrid (Spain) during autumn (2004 and 2005) and spring (2005 and 2006). Plants were arranged in plots and just before the formation of lettuce hearts they were infested with different densities of N. ribisnigri. Two days later, half of each plot was treated with tau-fluvalinate (Klartan24AF) and the other half remained as an untreated control. Economic thresholds were obtained from nonlinear regressions calculated between the percentage of commercial plants at the end of the crop cycle for both, treated and untreated semiplots, and the different initial densities of N. ribisnigri per plant. Two criteria were used to consider a commercial lettuce plant: a conservative estimate (0 aphids/plant) and a lax one (< 5 aphids/plant). Thus, an economic threshold was established for each season and criterium. The economic thresholds that were obtained with the most and least conservative criteria were in spring 0.06 and 0.12 aphids per plant, and in autumn 0.07 and 0.13 aphids per plant, respectively. These results show that to avoid cosmetic damage, insecticide sprays are required when a very low aphid density is detected in lettuce seedlings soon after transplant.