Probing behavior of aposymbiotic green peach aphid (Myzus persicae) on susceptible Solanum tuberosum and resistant Solanum stoloniferum plants

The green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae) is one of the potato important pests; it is the most efficient vector of potato viruses. Myzus persicae harbors the endosymbiotic bacteria Buchnera aphidicola which supplements their diet. There is increasing evidence that B. aphidicola is involved in plant–aphid interactions and we previously demonstrated that B. aphidicola disruption (aposymbiosis) affected the probing behavior of M. persicae on radish plants, delaying host plant acceptance. In this work, we evaluated the effect of aposymbiosis on the probing behavior of M. persicae on 2 Solanum species with different compatibility with M. persicae, Solanum tuberosum (susceptible) and Solanum stoloniferum (resistant) with the electrical penetration graph technique (EPG). To disrupt B. aphidicola, rifampicin was administered to aphids through artificial diets. Aposymbiotic aphids, on both plant species, showed increased pathway activities, mechanical problems with the stylets, and delayed salivation in the phloem. The extended time in derailed stylet mechanics affected the occurrence of most other probing activities; it delayed the time to the first phloem phase and prevented ingestion from the phloem. The effect of aposymbiosis was more evident in the compatible interaction of M. persicae–S. tuberosum, than in the incompatible interaction with S. stoloniferum, which generated the M. persicae–S. tuberosum interaction to become incompatible. These results confirm that B. aphidicola is involved in the plant–aphid interaction in relation to plant acceptance, presumably through a role in stylets penetration in the plant.     

Ultrastructure of the stylet pathway of Brevicoryne brassicae in host plant tissue, Brassica oleracea

Stylets and salivary sheaths of the cabbage aphid, B. brassicae (L.) were studied in leaf tissue of B. oleracea (L.) with Transmission Electron Microscopy. Examples of inter cellular penetration are described by sections of stylets and saliva in air spaces or between adjacent cell walls. Intracellular penetrations are represented by stylets and saliva within damaged cells. High resolution microscopy reveals a third route, where stylets and saliva lie between cell walls and plasmalemmas. This route is called intramural.The results are discussed with particular reference to signals of Electrical Penetration Graphs and at wider level to aphid-host plant selection and phloem location.

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Résumé Les stylets et les gaines salivaires de B. brassicae ont été examinés au microscope électronique à intérieur de feuilles de B. oleracea. Des sections de stylets et de salive dans les espaces intercellulaires et entre les parois de cellules adjacentes ont fourni des exemples de pénétration intercellulaire. Les pénétrations intracellulaires de stylets et salive ont été observées dans des cellules abimées.Une trosième voie a été mise en évidence lorsque les stylets et la salive sont coincés entre la paroi cellulaire et le plasmalesme; cette voie est baptisée intrapariétale.La discussion de ces résultats se réfère aux conditions d'obtention des signaux fournissant les images microscopiques. Ils peuvent servir à expliquer la sélection des plantes par les pucerons et leur aptitude à localiser le phloème.

     

Effect of Manipulations on the Host Selection Behavior of Sitobion avenae (Homoptera: Aphididae)

The study of aphid host selection and feeding behavior is difficult because aphids have to penetrate the plant to reach their feeding site, phloem tissue. The activity of the stylets, salivation or food intake, can not be observed externally and requires an indirect visualization technique such as the Electric Penetration Graph (EPG). The plant selection behavior of Sitobion avenae on potato varied depending on whether an ethological or EPG method was used to study it. A similar variation did not occur with Myzus persicae or Rhopalosiphum padi. The application of water-based silver conductive paint onto the thorax, as normally used for EPG, or onto the abdomen of Sitobion avenae alates resulted in increased duration and frequency of probing compared to results from ethological observations. Our results indicated that EPG manipulations might have different effects on different species of aphids and that a comparison of EPG and ethological data is required to confirm that the EPG method does not bias aphid feeding behavior.     

Stylet penetration by two aphid species on susceptible and resistant lettuce

The probing behavior of two aphid species, Myzus persicae (Sulz.) and Nasonovia ribisnigri (Mosley), was electronically monitored on susceptible and resistant lettuce lines using a DC amplifier. A waveform pattern associated with extracellular stylet pathway activities, pattern C, occurred for longer periods when either aphid species probed resistant plants. This pattern is usually regularly interrupted by drops in electrical potential lasting a few seconds, reflecting cell membrane punctures followed by rapid withdrawal of the stylet tips. For M. persicae on resistant lettuce a large increase in pattern C without these potential drops accounted for the increased duration of this pattern. For N. ribisnigri the increase in pathway activity on resistant plants was due to an increase in the more typical pattern C with potential drops, as well as to an increased duration of pattern F, associated with a curious type of stylet penetration within cell walls. Both aphids made more but shorter probes on resistant than on susceptible plants, and these probes led less frequently to periods of sieve element contact and ingestion. The effects of resistance appear to involve both mesophyll and phloem factors. The underlying mechanisms, however, remain unclear. The results indicate which stylet penetration activities or waveform patterns are of interest for further investigation of resistance mechanisms.

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Résumé Le comportement de sondage de variétés sensibles et résistantes de laitues par deux espèces de pucerons, Myzus persicae (Sulz.) et Nasonovia ribisnigri (Mosley), a été enregistré électroniquement en utilisant un amplificateur DC. Les deux espèces de pucerons ont présenté une amplification de l'onde de type C associée au cheminement extracellulaire des stylets lors du sondage des variétés résistantes. Cette onde est normalement interrompue par des chutes de quelques secondes du potentiel électrique, traduisant des ponctions de la membrane cellulaire suivies par un rapide retrait de l'extrémité des stylets. La forte augmentation de l'onde C mais sans chutes de potentiel rend compte de la plus longue durée de cette onde chez M. persicae sur laitue résistante. L'accroissement du cheminement des stylets chez N. ribisnigri sur plantes résistantes est dû à une augmentation de l'onde C typique avec chutes de potentiel, ainsi qu'à une prolongation de l'onde F liée à la pénétration des stylets dans les parois cellulaires. Les deux espèces font des sondages plus brefs et plus nombreux sur variétés résistantes, et ces sondages entraînent des contacts moins fréquents avec les éléments criblés et débouchent moins souvent sur de l'ingestion. Les effects de la résistance semblent impliquer des facteurs liés à la fois au mésophylle et au phloème. Les mécanismes sous-jacents, cependant, ne sont pas encore clairs. Ces résultats Montrent que l'examen des ondes liées à la pénétration des stylets est important pour des études ultérieures sur les mécanismes de résistance.

     

Comparative life table statistics of Apoanagyrus lopezi reared on the cassava mealybug Phenacoccus manihoti fed on four host plants

The demographic statistics of Apoanagyrus lopezi De Santis (Hymenoptera: Encyrtidae) were studied in the laboratory on its host, the cassava mealybug Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae), reared on four host plants characterized by different levels of antibiotic resistance to the mealybug: two cassava varieties, Incoza and Zanaga (Manihot esculenta Crantz: Euphorbiaceae), the faux-caoutchouc (hybrid of M. esculenta × M. glaziovii Muel. Arg.) and Talinum (Talinum triangulare Jack:Portulacaceae).Total and daily mean fecundities of female parasitoids were strongly influenced by the host plant but there was no link with antibiotic resistance. The mean duration of the oviposition period was also significantly modified by the host plant and was again not correlated to the level of antibiotic resistance. Net reproduction rates of female parasitoids were nearly 2 times higher on hosts fed on Zanaga and Talinum than on Incoza and Faux-caoutchouc, and were not related to the total fecundities. The generation time was significantly longer on Faux-caoutchouc than on the other three plants. The intrinsic rate of increase varied significantly with host plants and was higher on Talinum and Zanaga than on Incoza and Faux-caoutchouc. With the varieties of cassava, Incoza and Zanaga, it was observed that antibiosis had a significant negative effect on the survival of A. lopezi, which would influence the effectiveness of the parasitoid.     

Ultrastructure of sieve element penetration by aphid stylets during electrical recording

The probing activities of Brevicoryne brassicae (L.) on a host plant Brassica oleracea (L.) were electronically monitored using a direct current (DC) device. Two electrical penetration graphs are described, including a pattern, D+E, at two different potential levels; the normal level of pattern, C, D+E(c), and the potential drop level, D+E(pd). Stylets were amputated during these patterns, sap exudation from the stumps was scored, and the penetrated leaf area with the remnants of the stylets were processed for EM. Sap was not produced from stylets cut during D+E(c). About 70% of stylets cut during the D+E(pd) pattern produced sap and in micrographs, stylet tips had pierced the sieve element plasmalemma and were found inside the cell lumen. This supports the earlier hypothesis that the D+E(pd) pattern is related to intracellular sieve element penetration. The relation of D+E(c) and D+E(pd) with ingestion is discussed.

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Résumé Les pénétrations par Brevicoryne brassicae de la plante-hôte Brassica oleracea étaient enregistrées électriquement au moyen d'un montage au courant continu. Deux signeaux sont décrits avec l'onde D+E à deux niveaux de potentiel électrique: le niveau normal de l'onde C, D+E(c), et un niveau bas après une chute de potentiel, D+E(pd). Au cours de ces deux ondes les stylets étaient amputés, on a pu observer le jaillisement de sève hors du moignon; on a ensuite préparé une partie de la feuille avec les restes des stylets pour le microscope électronique. Au cours de l'onde D+E(c) on observait aucune production de sève. Environ 70% des stylets amputés pendant l'onde D+E(pd) produisaient de la sève et dans deux de ces cas, dont les signeaux sont décrits, les pointes des stylets se trouvaient dans une cellule des vaisseaux conducteurs en ayant pénétrés le plasmalemme. Ces résultats confirment l'hypothèse antérieure, selon laquelle l'onde D+E(pd) est liée à une pénétration intracellulaire d'un vaisseau conducteur. La relation de l'onde D+E(c) et D+E(pd) avec l'ingestion par l'aphide est discutée.

     

Field placement of mealybug-infested potted cassava plants for the study of parasitism of Phenacoccus herreni

During 4-day field exposure intervals, Phenacoccus herreni Cox & Williams, third-instar mealybugs on artificially infested potted cassava plants were reduced 73% from initial densities of 235.7–1100.2/infested plant to 63.7–314.2/plant due to dispersal, rain, and predation. Frequency of parasitoid attack did not differ between high and low mealybug densities. The same species of parasitoids, in approximately the same proportions, were reared from mealybugs on artificially infested potted plants as from field mealybugs (Acerophagus coccois) Smith, 88.1% and Epidinocarsis diversicornis (Howard), 11.3%. Parasitoid attack rates on artificially exposed mealybugs, when combined with a simulation model of parasitism, accurately predicted observed levels of stage-specific parasitism in the field population.

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Résumé Pendant des expositions de 4 jours dans la nature de plants en pots de M. esculenta contaminés artificiellement par des P. herreni du troisième stade, les densités de cochenilles ont été réduites de 73%: de 235.7–1100.2/par plant contaminé à 63.7–314.2/par plant, par suite des évasions, de la pluie et de la prédation. Les attaques de parasitoïdes ne changent pas suivant les densités de cochenilles. Les mêmes espèces de parasitoïdes, ont été élevées dans à peu près les mêmes proportions (Acerophagus coccois, 88.1%, et Epidinocaris diversicornis, 11.3%) à partir de cochenilles sur pots artificiellement contaminés ou de manioc de la nature. Les taux d'attaques par les parasitoïdes sur les cochenilles artificiellement exposées, associés à un modèle de simulation du parasitisme, ont permis de prévoir avec précision les niveaux de parasitisme observés pour chaque stade dans la population de la nature.

     

Mouthparts and stylet penetration of the lac insect Kerria lacca (Kerr) (Hemiptera:Tachardiidae)

Hitherto less known aspects on mouthpart morphology and penetration mechanism of the lac insect Kerria lacca have been explored. Unique details of the mouthparts, i.e. morphology of labium and stylets and salivary sheath have been brought out. The gross morphology of the mouthparts though resembled other plant sucking homopterans; a two-segmented labium with symmetrically distributed six pairs of contact-chemoreceptors on its surface was distinct; the mandibular stylets had serrations on its extreme apical region, while the maxillary stylets had their external surface smooth with parallel longitudinal grooves on their inner surface. Formation of flanges, salivary sheath and penetration pathway observed along with probing and penetration of the stylets intracellularly up to the phloem cells, as illustrated herein, are the addition to the existing knowledge on the structural details of the mouthparts and the feeding behavior thereupon.     

Feeding responses of two grain aphids to barley yellow dwarf virus-infected oats

The probing behavior of greenbugs, Schizaphis graminum (Rondani), and oat-bird cherry aphids, Rhopalosiphum padi (L.), was electronically monitored on healthy oats, Avena sativa (L.), and on oats infected with the RPV-NY isolate of barley yellow dwarf virus (BYDV). S. graminum fed better on infected oats than on healthy oats. This was manifested by a shorter time before initiation of committed phloem ingestion, and by increased duration of ingestion from phloem of infected compared to healthy plants. In addition, on infected oats S. graminum made fewer interruptions in their probing once their stylets were inserted into tissues, including phloem. R. padi fed similarly on infected and healthy oats, except that these aphids made fewer short probes to the phloem (lasting <15 min) on infected compared to healthy oats. BYDV infection of oats increased the rate of population growth of S. graminum relative to that on healthy oats but had no effect on the population growth of R. padi. The proportion of aphids of R. padi which developed into alatae on BYDV-infected oats was significantly greater than on healthy oats, but S. graminum showed no such response.

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Résumé Le comportement de sondage de S. graminum (Rond.) et de R. padi (L.) a été suivi électroniquement sur avoine (Avena sativa L.) saine ou contaminée par l'extrait RPV-NY du virus jaune du nanisme de l'orge (BYDV). S. graminum s'est mieux alimenté sur avoine contaminée que saine; ceci se traduisait par un temps de latence inférieur avant l'ingestion de phloème et par une prolongation de la période d'ingestion. De plus, sur avoine contaminée, S. graminum a moins souvent interrompu le sondage, une fois que les stylets avaient étè insérés dans les tissus, y compris le phloème. Les résultats avec R. padi présentaient les mêmes tendances, mais les améliorations des performances alimentaires de cette espèce sur avoine contaminée ont été moins nettes que pour S. graminum. La contamination de l'avoine par BYDV a accru le taux de croissance de la population de S. graminum par rapport à ce qui a été observé sur avoine saine; l'effet était moins prononcé pour R. padi. La proportion de pucerons devenant aliés était plus élevée sur avoine contaminée que saine, rien de tel n'a été observé avec S. graminum.

     

Searching and Oviposition Behavior of Anagrus atomus L. (Hymenoptera: Mymaridae) on Four Host Plants of Its Host, the Green Leafhopper Empoasca decipiens Paoli (Homoptera: Cicadellidae)

Anagrus atomus L. is an important egg parasitoid of the green leafhopper Empoasca decipiens Paoli. In this study the ability of the parasitoid to locate and parasitize its host was investigated on four host plants, i.e., broad beans (Vicia faba L.), sweet pepper (Capsicum annuum L.), cucumber (Cucumis sativus L.), and French beans (Phaseolus vulgaris L.). For each plant species, the behavior of the parasitoid was observed on E. decipiens infested and noninfested plants. Searching and oviposition behavior were characterized by drumming, probing, and resting. Parasitoids spent significantly less time on non-infested than infested plants, 274.5 and 875.7 s, respectively, and no probing behavior was observed on non-infested plants. Frequency of resting behavior was significantly greater on non-infested than on infested plants. Total foraging time was significantly longer on infested than on non-infested plants, indicating that A. atomus females can efficiently discriminate between leaves with and without infestation. Parasitism of A. atomus was influenced by parasitoid density, with the highest parasitism rate (64.0%) obtained at a density of 10 A. atomus females/0.1356 m² but the number of parasitized eggs per female and the searching efficiency decreased with increasing parasitoid density.     

Plant Finding and Acceptance Behaviors of Anasa tristis (DeGeer)

Finding and acceptance behaviors of Anasa tristis were investigated using plants that are suitable (pumpkin) or unsuitable (cucumber, coleus) for development. A. tristis found plants in the absence of non-contact cues. Plant finding is enhanced by olfaction and vision. A. tristis found suitable and unsuitable plants equally well. The importance of olfaction in plant acceptance is suggested by intense antennation while on plant and change in behavior on pumpkin and coleus after removal of antennae. Insects on suitable and unsuitable plants dab the surface, contact the plant surface with the labium and insert stylets. Post ingestive effects are important for plant acceptance and rejection. Ingesting water and some nutrients from plants unsuitable for development can be important in extending the time available to move between resource patches.     

Leafhoppers on leaves: An analysis of feeding behavior using conditional probabilities

Feeding behavior of four deltocephaline leafhoppers,Graminella nigrifrons, G. oquaka, Amblysellus grex, andDalbulus maidis on maize and johnsongrass was analyzed using an electronic monitoring device. Five distinct waveform patterns were identified: secretion of sheath saliva (salivation), nonvascular probing, nonsieve element ingestion, x-waveform, and phloem ingestion. Waveforms were associated with feeding activities by correlation with light microscopic examination of salivary sheath termination points in leaf tissue and analysis of honeydew excreted by monitored leafhoppers. In previous studies x-waveforms have been reported to occur only when the stylets of homopterans are in contact with the phloem; the function of x-waveforms is poorly understood. There were no differences in time spent salivating or ingesting from nonsieve elements amongG. nigrifrons, G. oquaka andA. grex on either plant.D. maidis differed from other species in phloem probing and feeding behavior; only a small proportion produced x-waveforms, although those that did spent significantly more time in this behavior than other species. Also,D. maidis spent more time than other leafhoppers ingesting from tissues other than sieve elements. Kinetic diagrams of transition probabilities show that probing activities of all species were not random regarding the sequence of behaviors culminating in phloem ingestion. Thirty-five percent ofG. nigrifrons x-waveforms were followed by nonsieve element ingestion. This was consistent with observations showing that salivary sheaths of leafhoppers producing x-waveforms sometimes do not terminate in the phloem, but rather in nearby cells. Phloem ingestion was always preceeded by x-waveforms. The quantitative differences in probing behavior are discussed in relation to ability of these leafhoppers to transmit the phloem-associated maize chlorotic dwarf waikavirus.     

Prey Habitat Location by the Cassava Mealybug Predator Exochomus flaviventris: Olfactory Responses to Odor of Plant, Mealybug, Plant–Mealybug Complex, and Plant–Mealybug–Natural Enemy Complex

Exochomus flaviventris Mader is considered to be the most active predator of the cassava mealybug Phenacoccus manihoti Matile–Ferrero in Central Africa. The response of experienced gravid female coccinellids to the odor of cassava plant (var. Zanaga), unparasitized mealybugs, plant–mealybug complex with or without feeding prey (parasitized or not), and plant–mealybug complex with or without conspecific coccinellids was investigated in a Y-tube olfactometer. The odor of uninfested cassava plants was not more attractive than clean air. Dual-choice tests revealed that mealybug-infested plants were preferred to mealybugs alone and mealybug-damaged plants and were the major sources of volatiles that attract females coccinellids to the microhabitat of its prey. The emission of volatile chemicals did not appear to be limited to the infested parts of the plant but did occur systemically throughout the plant. The presence of conspecific coccinellid larvae or adult males did not modify the attractiveness of the mealybug-infested plants. However, when an infested plant with conspecific predator females (alone or with conspecific males) was compared to an infested plant or infested plant with conspecific males, E. flaviventris females showed a preference for the last two sources of odor. The uninfested plant with conspecific males was also preferred to the uninfested plant with conspecific females. In addition, the odor of conspecific males was preferred over that of conspecific females. Female predators preferred the plant infested with unparasitized mealybugs over the plant infested with mealybugs previously parasitized. These results showed that E. flaviventris females use herbivore-induced plant volatiles during foraging and can detect via olfaction the presence of conspecific gravid females and parasitized prey, thus assessing patch suitability from a distance.     

The use of polyporus for the investigation of stylet behaviour in the Hemiptera

The hardness (i.e. resistance to penetration) of water-saturated polyporus and of various plant structures, determined by a 0.254 mm diameter needle-penetrometer, ranged from 12–50 and 11–112 g/mm respectively. The isolated mandibular stylets of three species of Homoptera (Typhlocyba rosae, Philaenus spumarius, Macropsis fuscula) possessed tapered but non-curved apices while the maxillaries were incurved to varying extents: this condition was reversed in two Heteroptera (Orthotylus ochrotrichus, Tingis cardui) where the mandibulars were incurved and the maxillaries straight. Insertion of the isolated stylets into wet polyporus produced curved paths closely related to the stylet structure with the average radius of curvature of the path decreasing by half in the hard (50 g/mm) compared with the soft (12 g/mm) polyporus. These results are related to published information on the stylet paths of these and related insects when they are feeding. Directional control of the stylets in plant tissue is considered to be primarily a function of the mandibular stylets in the Heteroptera and of the maxillaries in the Homoptera, for species in which the maxillary stylets are independently-moving.

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Résumé La dureté (c'est-à-dire la résistance à la pénétration) de petits morceaux de polypore saturés d'eau est évaluée d'après la force nécessaire pour assurer l'enfoncement d'une aiguille d'un diamètre de 0,254 mm; celle-ci est comprise entre 12 et 50 g/mm. La dureté moyenne ainsi établie est évidemment plus grande que celle obtenue avec une aiguille de 0.142 mm; par ailleurs, si le polypore est remouillé, après avoir été séché, il ne montre aucun changement significatif de sa dureté; celle-ci est inversement proportionnelle à la teneur en eau.La dureté de divers organes des plantes, déterminée avec une aiguille de 0,254 mm, varie de 11,3 g/mm (nervure principale d'une feuille de tabac) à 112,2 g/mm (tige de chèvrefeuille), bien que certains tissus puissent avoir des valeurs moyennes bien au-dessus ou bien audessous de celle de l'organe entier. Comme avec le polypore, il y a une relation inverse entre la dureté et la teneur en eau.L'examen des extrêmités des stylets isolés chez trois espèces d'Homoptères et deux espèces d'Hétéroptères, montre que chez les Homoptères les mandibules sont droites et acérées alors qu'elles sont courbes chez les Hétéroptères; la condition inverse s'observe pour les maxilles. La courbure des stylets qui se voit chez l'insecte vivant quand leur extrémité est prolongée au-dela du faisceau, varie avec les espèces. Pour les maxilles, elle est plus grande chez Typhlocyba rosae que chez Philaenus spumarius et Macropsis fuscula (Homoptères), et pour les mandibules, elle est plus grande chez Tingis cardui que chez Orthylus ochrotrichus (Hétéroptères).Des stylets isolés enfoncés dans du polypore tendre ou dur (12 et 50 g/mm respectivement) suivent des trajets courbes en rapport avec la structure des stylets et avec la dureté du substrat.Des résultats publiés pour d'autres espèces d'Hémiptères au sujet de la pénétration des stylets dans les tissus des plantes sont confrontés aux résultats précédents. On en conclut que les trajets sinueux observés pour les stylets des Hétéroptères (O. ochrotrichus et T. cardui) et de petits Homoptères (Typhlocyba rosae) sont surtout le fait des stylets mandibulaires et maxillaires respectivement présentant une courbure. La courbure moins accentuée des stylets maxillaires de plus grands Homoptères (P. spumarius et M. fuscula) est à rapprocher du trajet plus droit des stylets de ces espèces, avec un changement de direction limité aux tissus vasculaires les plus durs.

     

Electrically recorded feeding behaviour of cassava mealybug on host and non-host plants

The feeding behaviour of cassava mealybug (Phenacoccus manihoti) was analysed by the electrical penetration graph technique (EPG, DC-system) and by light microscopy. This study confirmed a typical phloem-feeding behaviour with an aphid-like predominance of extracellular pathways of stylets. Similarities of EPGs from mealybugs with those of aphids and whiteflies allowed adoption of standard pattern labelling. The main differences with aphid EPGs consisted of fewer but longer intracellular punctures (pd duration of 20 svs 5–7 s for aphids), longer times to the first phloem-ingestion period and a lower motility of stylets within the phloem searching process. Comparison of the feeding behaviour on 6 plant types, including two occasional hosts (Talinum and poinsettia), one cassava hybrid and three common cassava cultivars (Manihot esculenta), allowed differentiation of penetration profiles. Intracellular punctures were shorter on occasional hosts, on which phloem finding and subsequent ingestion were markedly delayed although a great variability within plants persisted. Chemical analysis of some secondary plant substances revealed that none of the plants tested contained detectable amounts of alkaloids, and that cyanides were restricted to true hosts (cassavas andManihot hybrid). Levels of total flavonoids did not differ between hosts and non-hosts, in contrast to those of phenolic acids. Preliminary correlations with EPG paterns are discussed.

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Résumé Le comportement alimentaire de la cochenille du manioc (Phenacoccus manihoti) a été étudié par électrographie de pénétration (EPG, analyse en courant continu) et par microscopie optique. Cette étude a permis de confirmer le caractère typiquement phloémophage de l'alimentation de cet Homoptère, présentant une large prédominance du trajet extracellulaire des stylets, comme cela est également observé chez les pucerons. Les similitudes des tracés EPG effectués sur cochenille et sur pucerons ou aleurodes ont permis d'adopter la même typologie du signal. Les principales différences avec les pucerons concernent le nombre moins important et la durée plus longue des ponctions intracellulaires (20 s en moyenne contre 5 à 7 s chez les pucerons), l'allongement du temps d'accès au phloème et une moins grande mobilité des stylets lors de la phase de recherche du phloème. La comparaison du comportement alimentaire sur 6 plantes, parmi lesquelles deux h?tes occasionnels (Talinum et poinsettia), un hybride et trois variétés courantes de manioc (Manihot esculenta), a permis de différencier plusieurs profils de pénétration. Les ponctions intracellulaires se sont révélées plus courtes sur h?tes occasionnels, qui induisent également, malgré la variabilité observée, un allongement net de la période de localisation du phloème et donc un retard à l'ingestion. Aucune des plantes testées ne contient d'alcalo?des, et les composés cyanés se restreignent au genreManihot, h?te naturel de la cochenille. Les flavono?des totaux ne permettent pas de distinguer plantes h?tes et non-h?tes, à la différence des acides phénoliques.

     

Locating a resistance mechanism to the cabbage aphid in two wild Brassicas

Feeding behaviour of the cabbage aphid,Brevicoryne brassicae, was monitored electronically on two resistantBrassica species,B. fruticulosa andB. spinescens, and compared with a susceptible controlB. oleracea var.capitata cv. Offenham Compacta. Aphids, monitored for 10 h on the under side of leaves, performed recognizable feeding behaviour on all species. Electrical Penetration Graphs (EPGs) of aphids on resistant and susceptible plants showed no difference in behaviour for aphids on resistantBrassica species compared to susceptible until stylets penetrated the phloem sieve elements when a large reduction in the duration of passive phloem uptake (E₂ pattern) onB. fruticulosa was indicated. Although feeding behaviour on 6 week-old plants ofB. spinescens was similar to the susceptible controls, behaviour on 10 week-old plants was similar to that recorded forB. fruticulosa. The mechanism of resistance is thought to be located in the sieve element as the normal sieve element salivation (E₁) signal was either quickly terminated by withdrawal of the stylets from the sieve element or continued as a disrupted E₂ pattern. Analysis of secondary plant compounds in the threeBrassica species only identified significant differences in the glucosinolate profile. No reproducible differences were detected in the concentration of phenolics or anthocyanins. The major glucosinolate component ofB. fruticulosa andB. spinescens was gluconapin rather than glucobrassicin and glucoiberin as found in the susceptible host plant. However, both pure glucosinolates and glucosinolate extracts from all three species did not reduce aphid survival on chemically-defined artificial diets. These results suggest that the mechanism of resistance may be a mechanical blocking of the sieve element or stylets rather than a difference in the secondary plant chemistry of glucosinolates and phenolics.     

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.     

Pathway phase waveform characteristics correlated with length and rate of stylet advancement and partial stylet withdrawal in AC electrical penetration graphs of adult whiteflies

A technique was developed for measuring the length of stylet insertion during adult whitefly probing. The distance that the labium shortens during a probe was shown to be equal to the length of stylets that were inserted into the plant tissue. The length of labial shortening then was measured in high-magnification video recordings of adult female silverleaf whitefly, Bemisia argentifolii, in conjunction with recording electrical penetration graphs (EPGs – AC method). Using a split-screen device, video images of the whitefly's labium during a probe and the EPG waveforms produced during the probe were recorded simultaneously on the same video tape. On playback, changes in labial length could be measured during specific EPG waveforms to determine the length of stylet insertion that occurred during the waveforms. The focus of the study was on two characteristics of the pathway phase sawtooth waveform: the frequency of voltage peaks and the increase in voltage level that occurs over time during sawtooth waveforms. The rate of stylet penetration was significantly and positively correlated with frequency of sawtooth waveform voltage peaks (r ²=0.33) and the length of stylet penetration was significantly and positively correlated (second-order polynomial) with the relative difference in voltage level between the beginning and end of the sawtooth waveform (r ²=0.43). Stylet advancement did not appear to occur during the few low-flat waveforms (unknown behavioral correlation) and high-flat waveforms (phloem phase) that were observed. Voltage drops occur sporadically during sawtooth waveforms, and these were associated with partial stylet withdrawal (indicated when the labium increased in length, but the probe was not terminated) with an accuracy of 99%.