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Pymetrozine, a pyridine azomethine compound, represents a novel insecticide with a selective activity against homopteran insects. It acts in a unique way: aphids are not knocked down on contact but seem to die of starvation. This implies an effect of pymetrozine on feeding behaviour. The aim of the present work was to elucidate how and at which step pymetrozine interferes with the complex mechanisms underlying phloem feeding. The effect of pymetrozine, applied in various ways, on different phases of stylet penetration and feeding activity of individual aphids was studied using the Electrical Penetration Graph technique (EPG). Initial choice experiments indicated that pymetrozine does not have a deterrent or antifeedant action. Topical application (150 ng pymetrozine mg -1 fresh weight) inhibited stylet insertion into the plant. When injected, less than 30 ng mg -1 was sufficient to produce the same effect. When pymetrozine was systemically applied via plant spraying or root uptake, aphids started feeding normally. After some time, however, they withdrew their stylets from the phloem and walked around with unaffected locomotion. At low doses aphids eventually recovered and resumed feeding. High doses, however, irreversibly disrupted feeding and prevented stylet reinsertion. Aphid motility was not affected up to an estimated haemolymph concentration of 1 mM pymetrozine. Aphids which eventually stopped feeding on pymetrozine-treated plants showed EPGs with distorted salivation/ingestion patterns. It is concluded that pymetrozine does not have a general toxic effect on aphids but selectively interferes with the nervous regulation of feeding behaviour which consequently results in death due to starvation after a few days.

The production of volatile secondary plant substances during the evolution of terrestrial plants is reviewed in regard to the defensive systems of plants to microorganisms and herbivores. Plant volatiles can be produced by both anabolic and catabolic processes. Although attraction of pollinators is a well-studied phenomenon, functions of volatiles range from excretion of waste products to the production of compounds attracting natural enemies of herbivores. During the evolution of the angiosperms a diversity of volatiles were selected to defend generative parts against microorganisms. Many of these allomones were related to or even identical with sex pheromones of insects. As a result flowers of angiosperms became utilized as a mating site. Consequently insects visiting flowers became involved in pollination, facilitating the steps from anemophily to entomophily. The efficiency of entomophily was increased because of nutritional rewards. An evolutionary scenario for the impact of plant volatiles on insects is presented and the role of volatile allomones in the establishment of plant-insect relationships is emphasized by (1) their strong antimicrobial properties, (2) strategies to protect symbiotic microorganisms, (3) their function as repellents and deterrents, (4) the use of volatile allomones as kairomones. These facts speak for an adaptation of insects to plant physiology and a limited importance of phytophagous insects in selection pressure upon plants. Herbivorous insects have realized specific adaptations to be able to discriminate between complex odour blends, but the utilization of chemical groups among insect taxa is different. The main theories on plant chemical defence do not discuss the impact of volatiles on host plant selection and may be apt to revision when pheromones, allomones, kairomones and synomones are not taken into account.

Substances secreted by aphids into diets through a rubber-wax membrane can be detected by their absorption of low wavelength ultraviolet light. Their detection can be simplified by presenting water as diet. Of the substances secreted into water by non-crowded aphids within 24 h, ca 35–55% of the absorbance at 200 nm can be accounted for by reactions with protein reagents and/or electrophoresis. Macrosiphum euphorbiae secreted over 30 ng of salivary protein in bovine serum albumin equivalents per mg fresh body weight. SDS-PAGE revealed that the proteins initially secreted had subunits mostly of > 100,000 Daltons. When dead or dying aphids were present on the membrane, however, e.g. due to crowding or prolonged confinement in the feeding chamber, additional subunits of < 30,000 Daltons also appeared in the water. When feeding continued for more than 24 h, proteins separable by electrophoresis began to disappear, possibly as a result of leakage of lyzing enzymes into the water from aphids that had died with their mouth parts still inserted through the membrane. The initial secretions possessed oxidase activity. Invertase was not detected but the secretion caused non-enzymatic reduction of a copper reagent used for detection of reducing sugars. Marked contrasts were noted between the electrophoretic protein patterns of the saliva of different species of aphids. The saliva of Macrosiphum euphorbiae and Myzus persicae contained over twice as much catechol oxidase activity per unit of salivary protein as that of Nasonovia ribisnigri and Aphis fabae. The significance of these findings in relation to use of artificial diets for colony maintenance and feeding experiments and for studies of aphid-plant interactions is discussed.

Electrical registration and visual observation of penetration behaviour combined with ELISA-technique showed that laboratory-made webs of synthetic fibers were very promising to protect plants against virus infection by aphids. It was demonstrated that stylet penetration by aphids into potato leaves could be prevented by covering plants with these materials. The recent mass production of polypropylene sealed fiber fleeces and polyethylene nettings have made field experiments possible to determine their effect on aphid probing behaviour and virus transmission. Field experiments with potatoes showed that one particular fleece completely protected the plants against virus transmission (PVY and PLRV) and with a second fleece and a netting a small proportion of plants were infected. In uncovered plots up to 25% of plants became infected. A seed potato crop covered with polypropylene fleece which is inspected regularly for damage will be fully protected against virus transmission. Resume Utilisation de toiles de polymeres afin d'empecher la transmission de virus par les pucerons chez les pommes de terres semencieres Des enregistrements electriques et des observations visuelles du comportement de penetration, combines avec l'usage de la technique ELISA, ont montre que l'usage de toiles de fibres synthetiques elaborees au laboratoire etait tres prometteur pour la protection des plantes contre des infections virales transmises par des aphides. On a pu demontrer que la penetration du stylet dans des feuilles de pomme de terre pouvait etre empechee en couvrant les plantes avec ces materiaux. La production en masse recente de nappes en fibres de polypropylene soudees et de filets de polyethylene a rendu possibles des experiences de terrain pour determiner leur effet sur le comportement de sondage des pucerons et sur la transmission des virus. Des essais en plein champ avec des pommes de terre ont montre qu'un type de nappe en particulier protegeait completement les plantes contre la transmission de virus (PVY en PLRV) et qu'avec un second type de nappe et un filet une petite proportion de plantes etait infectee. Dans des parcelles non couvertes la proportion de plantes infectees sx'elevait jusqu'a 25%. Une recolte de pommes de terre semencieres couverte d'une nappe de polypropylene regulierement inspectee afin de remedier aux deteriorations eventuelles sera totalement protegee contre la transmission de virus.

Pymetrozine inhibits feeding in aphids immediately after application without producing visible neurotoxic effects, as previously reported. In the present work, Locusta migratoria, though not a plant-sucking insect, was found to respond to pymetrozine by displaying unique symptoms, which were lifting and stretching of the hindlegs, in addition to inhibition of feeding. In locust, pymetrozine enhanced spontaneous spike discharge of the metathoracic and suboesophageal ganglia in situ at nanomolar concentrations. Similarly, pymetrozine increased the spontaneous rhythmic contractions of the isolated foregut with maximal effects also in the nanomolar range. The actions of pymetrozine were counteracted by biogenic amine receptor antagonists and mimicked by serotonin, not by dopamine and octopamine. Moreover, pymetrozine and serotonin strongly potentiated the effects of each other. Pymetrozine was inactive at all neurotransmitter receptors present on isolated locust neuronal somata, and at all other examined neuronal sites. In Myzus persicae, electrical penetration graph experiments revealed that serotonin, like pymetrozine, inhibited stylet penetration, and strongly enhanced the action of pymetrozine, comparable to the locust. Amine receptor antagonists were not specifically active in the aphid. We conclude from the present results that pymetrozine acts via a novel mechanism that is linked to the signalling pathway of serotonin.

There is a search for natural bioactive terpenoids by scientists of several disciplines. Biologists seek for bioactive essential oils and other plant-derived terpenoids, biochemists try to elucidate biosynthetic routes and to reveal the structure and function of the corresponding enzymes, gene technologists trace the responsive genes. Biologists and pharmacologists organize expeditions to rain forests and other regions of the world in search for new compounds. Not everyone has to go a long way: Indian scientific journals regularly report the isolation of bioactive terpenoids not seen before. Many novel terpenoids are discovered by marine biologists. In chapter 4.1 and section 7.1.8 terpenoids with anti-tumour functions have been reported to originate from marine biology. Once their modes of action have been elucidated they can be tested for different purposes. The marine pulmonates Trimusculus reticulatus and T. conica (Trimusculidae) have a unique lifestyle. The adults are essentially sessile and feed by filtering a mucous net produced by subepithelial glands on the mantle and sides of the foot. Being sessile they cannot escape from predators such as seastars, and their way of collecting and ingesting food (mostly microalgae) makes that they have to cope with intruders of their habitat. There is little space in the intertidal zone and much competition. Manker and Faulkner (1996) investigated the distribution and function of secondary metabolites from trimusculid limpets and found two diterpenes highly concentrated in the mantle, foot and mucus of T. reticulatus (Fig. 8.1).

Within organisms, terpenoids are known for their physiological activities on cell level. They can influence cell stage and mitosis, resulting in changes in morphology and differentiation. Terpenoid endproducts and certain precursors of the mevalonate metabolism and other exogenous terpenoids can affect the synthesis of endproducts and intermediates of other branches or interfere with gene expression, or more directly, act as enzyme or key enzyme regulators. The preciseness with which the enzymes of the various steps of the biosynthesis of terpenoid endproducts operate and the effective feedback systems acting on enzymes and key enzymes are the result of a long evolutionary process. Between organisms, functions of specific terpenoids as messengers are likewise the outcome of a long process of selection of adequate production and reception. This chapter covers the basic regulatory processes involved in the biosynthesis of isoprene units in plants, insects and vertebrates. As in many biochemical processes, overproduction of endproducts is in some way prevented. Once produced in an efficient way, the same terpenoid can acquire a number of messenger functions in completely different organisms. Chapter 3 will discuss the development of regulatory mechanisms during evolution, whereas chapter 4 focusses on the relationship between chemical and physical characteristics of terpenoids and their effects on the receptor sites.

Feeding behavior and scar production of male and female F. occidentalis (Pergande) (Thysanoptera: Thripidae) were studied in relation to transmission of tomato spotted wilt tospovirus (TSWV). Electrical penetration graph (EPG) analysis showed that females feed more frequently and intensively than males. The feeding intensity, reflected by silvery scar production and studied by an image analysis system, demonstrated that females induced more numerous scars than males. At the same time, males transmitted TSWV with a higher efficiency than females, indicating that TSWV transmission and scar production are not positively correlated. Furthermore, males produced significantly more local lesions of TSWV than females. These quantitative differences in scar production and transmission of TSWV can be explained by the lower mobility and higher consumption rate of females. The influence of the sex-ratio on crop damage and virus transmission, and thus to the spread of TSWV, is emphasized.

In this study on the air-borne viruses in Lilium, mainly lily symptomless virus (LSV) and tulip breaking virus (TBV), various factors involved in the efficiency of control are dealt with. The incidence of LSV was reduced very substantially from the old 100␛ate. TBV occurs at fairly low rates. Lily virus X (LVX) and cucumber mosaic virus (CMV) occurred at low level in the last decade. The symptoms of TBV to induce the roguing of diseased plants became less important, if the ELISA-testing of bulbs before bulk propagation by the scaling of bulbs and by tissue culture procedures was effectively applied. The reduction in bulb yield and the relative reduced quality of cut flowers indicate the need for virus control. The virus spread as affected by the elimination of virus source plants strongly became dependent on the ELISA-testing. The speed of spread was generally experienced to be differentially rapid over the season, while a range of c. 25 aphid species were able to transmit LSV and TBV. The chemical control of spread is efficiently done by the routine spraying of mineral oil plus pyrethroid. Terpenoids were not effective. Some effect was observed from a pheromone, plant oil, and -pinene polymer emulsion. Deltamethrin and zetamethrin sprays were most effective. The non-chemical control by the polymer web coverage of crops proved impractical under field conditions. The factors in the chain of control which have a weak impact, e.g., delayed elimination of TBV-diseased plants in the field if the ELISA-bulb testing prior to planting is routinely unapplicable in varieties of L. longiflorium and Oriental hybrids, or strong impact, e.g., testing of bulbs by ELISA of a large part of the assortiment of cultivars, chemical control of virus spread, and rapid propagation procedures, on the efficiency to improve the health situation in the lily culture were indicated.

Plant penetration by western flower thrips (Frankliniella occidentalis (Pergande)) was analysed with the electrical penetration graph technique (EPG, DC-system). Thrips attached to a gold wire were included in an electrical circuit to record EPGs when penetrating the plant tissues with their stylets. Three basic EPG waveforms have been distinguished, correlated with stylet penetration into cells, salivation, and ingestion, respectively. The main difference with EPGs of Homoptera is the occurrence of continued separate penetrations that are not necessarily followed by ingestion. Insertion of the stylets causes strong voltage fluctuations in the EPG. We could confirm earlier evidence that penetration of cells and subsequent ingestion of (part of) the protoplast takes less than 20 seconds. Repeated short penetrations can be followed by a continuous feeding pattern during which the stylets are not withdrawn. The same sequence of waveforms is produced on other plant parts such as fruits or pollen grains. The specific waveforms are mainly caused by electromotive force (emf). The emf component was recorded with high resolution and the correlation of waveform details with activities of the cibarial muscle system is discussed.

Although the biology of many thrips species is well-studied, there is a lack of knowledge on the physiology of feeding. In contrast to aphids (Homoptera) that mainly feed on phloem elements in which the sap is under pressure, thrips (Thysanoptera) have to actively imbibe the contents of separate cells or at best suck the sap of damaged tissues as it exudes from wounds (Lewis, 1973). Thrips feeding mechanisms are still incompletely understood and this hampers studies on the effect of host plant resistance factors on the behaviour of thrips species which cause considerable problems in horticulture and agriculture. Comparison of optimal and suboptimal plants could be facilitated if it were possible to elucidate how stylet penetration and feeding behaviour are arranged in time and how these activities are affected by host-plant factors.

The selection of resistant lines of lettuce (Lactuca sativa L.) infested by several aphid species has resulted in the transfer of a factor, from wild species of L. virosa,that confers resistance to Nasonovia ribisnigri (Mosley). Isogenic plant material with complete resistance to this aphid could be obtained based upon a dominant gene (Nr-gene). As Macrosiphum euphorbiae (Thomas) can thrive on lettuce selections with a considerable level of resistance to both N. ribisnigri and Myzus persicae (Sulzer), the objective of this study was to determine the effect of the Nr-gene on the probing behaviour of M. euphorbiae.

The papers in this book are organized as follows: insect-plant communities, host-plant selection, genetics and evolution, host-plant resistance and application of transgenic plants, and multitrophic interactions. Besides seven invited papers and a paper with concluding remarks, this volume also contains the short communications of all 115 oral presentations and posters. Included, too, are the summaries of four European Science foundation workshops held over the past two years, where European scientists discussed the state of the art and the future of major topics in insect-plant interactions in order to develop better integrated research programmes. The field of insect-plant interactions nowadays includes almost all of biology, as well as parts of chemistry and physics. It takes a central position in biology because insects are the most abundant animal group, half of them are herbivores and they dominate all terrestrial ecosystems. Knowledge of insect-plant interactions is thus fundamental to an understanding of the evolution of life on Earth. Two major topics of world-wide concern give this field an extra dimension. First, large amounts of food crops are still lost due to insect pests. With the increasing concern for environmental pollution and the subsequent plans to drastically reduce pesticides, integrated pest management and development of resistant crops become a major focus in agriculture. The importance of the study of insect-plant relationships is thus continuously augmented. Clearly, successful pest control demands sufficient fundamental knowledge of post-host interactions. Second, such work can contribute towards stopping or even counterbalancing the threatening biodiversity crisis thanks to an understanding of how the interaction of insects and plants has influenced and still influences the diversification and speciation (evolution) of both groups. These problems should, of course, be approached at a multitrophic level.

To determine whether first-instar Tipula oleracea L. preferentially feed on either grass roots or leaves, we developed a differential staining technique for grass roots. A 0.15% aqueous solution of trypan-blue stained the root-system of ryegrass sufficiently to identify root tissue in the gut system of leatherjackets. It did not stain leaf tissue of the same plants. Using stained grass plants, it was shown that young larvae of T. oleracea in grasslands preferentially feed on leaf tissue and not on root tissue as previously assumed.

Presumptive alatiform larvae deposited by groups of plant-reared apterous Myzus persicae can be diverted to an apterous course of development on artificial diets, which, although providing all essential nutrients, have an amino nitrogen ratio suboptimal for growth. These results are in contradiction to a widely accepted assumption that poor nutrition induces alata-production. Experiments with a newly developed continuous-flow artificial feeding apparatus for aphids showed that a nutritionally imbalanced diet may indeed favour wing production in case such a diet stimulates restlessness of the aphids. The tendency to follow an apterous course of development can be neutralized by inhibitors of the synthesis of indole alkylamines in new-born larvae, reared under conditions of controlled crowding stimuli. The possible significance of monoamine metabolism in wing formation of M. persicae is discussed. Resume SIGNIFICATION FONCTIONNELLE DES INDOL-ALKYLAMINES JOINTS AUX ELEMENTS NUTRITIFS POUR LA FORMATION DES AILES DE MYZUS PERSICAE Des larves presumees alatiformes, nees d'apTEres de Myzus persicae elevees sur des plantes, peuvent donner des apTEres quand elles sont transferees sur des milieux artificiels d'une composition appropriee. Ces milieux contiennent tous les elements nutritifs essentiels pour la croissance, seule la proportion des acides amines est suboptimale. Ces resultats sont en contradiction avec l'opinion generale qu'une nutrition deficiente a pour consequence la production d'alatiformes. A l'aide d'un appareil nouvellement mis au point et avec lequel il est possible d'assurer une alimentation continue des pucerons, on a pu mettre en evidence que la production d'alatiformes est possible sur un milieu nutritif suboptimal, mais il s'est revele que cela est du a l'augmentation de l'agitation des larves. Dans les larves nouvellement nees et elevees dans des conditions d'une surpopulation controlee, la tendance des pucerons a devenir apTEres peut etre neutralises par des inhibiteurs de la synthese des indol-alkylamines. L'importance du metabolisme des mono-amines dans la formation des ailes est discuTEe.

Raparadish, x Brassicoraphanus, the amphidiploid hybrid between Brassica rapa (syn. B.campestris) and Raphanus sativus (fodder radish) was made by Dolstra (1982). Primary hybrid plants grew vigorously, suggesting that the amphidiploid AARR might be useful as a fodder crop. Three populations of this new material were studied, with special attention to improvement of fertility and resistance to beet cyst nematode (Heterodera schachtii), whilst preserving genetic variability. For lack of progress one of the populations was abandoned after the fourth generation. The other two populations were observed through nine or ten generations. Apart from the last two generations mass selection for seed set was carried out on the basis of single plants. This led to a considerable increase in average seed production, without losing a wide variation for this trait. Thus more progress is being expected. Five cycles of mass selection for resistance to beet cyst nematodes led to a considerable increase of the level of resistance of both populations. The prospects of this new agricultural crop are discussed.

Oospore production inPeronospora farinosa f. sp.spinaciae was induced in spinach seedlings. This was done in detached cotyledons as wel as in undetached cotyledons on living plants, and in growth chambers as well as in the field. In young leaf tissue the fungus, produced oospores more often than in old tissue. The four spinach cultivars tested, viz. Breedblad Scherpzaad, Symphony, Novires and Huro, did not show significant differences in numbers of cotyledons with oospores, nor in numbers of oospores per cotyledon, though cv. Breedblad Scherpzaad was less susceptible to oospore formation in the field. Comparison of various inoculum densities showed a tendency to a smaller number of cotyledons with oospores with a lower inoculum density. Oospores were abundantly formed, when the plants were exposed to stress conditions in the second half of the latent period.

The assumption that the first flight of aphids from their secondary host is several kilometers long, followed by short hops, is mainly based on research done withAphis fabae. The validity of these observations has not been proved satisfactorily in the case of the peach aphidMyzus persicae. By using radiolabelled aphids their behaviour during the maiden flight was studied. The results indicate that the first flight can be as short as 1 to 100 m, which might have consequences for our views on the spread of viruses in the crop.

Less than ten years ago research on the feeding physiology of mites and aphids was hardly developed. Since then there have been a number of investigations about the influence of changes in the chemical composition of the leaves on the population development of insects and mites. It has nevertheless not yet been possible to acquire a useful understanding of this particular problem. This is parry due to the great diversity of the methods applied and parry to the fact that the special food requirements of the majority of parasites are as yet unknown and those known vary greatly. A comparison of the results is therefore extremely difficult and hampers an evalution of the data obtained. Within the framework of the Dutch Study Group on Integrated Control of Pests, the authors have studied the influence of the host plant's physiological condition 2 on the population development of phytophagous insects and mites. By changing the ion-ratio of the nutrient solution on which the plants are grown, chemical changes in the leaves are induced and differences in population development of the parasites can be evaluated. The experiments were set up as balance studies, that is to say, the whole of nutrient solution together with the plants and parasites was arranged as a closed system for certain nutrient elements. The distribution of a certain element over the three components, viz. nutrient solution, plant and phytophagous organism, is influenced by the composition of the solution, the ion uptake capacity of the plant and the food uptake and population density of the parasite. As a closed system was being considered, the initial quantity of the testelement had to equal the sum of the quantities recovered from the three corn

A number of essential processes in the development of Myzus persicae are initiated in early larval life. During this period the food quality often affects further development. This paper deals with the question whether amino acids, known to be necessary for developmental processes, play a specific role in diet acceptance. The amino acid methionine appeared to stimulate food uptake to a great extent. Experiments with diets containing 32P showed that food uptake within 1 hr by first-instar larvae on diets of unequal nutritive value was markedly different. Resume RELATION DE LA PERCEPTION GUSTATIVE DE MYZUS PERSICAE AVEC LA PRISE DE NOURRITURE ET SES CONSEQUENCES SUR LES MODALITES DU DEVELOPPEMENT. La qualiTE de la nourriture que recoit Myzus persicae pendant le debut de sa vie larvaire determine souvent les modaliTEs de son developpement ulTErieur (production de formes apTEres ou ailees par exemple). On a cherche a determiner si les acides amines, recormus comme etant necessaires au developpement, jo***uent seulement un role specifique dans le metabolisme des Aphides ou si ils interviennent comme facteurs stimulant la prise de nourriture. La methionine en particulier se revele avoir un role important comme phago-stimulant; en l'absence de methionine la quantiTE de nourriture absorbee est diminuee. Par contre sur des aliments depourvus d'histidine et d'isoleucine, le taux de nourriture absorbee ne change pas. Des experiences de choix alimentaire, entre deux substrats places cote a cote, ou disposes en couches superposees, montrent que dans le ler cas les Aphides peuvent se localiser sur le milieu le mieux equilibre en ces acides amines, et dans le second cas peuvent enfoncer leurs stylets jusqu'au milieu le plus favorable. L'adjonction de 32P a l'un des milieux permet d'apprecier la quantiTE de nourriture prelevee.

The fecundity and reproduction rate of caged Myzus persicae on leaves of potato plants growing on nutrient solutions with different anion and cation ratios were measured. The levels of total and soluble N in the leaves and the reproduction rate were found to be positively correlated with the amounts of N in the nutrient solutions. Longevity and reproduction rate were considerably less on 4-week-old plants than on 8-week-old plants; this difference is not correlated with the total N or soluble N-content of the leaves. When comparing aphid development on plants, daily variations in the amounts of N compounds in the leaves could provoke an erroneous basis for assessing the physiological condition of the host plant. Resume RELATION ENTRE LA NUTRITION MINERALE DE LA POMME DE TERRE ET LE POTENTIEL DE REPRODUCTION DE MYZUS PERSICAE Des pucerons Myzus persicae sont eleves individuellement dans des cages sur des feuilles de pommes de terre. Chaque experience comporte deux groupes de plantes les unes âgees d'un mois, les autres de deux mois. Les plantes sont alimentees avec des solutions nutritives dans lesquelles le rapport d'anions ou cations est different, mais qui demeurent ioniquement equilibrees. La reproduction de Myzus persicae, sur les plantes du meme âge, montre une correlation avec la quantite de N dans la solution nutritive et avec la teneur des feuilles en N total ou en N soluble. Le taux reproduction sur des plantes âgees de deux mois est en moyenne plus eleve que sur les plantes âgees d'un mois. Cette difference n'est pas en correlation avec la teneur des feuilles en N total ou N soluble. Sur des plantes âgees de deux mois le nombre de jours productifs par femelle est plus eleve que sur des plantes âgees d'un mois. La teneur des feuilles en composes azotes peut montrer des fluctuations quotidiennes importantes; pendant la duree de l'experience un echantillonnage frequent est indispensable a l'obtention de donnees reproductibles. Le rapport de K/Mg dans les solutions nutritives n'a pas une influence significative sur la fecondite des pucerons. La teneur en eau des feuilles n'est pas influencee par les differentes solutions nutritives ni par l'âge de la plante. Si on permet aux pucerons de se developper librement sur les plantes, on constate que la densite de population est la meme sur les differentes solutions nutritives. La nutrition minerale n'influence pas de facon significative les pourcentages de production d'insectes ailes.

Downy mildew resistance originating from Allium roylei Stearn provides a complete resistance to onions and is based on one, dominant gene. Since A. roylei can successfully be hybridized with onion (A. cepa L.), a breeding scheme aimed at the introgression of this gene was initiated ca. 20 years ago. Several setbacks in this programme were encountered, firstly the identified molecular marker linked to the downy mildew resistance locus became increasingly difficult to use and finally lost its discriminating power and secondly the final step, making homozygous introgression lines (ILs), turned out to be more difficult then was hoped. GISH analysis showed that the chromosomal region harbouring the resistance locus was the only remaining piece of A. roylei in the nuclear background of onion and it also confirmed that this region was located on the distal end of chromosome 3. It was hypothesized that some factor present in the remaining A. roylei region was lethal when homozygously present in an onion genetic background. The identification of an individual with a smaller and more distally located introgression fragment and homozygous ILs in its progeny validated this hypothesis. With the help of these nearly isogenic lines four AFLP® markers closely linked to the resistance gene were identified, which can be used for marker-aided selection. The introduction of downy mildew resistance caused by Peronospora destructor into onion is a significant step forward in the development of environmentally-friendly onion cultivars.Downy mildew resistance originating from Allium roylei Stearn provides a complete resistance to onions and is based on one, dominant gene. Since A. roylei can successfully be hybridized with onion (A. cepa L.), a breeding scheme aimed at the introgression of this gene was initiated ca. 20 years ago. Several setbacks in this programme were encountered, firstly the identified molecular marker linked to the downy mildew resistance locus became increasingly difficult to use and finally lost its discriminating power and secondly the final step, making homozygous introgression lines (ILs), turned out to be more difficult then was hoped. GISH analysis showed that the chromosomal region harbouring the resistance locus was the only remaining piece of A. roylei in the nuclear background of onion and it also confirmed that this region was located on the distal end of chromosome 3. It was hypothesized that some factor present in the remaining A. roylei region was lethal when homozygously present in an onion genetic background. The identification of an individual with a smaller and more distally located introgression fragment and homozygous ILs in its progeny validated this hypothesis. With the help of these nearly isogenic lines four AFLP (R) markers closely linked to the resistance gene were identified, which can be used for marker-aided selection. The introduction of downy mildew resistance caused by Peronospora destructor into onion is a significant step forward in the development of environmentally-friendly onion cultivars.

Suboptimal nutrition is often thought to stimulate wing formation in aphids by influencing the “milieu interieur” in a way that stimulates the process of wing determination.

1. 1. Wing dimorphism of the aphid Myzus persicae which normally is regulated by a number of exogenic factors can be influenced by neuropharmaca. 2. 2. With micro-injections of drugs that are known to act on monoamine metabolism it is possible to direct the course of development towards the alate or apterous morph. 3. 3. Serotonergic activity inhibits elongation of epidermal wing bud cells; this effect is antagonized by adrenergic compounds. 4. 4. The effects of the drugs tested suggest that factors in the “milieu interieur” regulating differentiation of the wing bud are mediated by neurotransmitters.