Shortly after its invasion into Europe, the western flower thrips, Frankliniella occidentalis (Pergande), became a more severe pest of greenhouse crops than the onion thrips, Thrips tabaci Lindeman. To test whether this differential pest status was due to a larger capacity of population increase, a comparative life history study was carried out on cucumber (Cucumis sativus cv. Corona). Experiments at 25°C showed that the egg-to-egg period of F. occidentalis was shorter, but its peak ovipositional rate was lower and its offspring sex ratio more male biased. These differences resulted in a slightly lower intrinsic rate of population increase (rm) for F. occidentalis than for T. tabaci (0.166 vs. 0.176/day). It was shown experimentally that between 15 and 28°C, developmental rate of F. occidentalis is linearly related to temperature, with a theoretical threshold temperature similar to the value reported for T. tabaci (10.9 vs. 11.5°C). It is argued that the rm-value of F. occidentalis will not be higher than that of T. tabaci for any temperature within this range. Alternative explanations for the difference in pest status between the two thrips species are discussed.

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Summary. 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.

In this study genetic variation for resistance to the greenhouse whitefly (Trialeurodes vaporariorum) of four genotypes of tomato (L. esculentum) and two subspecies of L. hirsutum was investigated. Resistance was quantified by the whitefly life history components adult survival, oviposition rate, pre-adult survival and developmental period, measured on plants inoculated with whiteflies in clip-on cages.The largest differences between species were found when life history components were measured on adult plants of about four months old. On L. hirsutum f. glabratum whiteflies had the lowest adult survival, oviposition rate and pre-adult survival. On L. hirsutum these components were intermediate whereas on all L. esculentum genotypes they were highest. The variation between plants was low compared to the variation within plants. These results indicate that single plant tests can be used to determine accurately genetic variation between individual plants in a segregating population.

Variation in performance of western flower thrips populations on susceptible and partially resistant cucumber de Kogel, W.J.; van der Hoek, M.; Mollema, C. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Key words: Frankliniella occidentalis, Cucumis sativus, biotype, host plant resistance, life history, population growth rate AbstractBiotypic variation is of major concern in breeding for host plant resistance to insects. The existence or development of aggressive biotypes can lead to a rapid break-down of host plant resistance. Therefore the study of biotypic variation should be included in breeding programs for resistance to insects. In the present study we measured the reproduction of randomly collected females of ten different populations of the insect herbivore Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) on one susceptible and two resistant cucumber (Cucumis sativus L.) accessions. Significant differences between thrips populations were observed on all three cucumber accessions. None of the populations had a significantly higher reproduction than the Dutch reference population NL1. For three populations, the Dutch population NL1, a population from New Zealand (NZ), and an Italian population (IT), partial life history parameters, such as reproduction rate, developmental time and survival were determined and the relative rate of increase r r was calculated. On all three cucumber accessions the r r -value of population NZ was lower than of populations NL1 and IT. It is concluded that there is biotypic variation in F. occidentalis with regard to performance on cucumber plants with different levels of resistance. Reproduction is a good criterion for differentiating biotypes of F. occidentalis on cucumber.