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During 16 weeks absolute counts and random samples were carried out in a glasshouse to determine population density and distribution pattern of the greenhouse whitefly (Trialeurodes vaporariorum) and its parasite Encarsia formosa. The results obtained by both methods were compared, and it was concluded that the data from the random sampling reflected the actual numbers and the distribution of the whitefly and its parasite very badly. The results are too bad to recommend this method to growers for obtaining a reliable idea about the sites and the degree of an infestation on which they can base the number and the dates of parasite introductions. Zusammenfassung Zur Parasit-Wirt-Beziehung zwischen Encarsia formosa (Hym. Aphelinidae) und Trialeurodes vaporariorum (Homopt. Aleyrodidae). VIII. Vergleich und Bewertung zweier verschiedener Ermittlungen der Populationsdichte und des Verteilungsbildes Wahrend 16 Wochen wurden die Populationsdichte und das Verteilungsbild der Weisen Fliege (Trialeurodes vaporariorum) und ihres Parasiten (Encarsia formosa) in einem Gewachshaus unter Verwendung von zwei Methoden ermittelt: eine unmittelbare Zahlung aller Individuen und eine zufallige Stichprobennahme. Die Ergebnisse beider Methoden wurden verglichen. Es zeigte sich, das die Daten der zufalligen Stichproben die wirkliche Abundanz und das Verteilungsbild der Weisen Fliege und ihres Parasiten nur sehr schlecht wiedergaben. Um zuverlassige Daten uber die Lokalisation und Grose der verschiedenen Herden zu bekommen, auf denen die Zuchter die Anzahl und die Zeitpunkte der Parasiteneinfuhrungen grunden konnen, sollte man daher die Stichproben-Methode nicht wahlen.

We tested the hypothesis that two populations of the parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their behavioral interactions with the whitefly host Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). The two wasp populations were studied because previous work suggested large differences between the populations in performance on this host. In this study the populations differed behaviorally in both the number of hosts encountered and their reactions to hosts once encountered. The population reared for many years on Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) that previously performed more poorly on B. tabaci had a higher host encounter rate but rejected hosts more frequently. The population reared for a number of years on B. tabaci encountered fewer hosts but accepted a higher percentage of hosts for oviposition. The number of parasitized hosts did not differ between the two populations, however. These data demonstrate that there are heritable differences between these two populations of asexual wasps in host-associated behavioral traits. These behavioral differences in host acceptance do not explain performance differences seen in the earlier study, however, possibly due to different conditions between the two experiments.

International audience; With the establishment of the Plant Health Panel in 2006, EFSA became the body responsible for risk assessment in the plant health area for the European Union (EU). Since then more than 70 outputs have been produced dealing with the full range of organisms harmful to plant health across all crop types and plants in the environment. There has been an increasing trend towards producing scientific opinions which are full pest risk assessments for the whole EU territory. In its work, and as a contribution to the wider development of risk assessment methodology, the Panel has developed a series of guidance documents. These deal with the peer review of existing pest risk assessments, a framework for conducting risk assessments which harmonise standards set by the International Plant Protection Convention and the legislative requirements of the EU, and extension of this framework to include environmental risk assessment and the evaluation of risk reducing options. Quantitative approaches have become increasingly important during this time. The Panel has developed such methods in climatic mapping (in association with the Joint Research Councils), application of spatial spread models, re-evaluation of quantitative pathway analyses, and in statistical modelling of experimental data. A Plant Health Network has been established to facilitate interaction with EU Member States, especially in relation to data collection and co-ordination of risk assessment activities. At the current time a revision of the EU Plant Health Regime is being formulated. The legislative consequences of the revision will be of considerable significance for the work of the Plant Health Panel.

The interaction between the entomopathogenic fungus Aschersonia aleyrodis and the parasitoid Encarsia formosa on greenhouse whitefly as a host organism was studied, in particular, the survival of the parasitoid after treatment of parasitized hosts with fungal spores. The mean number of parasitized black pupae per parasitoid produced at 25°C was significantly reduced after spore treatment in the first three days following parasitization. Spore treatment four, seven or ten days after parasitization resulted in a mean number of parasitized pupae not significantly different from the number of black pupae in the control. The rather sudden change from low to high survival of parasitized hosts when treated with spores four days after parasitization in spite of high numbers of infected unparasitized larvae, coincided with the hatching of the parasitoid larva from the egg inside the host. Possible reasons for this decrease in susceptibility to infection after parasitoid egg hatch, such as induced changes in host cuticle or haemolymph, are discussed. Parasitoids emerged from treated hosts did not show differences in reproduction compared with parasitoids emerging from untreated hosts. Both natural enemies of whitefly are compatible to a great extent.

Invertebrate predators and parasitoids have long been characterized as having a hyperbolic (Type 2) functional response. Modifications were made to Holling's sand paper disc experiment which consisted of limiting the initial period of search during which a host must be contacted. Failure to contact a host during this initial period causes the predator to emigrate from the search area. The modification generated a sigmoid (Type 3) functional response. This response resulted from the low probability of encountering a host during the initial period of search at low host densities in the time allotted. A limited period of search has been found in several insect parasitoids. Such a strategy would minimize the time (energy) spent per offspring produced by minimizing the time invested in searching microhabitats in which hosts are scarce or absent.