Temporal and spatial variations in the parasitoid complex of the horse chestnut leafminer during its invasion of Europe

The enemy release hypothesis posits that the initial success of invasive species depends on the scarcity and poor adaptation of native natural enemies such as predators and parasitoids. As for parasitoids, invading hosts are first attacked at low rates by a species-poor complex of mainly generalist species. Over the years, however, parasitoid richness may increase either because the invading host continuously encounters new parasitoid species during its spread (geographic spread-hypothesis) or because local parasitoids need different periods of time to adapt to the novel host (adjustment-hypothesis). Both scenarios should result in a continuous increase of parasitoid richness over time. In this study, we reconstructed the development of the hymenopteran parasitoid complex of the invasive leafminer Cameraria ohridella (Lepidoptera, Gracillariidae). Our results show that the overall parasitism rate increases as a function of host residence time as well as geographic and climatic factors, altogether reflecting the historic spread of C. ohridella. The same variables also explain the individual parasitism rates of several species in the parasitoid complex, but fail to explain the abundance of others. Evidence supporting the “geographic spread-hypothesis” was found in the parasitism pattern of Cirrospilus talitzkii (Hymenoptera, Eulophidae), while that of Pediobius saulius, another eulophid, indicated an increase of parasitism rates by behavioral, phenological or biological adjustments. Compared to fully integrated host-parasitoid associations, however, parasitism rates of C. ohridella are still very low. In addition, the parasitoid complex lacks specialists, provided that the species determined are valid and not complexes of cryptic (and presumably more specialized) species. Probably, the adjustment of specialist parasitoids requires more than a few decades, particularly to invaders which establish in ecological niches free of native hosts, thus eliminating any possibility of recruitment of pre-adapted parasitoids

Thermomechanisches Verhalten gefügter Divertormodule unter fusionsrelevanten Belastungen

In the present work the thermomechanical behaviour of four single-tube divertor modules and a prototypical divertor plate under heat fluxes, which are expected for future fusion reactors as NET or ITER, has been characterized. The specimens were made up of a heat sink of the molybdenum-alloy TZM with a cooling tube of Mo41Re and armour tiles of multi-directional carbon fibre reinforced carbon material (CFC) brazed to the heat flux facing side. Two different designs (with and without a TZM-mterlayer between CFC-tile and heat sink, respectively) were investigated. The prototype consisted of 8 cooling channels connected by collectors. The specimens were exposed both to stepwise increasing as well as to cyclic heat loads in an ion beam facility with hydrogen and helium beam, respectively, and in two electron beam machines (scanned and stationary beam, respectively) with pulse lengths allowing quasistationary thermal conditions to be achieved. Whereas in the electron beam tests the divertor modules resisted up to 20.7 MW/m$^{2}$ in a single pulse and up to 15 MW/m$^{2}$ under cyclic loading (300 cycles) without failure, damage occurred at significantly lower heat fluxes and already after few pulses in the ion beam experiments. As an explanation for the different behaviour embrittlement of the zirconium high temperature braze between CFC and heat sink under the influence of the hydrogen ion beam was found. Thermal loading results in high vertical temperature gradients during the transient as well as the stationary phases, causing bending of the armour tiles. Thus high stresses in the bonding zones arise, which can lead to detachment of the tiles. This damage mechanism could be observed in the experiments and was confirmed by numerical analyses of the resulting temperature and stress fields by using a finite-element- code. The early failure of the divertor prototype could be attributed to unfavourableorientation of the CFC-tiles, melting of the brazed joints caused by heat absorption in the gaps between the armour tiles and deteriorated cooling efficiency