Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

Spatial and temporal variation in insect floral herbivory is common and often important. Yet, the determinants of such variation remain incompletely understood. Using 12 years of flowering data and 4 years of biweekly insect counts, we evaluated four hypotheses to explain variation in damage by the Eurasian flower head weevil, Rhinocyllus conicus, to the native North American wavyleaf thistle, Cirsium undulatum. The four factors hypothesized to influence weevil impact were variations in climate, weevil abundance, phenological synchrony, and number of flower heads available, either on wavyleaf thistle or on the other co-occurring, acquired native host plant (Platte thistle, Cirsium canescens), or on both. Climate did not contribute significantly to an explanation of variation in R. conicus damage to wavyleaf thistle. However, climate did influence weevil synchrony with wavyleaf flower head initiation, and phenological synchrony was important in determining R. conicus oviposition levels on wavyleaf thistle. The earlier R. conicus was active, the less it oviposited on wavyleaf thistle, even when weevils were abundant. Neither weevil abundance nor availability of wavyleaf flower heads predicted R. conicus egg load. Instead, the strongest predictor of R. conicus egg load on wavyleaf thistle was the availability of flower heads on Platte thistle, the more common, earlier flowering native thistle in the sand prairie. Egg load on wavyleaf thistle decreased as the number of Platte thistle flower heads at a site increased. Thus, wavyleaf thistle experienced associational defense in the presence of flowering by its now declining native congener, Platte thistle. These results demonstrate that prediction of damage to a native plant by an exotic insect may require knowledge of both likely phenological synchrony and total resource availability to the herbivore, including resources provided by other nontarget native species

Population Growth of \u3ci\u3eRhinocyllus conicus\u3c/i\u3e (Coleoptera: Curculionidae) on Two Species of Native Thistles in Prairie

The insect Rhinocyllus conicus Froehlich is a flowerhead weevil deliberately introduced into the United States for the biological control of invasive exotic thistles in the genus Carduus. This study documents the course and magnitude of the weevil population expansion onto nontarget host plants. No weevils were reared from either Platte thistle, Cirsium canescens Nuttall, or wavyleaf thistle, C. undulatum (Nuttall) Sprengel, from 1977 to 1993 at 2 Sandhills prairie preserves: Arapaho Prairie and Niobrara Valley. For Platte thistle, the number of both R. conicus adults on plants and the number of larvae developing in flowerheads increased significantly from 1993 to 1996. Population growth lagged at the north-central Niobrara Valley site, compared with the southwestern Arapaho Prairie site, but by 1996 the densities attained were similar. For wavyleaf thistle, a later flowering native species, the 1st weevils were also observed in 1993 at both sites. However, weevil densities on wavyleaf thistle grew more slowly and remained significantly lower than those on Platte thistle. The most likely hypothesis to explain the greater use of Platte thistle, compared with wavyleaf thistle at these sites, is greater phenological synchrony of its flowerhead development with R. conicus oviposition activity. The results suggest that inclusion of ecological characteristics, such as phenology, in prerelease studies and completion of long-term, follow-up studies on releases would improve our understanding and evaluation of risk to native species from potential biological control agents

Beschäftigungseffekte des Außenhandels. Ein kritischer Beitrag zum Stand der Forschung

"Eine systematische und empirisch gehaltvolle Theorie der Beschäftigungswirkungen des Außenhandels ist bisher erst in partiellen Ansätzen erkennbar, die auf bestimmte Aspekte dieses Problemkreises abstellen. Der vorliegende Artikel versucht einen Überblick über den Stand der Forschung zu vermitteln und die besonderen Probleme aufzuzeigen, die bei einer empirischen Erfassung der Zusammenhänge zwischen Außenhandel und Beschäftigung auftauchen. Der Schwerpunkt der Untersuchung liegt bei der Analyse der kurzfristigen Beschäftigungseffekte des Außenhandels. Dabei geht es nicht nur um die unmittelbaren Einflüsse des Außenhandels auf Output und Beschäftigung (primäre Beschäftigungseffekte), sondern auch um die außenhandelsinduzierten Einkommensänderrungen im Inland und ihre Wirkungen auf Output und Beschäftigung (sekundäre Beschäftigungseffekte). Die längerfristigen Beschäftigungseffekte des Außenhandels kommen vor allem über die Wirkungen von Exporten und Importen auf Wirtschaftswachstum und Wirtschaftsstruktur zum Tragen. Im Anschluß an die theoretische Analyse werden Möglichkeiten und Probleme der InputOutput-Analyse bei der Quantifizierung der kurzfristigen Beschäftigungseffekte des Außenhandels diskutiert und die Resultate vorliegender empirischer Untersuchungen ausgewertet. Die theoretische Analyse der kurzfristigen Beschäftigungseffekte des Außenhandels führte zum generellen Ergebnis, daß sich gleich große reale Veränderungen von Exporten und Importen in der Regel nicht in ihrer Wirkung auf Einkommen, Output und Beschäftigung der inländischen Wirtschaft kompensieren, da die Einkommens- und Outputwirkungen von Export- und Importveränderungen nicht unabhängig von der Export- und Importstruktur sind und da bei unterschiedlicher Arbeitsproduktivität in den einzelnen Wirtschaftszweigen bereits Veränderungen in der Struktur des inländischen Outputs mit positiven oder negativen Beschäftigungseffekten verbunden sind. Die Input-OutputAnalyse vermag zwar beim gegenwärtigen Stand der Forschung keine exakten Aussagen über die tatsächlichen kurzfristigen Beschäftigungseffekte des Außenhandels zu liefern, vermittelt aber zumindest eine Vorstellung von den Größenordnungen und Strukturen der kurzfristigen Beschäftigungswirkungen von Exporten und (mit Einschränkungen) von Importen. Die Analyse der längerfristigen Auswirkungen des Außenhandels steht auf einem recht schwachen theoretischen und empirischen Fundament. Soweit empirische Ergebnisse vorliegen, bestätigen sie jedoch die Vermutung, daß Wirtschaftswachstum und Beschäftigung in der Bundesrepublik Deutschland durch die Wirkungen der Außenhandelsentwicklung auf Investitionen, Arbeitsproduktivität und technischen Fortschritt im allgemeinen positiv beeinflußt wurden."Außenhandel, Beschäftigung

Biological Control of Weeds in Great Plains Rangelands

Chemical control of weeds has increased agricultural productivity, but complete reliance on chemicals has serious drawbacks. These include high cost per acre, decreasing effectiveness, negative effects on plant community diversity, and increased opportunities for environmental contamination. One alternative is biocontrol, the use of biological factors that naturally limit weed populations. Long-term research goals focus on improving our knowledge of the processes that control and limit potential plant pests naturally and to use that knowledge to develop predictable, sustainable, low-cost, biologically-based weed management strategies. This paper reviews the ecological underpinnings of classical biological control of weeds, including basic research on the interaction of natural enemies with native thistles in Nebraska. The fundamentals for developing a biological weed control program are summarized and research in the central and northern Great Plains on applying biological controls to limit introduced noxious rangeland weeds is reviewed. This is followed by a discussion of one of the problems associated with biological control, the potential secondary effects of biocontrol agents on non-target plant species. The evidence suggests that biological control is an ecologically sound, sustainable and economical option for limiting introduced rangeland weeds. Biological control represents an attractive technique for limiting the negative impacts of high weed densities, while not eradicating the introduced weed. Expanding interest in sustainable agricultural systems and more environmentally- friendly pest control and the prohibitive costs of chemical control suggest that biological weed control is likely to become an increasingly important part of rangeland management

\u3ci\u3eScaptomyza nigrita\u3c/i\u3e Wheeler (Diptera: Drosophilidae), a Leaf Miner of the Native Crucifer, \u3ci\u3eCardamine cordifolia\u3c/i\u3e A. Gray (Bittercress)

The biology of Scaptomyza nigrita on its host plant, a native crucifer (Bittercress) in the Rocky Mountains, is described. Development of each stage in the life history was studied both in the field and in the laboratory. This is the first documentation of a host for S. nigrita. We examined the activity of adult flies in two adjacent habitats, sun and adjacent willow shade. Adult flies were more abundant on bittercress plants in sun-exposed versus in shaded areas, and were most active from mid-day to late afternoon. Female flies were significantly larger than male flies, but there were no differences in size of adults between the two habitats. Larval damage to bittercress is generally much greater on plants in sunny areas than on those in the shade, possibly due to the increased activity of ovipositing flies in sun-exposed areas

Re-test of Rhinocyllus conicus host specificity, and the prediction of ecological risk in biological control

Biological control is proposed as an ecological strategy to manage the threat of invasive plants, especially in natural areas. To pursue this strategy, we need to know that the host specificity criteria used to evaluate ecological risk with deliberate introduction of an exotic insect for biocontrol are sufficient to predict potential impact on native species. Host specificity is defined by adult feeding and oviposition preferences and larval development. One way to evaluate the criteria is to re-examine case histories where ecological effects are recorded, such as that of Rhinocyllus conicus Frölich. This flower head weevil, released in North America in 1968 to control exotic thistles like Musk thistle (Carduus nutans L), is now reducing seed production by multiple native North American thistle species (Cirsium spp.), and local population density of Platte thistle (Cirsium canescens Nutt.). We hypothesized that host specificity of R. conicus has changed since pre-release testing, providing an explanation for the unexpected magnitude of the documented ecological effects. Instead, when we re-tested host specificity of weevils naturalized over 28 generations, we found that host specificity has not changed. Naturalized adults of R. conicus showed strong feeding and oviposition preference for Musk thistle over Platte thistle. In addition, larval development by these weevils was faster and more successful (to larger size) on Musk thistle than on Platte thistle. Thus, our results indicate that a change in host specificity cannot explain the unexpectedly large build-up of R. conicus and significant ecological effect on Platte thistle. We conclude that accurate prediction of the potential level of impact on native host plants in the field requires further ecological information in addition to host specificity

Patterns of Resource Use by a Drosophilid (Diptera) Leaf Miner on a Native Crucifer

Distribution and damage of Scaptomyza nigrita Wheeler on its host (bittercress, Cardamine cordifolia A. Gray), a native perennial crucifer, were examined over two growing seasons in relation to leaf position. Concentrations of defensive compounds (glucosinolates) and of nutritive compounds (total nitrogen, free amino acids, soluble carbohydrates) were also examined. The fly-host plant relationship was studied in sun and shade habitats at two sites. Oviposition and leaf-mining damage were concentrated on the lower central leaves of a stem in both habitats. These mature leaves have lower glucosinolate concentrations than new leaves. Adult densities and larval feeding damage were consistently and significantly greater on plants in the sun than on those in the shade. Higher S. nigrita densities in the sun habitat and slightly higher soluble carbohydrate concentrations in sun leaves at the beginning of the growing season, rather than variation in defensive glucosinolate levels, are the most likely mechanisms determining higher levels of leaf mining on host plants in the sun habitat

Evidence for Natural Biological Control: Insects Decrease Survival and Growth of a Native Thistle

Native thistles, in contrast to exotic species, are seldom noxious weeds. In this study, we evaluated one hypothesis for this difference: that the growth and fitness of native thistles are limited by natural enemies. Specifically, we tested the effect of insect foliage feeding on the survival and growth of large and small rosettes of tall thistle, Cirsium altissimum, using an insecticide exclusion experiment. Large juveniles were proportionally more damaged than were small ones. Insecticide reduced insect feeding, including overall intensity of plant damage and area removed or damaged on each of the two longest leaves, compared to controls treated only with water. As a result, both leaf growth and plant size increased significantly by the end of the growing season. Growth parameters showing major increases with the reduction in insect herbivory included upper rootcrown diameter, the total number of leaves, and the length of the two longest leaves on both large and small rosettes. Insecticide also reduced the mortality of large juveniles to half that of the controls. Thus, the study shows that the suite of naturally occurring, coevolved, foliage-feeding insects significantly reduces the growth and survival of rosettes of this native thistle under field conditions. The results provide strong support for the hypothesis underlying biological control of weeds programs

INVESTIGATION OF COMPRESSIBLE FLOW THROUGH A TURBINE BLADE CASCADE FOR VARIOUS TRANSONIC FLOW REGIMES

This paper deals with the numerical simulation of 2D transonic flow through the SE1050 turbine blade cascade at various flow conditions. The first one concerns the design operation with a zero incidence angle involved in the ERCOFTAC Database CFD-QNET and the second one with a +20° incidence angle corresponding to an off-design operation. Advanced mathematical models with two different models of the bypass transition to turbulence were applied for the simulation of different regimes of transonic flows as well as with attached and separated flows. Transition models proposed by Dick et al. [1] and by Menter and Smirnov [2] are based on transport equations for the intermittency coefficient. Numerical results were compared with experimental data based on the optical and pressure measurements