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

Variation in Herbivore-Mediated Indirect Effects of an Invasive Plant on a Native Plant

Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001–2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0–220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (≤ 5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle’s co-occurrence with the coevolved preferred exotic host plant and not from the weevil’s response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants

Variation in Herbivore-Mediated Indirect Effects of an Invasive Plant on a Native Plant

Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001–2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0–220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (≤ 5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle’s co-occurrence with the coevolved preferred exotic host plant and not from the weevil’s response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants

Sensitivity of Air Pollution-Induced Premature Mortality to Precursor Emissions Under the Influence of Climate Change

The relative contributions of PM2.5 and ozone precursor emissions to air pollution-related premature mortality modulated by climate change are estimated for the U.S. using sensitivities of air pollutants to precursor emissions and health outcomes for 2001 and 2050. Result suggests that states with high emission rates and significant premature mortality increases induced by PM2.5 will substantially benefit in the future from SO2, anthropogenic NOX and NH3 emissions reductions while states with premature mortality increases induced by O3 will benefit mainly from anthropogenic NOX emissions reduction. Much of the increase in premature mortality expected from climate changeinduced pollutant increases can be offset by targeting a specific precursor emission in most states based on the modeling approach followed here

Frustrated magnetic interactions and quenched spin fluctuations in CrAs

The discovery of pressure-induced superconductivity in helimagnets (CrAs, MnP) has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity. However, the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear. Here, we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure. Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV, which can be effectively described by the Heisenberg model with nearest neighbor exchange interactions. Most surprisingly, the spin excitations are largely quenched above the Neel temperature, in contrast to cuprates and iron pnictides where the spectral weight is mostly preserved in the paramagnetic state. Our results suggest that the helimagnetic order is driven by strongly frustrated exchange interactions, and that CrAs is at the verge of itinerant and correlation-induced localized states, which is therefore highly pressure-tunable and favorable for superconductivity.Comment: 6 pages, 4 figure