Summary 1.Parameter uncertainty challenges the use of matrix models because it violates key assumptions underlying elasticity analyses. We have developed a matrix model to compare Monte Carlo methods with elasticity analyses for estimation of the relative importance of factors in the asymptotic population growth rate, λ , of Cirsium vulgare (spear thistle) in Nebraska, USA. 2. We calculated λ for a base model using 11 parameter estimates available for Nebraska populations plus eight extracted from the literature, causing parameter uncertainty. We then calculated λ for 10 000 alternative models using Monte Carlo parameter estimation; parameters were drawn from the full range of each parameter in the literature and partial rank correlation analysis (PRCC) was used to order the parameters by the magnitude of their effect on λ . 3. Monte Carlo analysis found that insect floral herbivory, affecting the regeneration transition, was the most important parameter affecting λ , whereas elasticity analyses suggested that the transition from small to medium size was the most significant. Statistical comparison, using PRCC vs. lower level elasticity (LLE), showed that the Monte Carlo analysis provided a more accurate assessment. 4. As λ > 1 in 99% of the model runs even with significant floral herbivory, we added two parameters influenced by weed management (probability of large thistles dying without producing seed and proportion of seeds that failed to germinate). Simulations that included reductions in these parameters, along with floral herbivory, led to λ < 1 in 17% of the runs, suggesting these three factors interact to produce the low densities observed for this invasive thistle in our study area. 5. Synthesis and applications. This study demonstrates the utility of the Monte Carlo approach for modelling weed dynamics with parameter uncertainty and multiple, potentially interacting, parameters. Invasive population growth by C. vulgare could be limited by a combination of weed management practices and the biotic resistance imposed by native floral herbivores.

Insect herbivores can reduce growth, seed production, and population dynamics of host plants, but do not always do so. Big sagebrush (Artemisia tridentata) has one of the largest ranges of any shrub in North America, and is the dominant and characteristic shrub of the extensive sagebrush steppe ecosystem of the western United States. Nevertheless, the impact of insect herbivores on big sagebrush, its dominant and characteristic shrub, is largely unknown. Occasional large effects of insect herbivore outbreaks are documented, but there is little knowledge of the impact of the more typical, nominal herbivory that is produced by the diverse community of insects associated with big sagebrush in natural communities. In 2008, we removed insects from big sagebrush plants with insecticide to evaluate whether insect herbivores reduced growth and seed production of big sagebrush. Removal of herbivores led to significant and substantial increases in inflorescence growth (22%), flower production (325%), and seed production (1053%) of big sagebrush. Our results showed the impact of insect herbivory in the current growing season on the growth and reproduction of big sagebrush and revealed an unrecognized, significant role of non-outbreak herbivores on big sagebrush.

Postrelease studies can provide data with which to evaluate expectations based on prerelease tests of biological control insects. In 2004, we observed Trichosirocalus horridus Panzer (Coleoptera: Curculionidae), the rosette weevil introduced into North America against Eurasian thistles, feeding on native tall thistle, Cirsium altissimum L. Spreng., in tallgrass prairie. In this study, we examined the rosette weevil's use of tall thistle, compared with its use of the co-occurring exotic bull thistle, C. vulgare (Savi) Tenore. For both thistle species, we quantified weevil frequency, abundance, and seasonal variation in incidence, using both timed observations at two sites over two growing seasons (2004, 2005) and dissections of thistle flowering shoots from 13 sites (2005). Based on prerelease information, we expected the Eurasian thistle to be the quantitatively preferred host plant for this Eurasian weevil. Instead, we found that both the frequency of infestation and the mean number of adult rosette weevils per plant were at least as high, and sometimes higher, on the native thistle as on the exotic thistle. Furthermore, adult weevil phenology coincided on the two host species. This study provides new quantitative evidence of nontarget feeding by another weevil released for thistle biological control; and it raises important questions for further research. We conclude that continued new releases, as well as augmentation of existing populations, of T. horridus should wait until more research is done on the impact of the nontarget occurrence now reported for this biological control insect.