Centaurea Maculosa Research Articles

Neighbour Origin and Ploidy Level Drive Impact of an Alien Invasive Plant Species in a Competitive Environment.

Our understanding of the potential mechanisms driving the spread and naturalization of alien plant species has increased over the past decades, but specific knowledge on the factors contributing to their increased impact in the introduced range is still urgently needed. The native European plant Centaurea stoebe occurs as two cytotypes with different life histories (monocarpic diploids, allo-polycarpic tetraploids). However, only tetraploids have been found in its introduced range in North America, where C. stoebe has become a most prominent plant invader. Here, we focus on the ploidy level of C. stoebe and origin of neighbouring community in explaining the high impact during the invasion of new sites in the introduced range. We conducted a mesocosm experiment under open-field conditions with the diploid (EU2x) and tetraploid (EU4x) cytotype of Centaurea stoebe from its native European (EU) range, and with the invasive tetraploid (NA4x) cytotype from the introduced North American (NA) range in competition with EU (old) or NA (new) neighbouring plant communities. In the presence of competition, the biomass of EU neighbouring community was reduced to a comparable level by all three geo-cytotypes of C. stoebe. In contrast, the biomass of the NA neighbouring community was reduced beyond when competing with tetraploid, but not with diploid C. stoebe. The fact that the biomass of all three geo-cytotypes of C. stoebe was correlated with the biomass of the EU neighbouring community, but not with that of the NA neighbouring community suggests that different mechanisms underlie the competitive interactions between C. stoebe and its old vs. new neighbouring communities, such as competition for the same limiting resources at home vs competition through novel allelo-chemicals or differential resource uptake strategies in the introduced range. We therefore caution to simply use the ecosystem impact assessed at home to predict impact in the introduced range.

Linking Native and Invader Traits Explains Native Spider Population Responses to Plant Invasion.

Theoretically, the functional traits of native species should determine how natives respond to invader-driven changes. To explore this idea, we simulated a large-scale plant invasion using dead spotted knapweed (Centaurea stoebe) stems to determine if native spiders’ web-building behaviors could explain differences in spider population responses to structural changes arising from C. stoebe invasion. After two years, irregular web-spiders were >30 times more abundant and orb weavers were >23 times more abundant on simulated invasion plots compared to controls. Additionally, irregular web-spiders on simulated invasion plots built webs that were 4.4 times larger and 5.0 times more likely to capture prey, leading to >2-fold increases in recruitment. Orb-weavers showed no differences in web size or prey captures between treatments. Web-spider responses to simulated invasion mimicked patterns following natural invasions, confirming that C. stoebe’s architecture is likely the primary attribute driving native spider responses to these invasions. Differences in spider responses were attributable to differences in web construction behaviors relative to historic web substrate constraints. Orb-weavers in this system constructed webs between multiple plants, so they were limited by the overall quantity of native substrates but not by the architecture of individual native plant species. Irregular web-spiders built their webs within individual plants and were greatly constrained by the diminutive architecture of native plant substrates, so they were limited both by quantity and quality of native substrates. Evaluating native species traits in the context of invader-driven change can explain invasion outcomes and help to identify factors limiting native populations.

Flowering phenology influences bee community dynamics in old fields dominated by the invasive plant Centaurea stoebe

Wild and managed bees provide critical pollination services to both native and cultivated plants, and the invasion of exotic plants can have positive or negative effects on bee communities. In this study we investigated the influence of the exotic invasive Centaurea stoebe ssp. micranthos (spotted knapweed) on bee species diversity and abundance in old-fields (Michigan, USA). We conducted field observations in knapweed dominated fields and adjacent fields with greater forb diversity to determine whether the bee communities foraging in each field type differed in their composition, diversity, and abundance, and to determine how bees’ use of floral resources changed throughout the growing season. This was coupled with a common garden experiment that contrasted the attractiveness of C. stoebe to 12 native plant species, which occur in similar habitats. Both studies were conducted throughout the 2012–2013 growing seasons to examine the temporal effect of floral resource availability. C. stoebe was the most heavily-visited plant, in terms of total bee visitation and bee species richness in both studies. While C. stoebe-dominated old-fields had high floral resource levels during its peak bloom period, before and after this period these fields contained very few floral resources. In contrast, diverse fields had a number of flowering plant species that provided floral resources before, during, and after C. stoebe bloom. As a result, diverse fields contained significantly greater season-long floral resource availability and significantly greater season-total bee abundance, diversity and species richness. This greater species richness was driven at least in part by the ability of diverse flowering plant communities to support bee species that are active before and after the bloom period of C. stoebe. Our results suggest that efforts to manage C. stoebe should take into account the floral resources the plant is providing, and coincide with the restoration of diverse forb communities in order to enhance bee foraging habitat in old-fields.

The population genetics of the fundamental cytotype-shift in invasive Centaurea stoebe s.l.: genetic diversity, genetic differentiation and small-scale genetic structure differ between cytotypes but not between ranges

Polyploids are overrepresented in invasive species. Yet, the role of genetic diversity and drift in colonization success of polyploids remains unclear. Here, we investigate genetic diversity, genetic differentiation and small-scale genetic structure in our model system, the three geo-cytotypes of Centaurea stoebe: monocarpic diploids and polycarpic (allo)tetraploids coexist in the native range (Eurasia), but only tetraploids are reported from the invasive range (North America). For each geo-cytotype, we investigated 18–20 populations varying in size and habitat type (natural vs. ruderal). Population genetic analyses were conducted at eight microsatellite loci. Compared to diploids, tetraploids revealed higher genetic diversity and lower genetic differentiation, whereas both were comparable in tetraploids between both ranges. Within spatial distances of a few meters, diploid individuals were more strongly related to one another than tetraploids. In addition, expected heterozygosity in diploids increased with population size and was higher in natural than in ruderal habitats. However, neither relationship was found for tetraploids. The higher genetic diversity of tetraploid C. stoebe may have enhanced its colonization abilities, if genetic diversity is correlated with fitness and adaptive capabilities. Furthermore, the inheritance of a duplicated chromosome set as well as longevity and frequent gene flow reduces drift in tetraploids. This counteracts genetic depletion during initial introductions and in subsequent phases of small or fluctuating population sizes in ruderal habitats. Our findings advocate the importance of studying colonization genetic processes to gain a more mechanistic understanding of the role of polyploidy in invasion dynamics.

Larinus minutus (Coleoptera: Curculionidae) and Urophora quadrifasciata (Diptera: Tephritidae), Evidence for Interaction and Impact on Spotted Knapweed in Arkansas.

Interactions among seed-feeding insects are well noted in the literature, with many of these interactions being asymmetrical and causing a disadvantage for one of the species involved. While often effective, the use of the cumulative stress approach to biological control (where multiple natural enemy species are released) may increase the risk of interaction, which may lead to less effective biological control programs. Spotted knapweed (Centaurea stoebe ssp. micranthos (Gugler) Hayek) is an invasive plant that causes damage in rangelands and pastures. In Arkansas, two biological control agents for spotted knapweed, Larinus minutus Gyllenhal and Urophora quadrifasciata Meigen, are established. Both species attack the capitula of spotted and diffuse knapweeds. There is potential for interactions between these species, and this could affect the efficacy of the biological control program. To evaluate these potential interactions, we collected spotted knapweed capitula weekly from three sites in Arkansas from 2010 to 2012. Capitula were monitored for emergence of L. minutus and U. quadrifasciata, and the effect on seed reduction was analyzed. Over all three years, L. minutus and U. quadrifasciata were present in 35.8% (492) and 22.8% (313) of the capitula, respectively. We determined that the occurrences of L. minutus and U. quadrifasciata were not independent of each other, and that L. minutus is more effective at reducing spotted knapweed seed production than U. quadrifasciata within individual capitula. Seed production in capitula with both natural enemies present was no different from capitula with L. minutus.

Combined Herbivory by Targeted Sheep Grazing and Biological Control Insects to Suppress Spotted Knapweed (Centaurea stoebe)

AbstractThe use of biological control insects is a promising option for suppressing spotted knapweed, a nonindigenous perennial forb that infests more than 3 million hectares of North American rangeland. Efficacy increases when spotted knapweed is attacked by more than one phytophagous insect; however, combined herbivory by biological control insects has not achieved widespread suppression of spotted knapweed in North America. Here we expand the concept of combined herbivory beyond two or more species of biological control insects to include a vertebrate herbivore, specifically targeted grazing by domestic sheep. Our experiment on foothill rangeland in northwestern Montana evaluated spotted knapweed response to three treatments: (1) biological control insects only, (2) biological control insects + targeted sheep grazing applied in late July (spotted knapweed in late bud–early flower stage), and (3) biological control insects + targeted sheep grazing applied in mid-August (spotted knapweed in full-flower stage). We combined targeted sheep grazing with herbivory by three species of biological control insects: knapweed flower weevil, knapweed root weevil, and sulfur knapweed root moth. Treatments were applied during four consecutive years (2009 to 2012). Spotted knapweed fitness was suppressed more where targeted sheep grazing and biological control insects were combined vs. areas treated with biological control insects alone. Combined herbivory was effective when targeted sheep grazing was applied during either late July or mid-August, but July grazing was more effective. Spotted knapweed produced 96 to 99% fewer viable seeds in sheep-grazed areas. After 4 yr of treatment, total spotted knapweed plant density (seedlings, juvenile, and adult plants) was 86% less in July-grazed areas and 61% less in August-grazed areas than in areas treated with biological control insects alone. Combined herbivory by targeted sheep grazing and biological control insects reduced adult plant density and prevented compensatory recruitment of spotted knapweed, but treatment with biological control insects alone did not.

An exotic invasive plant selects for increased competitive tolerance, but not competitive suppression, in a native grass.

Exotic invasive plants can exert strong selective pressure for increased competitive ability in native plants. There are two fundamental components of competitive ability: suppression and tolerance, and the current paradigm that these components have equal influences on a species' overall competitive ability has been recently questioned. If these components do not have equal influences on overall ability, then selection on competitive tolerance and suppression may be disproportionate. We used naturally invaded communities to study the effects of selection caused by an invasive forb, Centaurea stoebe, on a native grass, Pseudoroegneria spicata. P. spicata plants were harvested from within dense C. stoebe patches and from nearby uninvaded areas, divided clonally into replicates, then transplanted into a common garden where they grew alone or competed with C. stoebe. We found that P. spicata plants collected from within C. stoebe patches were significantly more tolerant of competition with C. stoebe than P. spicata plants collected from uninvaded areas, but plants from inside invaded patches were not superior at suppressing C. stoebe. These results are consistent with the hypothesis that strong competitors may select for tolerance to competition more than for the ability to suppress neighbors. This has important implications for how native plant communities may respond to invasion over time, and how invasive and native species may ultimately coexist.

Temporal‐ and density‐dependent impacts of an invasive plant on pollinators and pollination services to a native plant

AbstractPollinators and pollination services are under threat globally, and invasive plants have been implicated in their decline. Results of previous studies suggest that consequences of invasion for pollinators and plant–pollinator interactions are context specific. Investigating factors such as the density of an invasive plant and its phenology may provide a nuanced understanding of invasive species impacts. We conducted a 2‐yr study in Montana to investigate how local pollinator abundance, richness, community composition, and visitation patterns varied with invasive Centaura stoebe density and phenology, and whether C. stoebe altered the reproduction of a co‐flowering native plant, Heterotheca villosa, through changes in pollinator visitation. In an observational study, we found that during its peak bloom in August, Centaurea stoebe provided abundant floral resources to late‐season pollinators. However, prior to C. stoebe bloom, native floral density and pollinator abundance and richness of these plots were lower compared to plots where C. stoebe was low or absent. Pollinator community composition in plots without C. stoebe was different compared to plots with C. stoebe (both high and low C. stoebe density), and these differences in pollinator composition strongly depended on the time of season. In an experimental study, we found that there was little evidence of competition between C. stoebe and H. villosa for pollinators at low relative densities of C. stoebe. Using experimental pollen supplementation, we observed no evidence of pollen limitation of seed set in H. villosa with increasing density of experimentally added C. stoebe. Our results suggest that the impact of an invasive plant on pollinators and plant–pollinator interactions depends on the relative density of the invasive plant and the timing of its bloom. Differences in pollinator visitation patterns over the growing season suggest that although C. stoebe provides abundant resources to late‐season pollinators, displacement of native plants at high C. stoebe density may indirectly harm pollinators that are active before C. stoebe blooms or that prefer native plants. Based on our results, restricting C. stoebe to low densities may help mitigate negative repercussions to native plant reproduction and may even be beneficial to some pollinators.

Simulated nitrogen deposition influences the growth and competitive ability of Centaurea stoebe populations

Simulated nitrogen deposition influences the growth and competitive ability of Centaurea stoebe populations

Effects of invasive knapweed (Centaurea stoebesubsp.micranthos) on a threatened native thistle (Cirsium pitcheri) vary with environment and life stage

We examined the type and magnitude of invasive plant (Centaurea stoebe subsp. micranthos (Gugler) Hayek) effects on the demographic parameters of a rare plant (Cirsium pitcheri Torr. & A.Gray), using two seed addition experiments with three treatments in two dune habitats — lakeshore linear dunes and high perched dunes on Lake Michigan, USA. Treatments were near Centaurea stoebe subsp. micranthos, near a native plant, or in the open. Seedling emergence, establishment, and juvenile survivorship were significantly higher on perched dunes, with higher surface soil moisture, than on lakeshore dunes. Treatment effects varied significantly with habitat and life stage. Seedling establishment decreased near Centaurea stoebe subsp. micranthos on lakeshore dunes, but increased near any neighbor on perched dunes. However, juvenile survival was significantly lower near Centaurea stoebe. subsp. micranthos in both habitats. The probability of reaching the adult flowering stage was much lower, and occurred later, for plants near Centaurea stoebe subsp. micranthos, especially on lakeshore dunes. Thus, Centaurea stoebe subsp. micranthos had a strong negative effect on establishment, survival, and flowering of Cirsium pitcheri, but interaction intensity varied with habitat and life stage. The results suggest that coupling seed addition with watering and invasive plant removal could bolster populations of this rare endemic plant.

Diversity Increases Indirect Interactions, Attenuates the Intensity of Competition, and Promotes Coexistence.

A fundamental assumption of coexistence theory is that competition inevitably decreases species diversity. Consequently, in the quest to understand the ecological regulators of diversity, there has been a great deal of focus on processes with the potential to reduce competitive exclusion. However, the notion that competition must decrease diversity is largely based on the outcome of two-species interaction experiments and models, despite the fact that species rarely interact only in pairs in natural systems. In a field experiment, we found that competition among native perennial plants in multispecies assemblages was far weaker than competition between those same species in pairwise arrangements and that indirect interactions appeared to weaken direct competitive effects. These results suggest that community assembly theory based on pairwise approaches may overestimate the strength of competition and likelihood of competitive exclusion in species-rich communities. We also found that Centaurea stoebe, a North American invader, retained strong competitive effects when competing against North American natives in both pairwise and multispecies assemblages. Our experimental results support an emerging body of theory suggesting that complex networks of competing species may generate strong indirect interactions that can maintain diversity and that ecological differentiation may not be necessary to attenuate competition.

Origin matters: diversity affects the performance of alien invasive species but not of native species.

At local scales, it has often been found that invasibility decreases with increasing resident plant diversity. However, whether resident community diversity similarly resists invasion by alien versus native species is seldom studied. We examined this issue by invading constructed native plant assemblages that varied in species and functional richness with invasive alien or native Asteraceae species. Assemblages were also invaded with spotted knapweed, Centaurea stoebe, a native European aster that has been previously used in diversity-invasibility experiments in North America. We also conducted a field survey to explore the generality of the patterns generated from our experimental study. Both experimental and observational work revealed that increasing diversity reduced the performance of alien but not native invaders. Centaurea stoebe invading its native community performed poorly regardless of resident diversity, whereas in a parallel, previously published study conducted in North America, C. stoebe easily invaded low-diversity but not high-diversity assemblages. Our results suggest that diversity is an attribute of resident communities that makes them more or less susceptible to invasion by novel invasive alien but not native plant species.

Native parasitoids associated with the biological control agents of Centaurea stoebe in Montana, USA

Under classical biological control, agents are released against target non-native organisms based on the assumption that invasiveness occurs due to a lack of adapted natural enemies. Biological control agents themselves, however, can become prey to native predators, parasites and parasitoids in their introduced environment with the potential to inhibit their effectiveness. The goal of this study was to identify parasitoids that attack the biological control agents of Centaurea stoebe L. in western Montana, United States. Roots and seedheads of C. stoebe were collected from 45 sites over a two-year period, and monitored for insect emergence. Of the thirteen biocontrol agents released against C. stoebe in Montana, ten were reared from the plant material collected. Nine species of parasitoid emerged, four of which were previously unknown associations with these biocontrol agents: Bracon sp., Pronotalia carlinarum Szelényi & Erdös, Eupelmus vesicularis Retzius, Scambus brevicornis Gravenhorst, Pteromalus cardui Erdös, Pteromalus elevatus Walker, two unknown species of Pteromalus, and one unknown species of Mesopolobus. Host associations were determined for three of the parasitoid species, and others were inferred based on previous studies. Parasitism rates of Urophora affinis by P. carlinarum were variable by location and time of sample collection, but were surprisingly high (reaching 100% in some cases) considering this is the first record of this host–parasitoid relationship and previous studies of U. affinis in this region found low levels of parasitism by other species. The long-term vulnerability of biocontrol agents to parasitism and predation by native organisms is a concern for the practice of classical biological control, especially for agents that have been established for several decades, and thus merits further research attention.

Community structure and diversity of vegetation and flower-visiting wild bees (Hymenoptera: Apoidea) in sandy dry grassland: are there congruent characteristics?

We studied community structures of mainly threatened types of sandy dry grassland, and those of flower-visiting wild bees, in successional, non-ruderal or ruderal gradients in the Upper Rhine Valley (Germany). We searched for congruent characteristics in the floristic/faunistic structure of plant and bee species, and for differences between two study years. We also asked which vegetation type or complex plays the most important role as a pollen or nectar resource for wild bees, and what conclusions can be drawn for nature conservation. The vegetation types or complexes range from basiphytic pioneer stages to consolidated basiphytic to slightly acidophytic grasslands (classes Koelerio-Corynephoretea and Festuco-Brom- etea): Koelerion glaucae complex (1), Armerio-Festucetum trachyphyllae (2a, typical; 2b, ruderalized), ruderalized ex-arable field (3), and Allio-Stipetum capillatae (4). We used a plot-based approach for recording vegetation data and flower-visiting wild bees. Releves were sampled and flower- visiting bees netted on defined flower resources (2004, 2005). Data were analyzed by ordination, mixed linear models and regres- sion analysis. Phytosociological table and DCA show clear floristic differentiations between the vegetation types and complexes. For the bee data there were weaker species-based differentiations. In the year 2004 there were two bee assemblages, in the year 2005 this dif- ferentiation was not detectable. The most species- and individual-rich wild-bee populations were detected in the vegetation types 2b, 3 and 4. These types were characterized by a high diversity of entomophilous plant species in combination with high flower density. Although ruderalized sandy grasslands have only moderate conservation value based on vegetation characteristics, they offer remarkable resources for species-rich populations of wild bees. Important flower resources for endangered wild-bee species are, among others, Centaurea stoebe s.l., Berteroa incana and Carduus nutans. The diversity characteristics of the studied vegetation types or complexes do not always correspond to those of the flower-visit- ing bees, which should be taken into account in developing conservation strategies.

Native North American pine attenuates the competitive effects of a European invader on native grasses

Strong direct competitive interactions between two species can be attenuated or exacerbated by the asymmetric effects of other species. In this context we explored a natural pattern in which an exotic invader, Centaurea stoebe, was much less abundant under ponderosa pines than in open grassland. In a field experiment Centaurea had strong competitive effects on the native Pseudoroegneria spicata in open grassland. Pines had no effects on either Centaurea or Pseudoroegneria when they were grown alone, but the strong competitive effects of Centaurea on Pseudoroegneria found in open grassland did not occur under pine canopies. In common garden experiments we found that shade mimicking one effect of pine canopies inhibited Centaurea but did not affect the native Festuca idahoensis. Shade did not alter competitive outcomes between the native and the invader. In the same experiment, soil from under pines increased Centaurea size and its competitive effects on Festuca, the opposite of what would be predicted from natural patterns in the field. However, pine litter strongly suppressed Centaurea in field and greenhouse experiments, inhibited growth of Festuca when grown alone, and importantly also eliminated competition between Festuca and Centaurea. Overall, our results suggest that ponderosa pine may resist Centaurea invasion both directly and by attenuation of the invader’s competitive effects. This biotic resistance appears to occur in part through asymmetric direct effects of pine litter, and in part from ways in which the litter, canopy effects, and soil under pines modify interactions between Centaurea and natives.

Phenology and dispersal of Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae), two biological control agents of Centaurea stoebe ssp. micranthos (spotted knapweed) in Michigan

Centaurea stoebe L. ssp. micranthos (Gugler) (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. The seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) are two of the most-widely released C. stoebe control agents, and have more recently been introduced into the eastern US. While there have been many studies focusing on their ability to impact C. stoebe in the western US and Canada, there have been few studies from eastern North America, and basic knowledge of important aspects of their biology is lacking. Here we report on the phenology and dispersal of L. minutus and L. obtusus in Michigan. We regularly sampled two established Larinus spp. populations in southern Michigan in 2012 and 2013, and found that while adult abundance fluctuates during the growing season, they remained at easily detectable levels from mid-June through the end of August. We also used previously established populations of L. minutus and L. obtusus released in 1996 (n=1), 2007 (n=2), and 2010 (n=5) to determine how dispersal of Larinus spp. into the surrounding landscape changes with time since release. Populations of Larinus spp. weevils showed little dispersal for 2years post-release. However, after initial establishment dispersal rates increased rapidly, resulting in average dispersal rates that increased exponentially with time since release. These findings can inform future biological control release and sampling programs for Larinus spp. in eastern North America.

Native Plant Establishment Success Influenced by Spotted Knapweed (Centaurea stoebe) Control Method

Invasive species frequently need to be controlled as part of efforts to reestablish native species on degraded sites. While the effectiveness of differing control methods are often reported, the impacts these methods have on the establishment of a native plant community are often unknown. To determine methods that effectively reduce spotted knapweed ( Centaurea stoebe ) while enhancing native species establishment, we tested 12 treatment combinations consisting of an initial site preparation (mowing, mowing + clopyralid, or mowing + glyphosate), in factorial combination with annual adult knapweed hand pulling and/or burning. We established 48 plots and applied site preparation treatments during summer 2008, seeded 23 native forbs and grasses during spring 2009, pulled adult knapweed annually from 2009–2012, and burned in the early spring 2012. During July of 2011 and 2012, percent cover of all species was visually estimated. By 2011, seeded species had established in all treatment plots, including plots that retained greater than 50% knapweed cover, indicating that native species successfully established despite knapweed dominance. Mowing alone had no longterm impacts on community development. Clopyralid favored non-native grass establishment, while glyphosate encouraged non-native forbs. Clopyralid had minimal impacts on native forb establishment, but did effectively control knapweed. Pulling reduced knapweed cover, increased non-native grass cover and enhanced native species establishment. Burning had little impact, possibly due to low intensity and unseasonable weather. On the heavily invaded site we studied, combining the use of clopyralid with hand pulling effectively controlled knapweed and favored the establishment of seeded native grasses and forbs.

Drought tolerance and plasticity in the invasive knapweed Centaurea stoebe s.l. (Asteraceae): effect of populations stronger than those of cytotype and range.

Spotted knapweed (Centaurea stoebe s.l., Asteraceae) is native to Europe, where it occurs as a diploid (2xEU) and tetraploid cytotype (4xEU), but so far only the tetraploid has been reported in the introduced range in North America (4xNA). In previous studies, significant range shifts have been found towards drier climates in 4xEU compared with 2xEU, and in 4xNA when compared with the native range. In addition, 4x plants showed thicker leaves and reduced specific leaf area compared with 2x plants, suggesting higher drought tolerance in 4x plants. It is thus hypothesized that the 4x cytotype might be better pre-adapted to drought than the 2x, and the 4xNA better adapted than the 4xEU due to post-introduction selection. Plants of the three geocytotypes (2xEU, 4xEU and 4xNA ), each represented by six populations, were subjected to three water treatments over 6 weeks in a greenhouse experiment. Plasticity and reaction norms of above- and below-ground biomasses and their ratio, survival rate, stomatal conductance and carbon isotope discrimination were analysed using linear and generalized linear mixed effect models. Above-ground and total biomasses of European tetraploids were slightly less affected by drought than those of European diploids, and 4xEU plants maintained higher levels of stomatal conductance under moderate drought than 4xNA plants, thus supporting the pre-adaptation but not the post-introduction evolution hypothesis. Plasticity indexes for most of the traits were generally higher in 2xEU and 4xNA than in 4xEU plants, but these differences were not or were only marginally significant. Interestingly, the effect of population origin and its interaction with treatment was more important than the effects of geocytotype and range. Population means for the control treatment showed several significant associations either with latitude or some aspect of climatic data, suggesting evolution of local adaptations, especially within the 2xEU and 4xEU geocytotypes.

Plant neighbours rather than soil biota determine impact of an alien plant invader

Summary Various factors have been shown contributing to the ecosystem impact of invasive alien plants, but their relative importance remains unclear. We focused on the effects of neighbouring plant community and soil biota as these biotic factors have been repeatedly put forward to explain invasion success (e.g. as components of the novel weapons and of the biotic release hypothesis). To assess their relative importance in explaining the high impact of Centaurea stoebe during the invasion of new sites in the introduced range, we conducted a greenhouse experiment with both European (EU) and North American (NA) tetraploid C. stoebe competing with/without EU vs. NA neighbouring community and with the two neighbouring communities growing without C. stoebe. Plants were grown in sterilized commercial soil inoculated with naïve soil (from rhizosphere of plants other than C. stoebe) originating either from the home EU or the introduced NA range and half of which was sterilized to remove soil biota. In the competition pots, relative competitive ability (difference between the relative growth rates of C. stoebe and neighbouring community) and impact level (biomass of the neighbouring community relative to that in non‐competition pots) of C. stoebe was significantly higher when grown with the NA than with the EU neighbouring community, although growth rates of EU and NA neighbouring communities did not differ in the non‐competition pots. Both soil origin and C. stoebe origin had no effect on these processes. Soil sterilization increased growth of both C. stoebe and neighbouring communities, but had only a moderate effect on impact level and type, and no effect on the relative competitive ability of C. stoebe. These results suggest that during the colonization of new sites in North American grasslands, the impact of C. stoebe is strongly driven by reduced competitive ability of NA neighbours compared with EU neighbours, while altered biotic soil conditions in the introduced range and post‐introduction evolutionary changes in the invader are of less importance. This differential impact appears to be due to inherently different mechanisms underlying the competitive interactions between EU and NA neighbouring communities when grown with C. stoebe.

Release and Monitoring ofLarinus minutus(Coleoptera: Curculionidae), a Biological Control Agent of Spotted Knapweed in Arkansas

Spotted knapweed (Centaurea stoebe ssp. micranthos (Gugler) Hayek) is an invasive plant from Eurasia that degrades pastures and rangelands in the western United States and Canada. A biological control program for spotted and diffuse (C. diffusa Lamarck) knapweeds was initiated in the 1960s, with 13 species of insects introduced and established in the United States and Canada. The program has largely been considered a success and Larinus minutus Gyllenhal (Coleoptera: Curculionidae) is considered to be one of the key agents responsible for the reduction of knapweeds. Spotted knapweed expanded its range into the southeastern United States, but biological control was not used against the weed in this region until recently. Larinus minutus were released in 6 counties in Arkansas from 2008 through 2012. Weekly sweep-net sampling to monitor weevil activity showed establishment of the weevils in 5 counties. The numbers of L. minutus captured at each site increased from 2010 to 2012. Larinus minutus in Arkansas was univoltine and sex ratios were 1:1, except during flowering, when the proportion of females captured increased. Eggs were not present in females until 1 week after flowering of spotted knapweed.