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Invasive species may alter selective pressures on native plant populations. Although there is some evidence that competition with invasive plants may lead to differences in competitive ability between populations that have experienced invasion and those that have not, previous results have varied among species but also among populations of the same species. We conducted a glasshouse experiment to determine whether there was variation in traits or in ability to tolerate or suppress an invasive species among populations of a common native grass that had different histories of exposure to competition from an invasive species. Specifically, we grew seeds of a native grass (Pseudoroegneria spicata) collected from 14 wild populations (six from invaded populations and eight from uninvaded populations) and a cultivar (Anatone) alone or in competition with the invasive aster (Centaurea stoebe) and measured traits of both species during and at the end of a 100‐day growing period. Pseudoroegneria spicata seedlings from invader‐experienced populations had more leaves than invader‐naive populations, and juvenile plants from experienced populations were less affected by competition with C. stoebe than were plants from naive populations. There were significant differences in traits among populations at the seedling and juvenile life stages, and at both life stages variation among populations was greater than variation among experience types. The most predictive traits of P. spicata tolerance to competition were number of leaves (seedling and juvenile stage) and total and root biomass (juvenile only). No traits significantly predicted suppression of C. stoebe. There was not a significant relationship between a population's suppression of C. stoebe and its tolerance of competition. Our results suggest that, in P. spicata, invasion selects for larger plants and traits that can influence tolerance of competition. If land managers are interested in identifying highly competitive seed sources for revegetation in invaded areas, both population and invader experience type should be considered. As tolerance and suppression do not appear to be related in P. spicata, seed source selection should be driven by the element of competitive ability (either tolerance or suppression) that is most important to project goals. A plain language summary is available for this article.

In greenhouse experiments Sclerotinia sclerotiorum has killed seedlings of Centaurea stoebe, an invasive plant that is commonly known as spotted knapweed in Pacific Northwestern North America. Sclerotinia sclerotiorum is widespread in this invaded range, yet mortality of the plant due to S. sclerotiorum has not been observed in the field. Our greenhouse experiments address this paradox. To explore multitrophic interactions, four levels of inoculum were used: zero (control), and 1, 5, or 10 alginate pellets. Combinations of four organisms were varied: two fungi (phytopathogenic Sclerotinia sclerotiorum and mycoparasitic Trichoderma harzianum), one fungivorous nematode (Aphelenchoides saprophilus), and the host plant (Centaurea stoebe). Soil of potted knapweed was inoculated with T. harzianum (10 pellets per 200 g of soil) either (i) one week before adding either the nematodes or S. sclerotiorum, or both, or neither, or (ii) the same day when the nematodes and/or S. sclerotiorum were added, or (iii) left untreated with T. harzianum. The first, and overarching result, was confirmation that either 5 or 10 pellets of Sclerotinia sclerotiorum killed all seedlings 15 days post-application. In contrast, no mortality of seedlings was seen in the control but there was also no mortality with low inoculum (i.e., 1 pellet). Secondly, of 12 whole/subplot treatment combinations in total, six included phytopathogenic S. sclerotiorum, and of these six, the plants were killed in five. However, in one treatment combination (i.e., the addition of Aphelenchoides saprophilus nematodes, 2000 per 200 g of soil, one week after inoculation with T. harzianum) plant mortality due to S. sclerotiorum was suppressed. These findings suggest that both low inoculum dose and specific, multi-trophic microbial interactions may contribute to the absence of Sclerotinia disease of knapweed in its invaded range in the Pacific Northwest.

Eurasian seed-head weevils, Larinus obtusus Gyllenhal and Larinus minutus Gyllenhal (Coleoptera: Curculionidae), were released annually from 2004-2013 across Wisconsin for biological control of the invasive forb spotted knapweed ( Centaurea stoebe L. ssp. micranthos (Gugler) Hayek). In 2014, Larinus spp. presence/absence surveys were conducted at 18 previous releases sites between 7/18/2014 – 8/16/2014 from 2007 (n=3), 2008 (n=3), 2009 (n=3), 2010 (n=6), 2011 (n=1) and 2012 (n=2). We found a significant, linear relationship between time since release and the natural log of the observed dispersal rates of Larinus spp. (F 1,10 = 18.8, P = 0.002, R 2 = 0.65), suggesting an increasing dispersal rate through time. Modeled result suggested the following relationship: LN (Dispersal distance in km) = 0.258*time since release + 0.741. Because the model intercept (i.e. dispersal rate) did not pass through zero at age zero, which was expected from a biological basis, I warn against extrapolating the modeled dispersal rate to the period from release (t=0) to the end of Year One. Applying these dispersal rates to all 326 previous Larinus spp. releases in Wisconsin we predict that by 2017 low level populations of Larinus spp could be found state-wide.

Primary colonisation in invasive ranges most commonly occurs in disturbed habitats, where anthropogenic disturbance may cause physical damage to plants. The tolerance to such damage may differ between cytotypes and among populations as a result of differing population histories (adaptive differentiation between ruderal verus natural habitats). Moreover, founder populations often experience inbreeding depression, the effects of which may increase through physical damage due to inbreeding-environment interactions. We aimed to understand how such colonisation processes differ between diploid and tetraploid Centaurea stoebe populations, with a view to understanding why only tetraploids are invasive. We conducted a clipping experiment (frequency: zero, once or twice in the growing season) on inbred versus outbred offspring originating from 37 C. stoebe populations of varying cytotype, range and habitat type (natural versus ruderal). Aboveground biomass was harvested at the end of the vegetation period, while re-sprouting success was recorded in the following spring. Clipping reduced re-sprouting success and biomass, which was significantly more pronounced in natural than in ruderal populations. Inbreeding depression was not detected under benign conditions, but became increasingly apparent in biomass when plants were clipped. The effects of clipping and inbreeding did not differ between cytotypes. Adaptive differentiation in disturbance tolerance was higher among populations than between cytotypes, which highlights the potential of pre-adaptation in ruderal populations during early colonisation on anthropogenically disturbed sites. While the consequences of inbreeding increased through clipping-mediated stress, they were comparable between cytotypes, and consequently do not contribute to understanding the cytotype shift in the invasive range.

International audience; 1. Plant neighbours can strongly influence the interactions between herbivores and focal plants, for instance by providing food of different quality (consumptive effects) or by changing the behaviour and metabolism of the herbivore and the focal plant without being consumed (non-consumptive effects). Determining the species-specific contributions of consumptive and non-consumptive effects is important to understand the ecophysiological mechanisms which underlie neighbourhood effects. 2. We quantified the impact of nine different grassland plant species on the interaction between Taraxacum officinale and the root-feeding insect Melolontha melolontha. We investigated the contribution of consumptive and non-consumptive effects to the observed patterns, and evaluated the impact of neighbouring plants on the growth and physiology of T.officinale upon M.melolontha attack. 3. Melolontha melolontha growth was strongly affected by the presence of different neighbouring species. The three grass species increased larval growth when growing with T.officinale, with Poa pratensis having a synergistic effect in the mixture compared to both monocultures. The forb Centaurea stoebe reduced larval growth when growing with T.officinale or alone. The other five species had no measurable impact on larval performance. Based on these results, P.pratensis and C.stoebe were selected for further experiments. 4. Diet-mixing experiments showed that P.pratensis increased M.melolontha growth when offered together with T.officinale, while C.stoebe suppressed it. When feeding was restricted to artificial diet, larval growth was not changed by the presence of P.pratensis or C.stoebe. However, when feeding was restricted to T.officinale, larval growth was increased by both heterospecific neighbours. Biomass and primary metabolism of T.officinale under attack by M.melolontha was also altered by the presence of C.stoebe and P.pratensis. Together, these results show that consumptive and non-consumptive effects can explain the positive effect of P.pratensis. In contrast, the negative effect of C.stoebe is likely driven exclusively by intoxication. 5. Synthesis. The performed experiments suggest that different combinations of consumptive and non-consumptive effects are likely to contribute to the diversity of neighbourhood effects in nature. Furthermore, they show that neighbourhood effects are important factors in below-ground plant-insect interactions.

Volatile organic compounds (VOCs) emitted by plant leaves can influence the physiology of neighbouring plants. In contrast to leaf VOCs, little is known about the role of root VOCs in plant–plant interactions. Here, we characterize constitutive root VOC emissions of the spotted knapweed (Centaurea stoebe) and explore the impact of these VOCs on the germination and growth of different sympatric plant species. We show that C. stoebe roots emit high amounts of sesquiterpenes, with estimated release rates of (E)‐β‐caryophyllene above 3 μg g−1 dw hr−1. Sesquiterpene emissions show little variation between different C. stoebe populations but vary substantially between different Centaurea species. Through root transcriptome sequencing, we identify six root‐expressed sesquiterpene synthases (TPSs). Two root‐specific TPSs, CsTPS4 and CsTPS5, are sufficient to produce the full blend of emitted root sesquiterpenes. VOC‐exposure experiments demonstrate that C. stoebe root VOCs have neutral to positive effects on the germination and growth of different sympatric neighbours. Thus, constitutive root sesquiterpenes produced by two C. stoebe TPSs are associated with facilitation of sympatric neighbouring plants. The release of root VOCs may thus influence plant community structure in nature., Centaurea stoebe plants emit high amounts of root sesquiterpenes, which are produced by two root‐specific sesquiterpene synthases. Exposure to C. stoebe root volatiles increases the germination and growth of sympatric neighbours, thus highlighting the potential of root volatiles to mediate plant–plant interactions.

Spotted knapweed (Centaurea stoebe) is an invasive plant introduced into North America from Europe that has colonized diverse plant communities across the temperate zone of North America. The impacts of classical biological control insects on this plant have been identified as harmful (benefiting the plant), inconsequential, or exhibiting effective controls. Here we present results that measured C. stoebe densities in Colorado subjected to small numbers of insect releases in 2001 and monitored for density effects on the plant from 2007 to 2020. Seed head weevils (Larinus spp), gall flies (Urophora spp) and root weevils (Cyphocleonus achates) were also measured. In one monitoring transect with ample native perennial and introduced grasses, knapweed stem densities declined over a 14-year interval from about 50 stems/m2 to 0/m2. At a second site that was disturbed by flood scour midway through the study, stems declined over a 13 year interval from about 40 stems/m2 to under 20 stems/m2. At a site lacking C. achates, stem densities were measured at 40 stems per m2 in 2020, the same densities reported at other sites in 2008. No statistically detectable directional shifts in growing season precipitation or temperatures have occurred across the 2001–2020 interval, and the strongest relationships among variables measured here were directional declines in stem and rosette densities of the knapweed. Seeds/seed head showed positive relationships with precipitation but negative relationships with seed head Larinus spp densities, which in turn showed no directional trends in abundance. The outcome of biological control releases at these Colorado sites for C. stoebe ranged from excellent to negligible, and our work supports the context specific model of biological control, i.e., biological control in conjunction with plant competition are the essential components for reduction of C. stoebe densities.

Spotted knapweed, Centaurea stoebe L. (Asteraceae), is an invasive perennial forb that has become economically and ecologically damaging in North America. The weevil Larinus minutus Gyllenhal (Coleoptera: Curculionidae), a biological control agent of invasive knapweeds, was introduced to 37 sites in northwest Arkansas since 2008 as part of a biological control program for spotted knapweed. In 2011 and 2012, 25 of these release sites were surveyed to monitor how L. minutus infestation rates changed in relation to distance and time from release. The initial L. minutus introductions at these sites occurred from 2008 to 2011. Transects were used from the point of initial weevil introduction to establish sampling quadrats in which capitula were collected to document weevil presence and infestation rates. The mean maximum distance of weevil colonization and mean local abundance (within the first 50 m from the release point) were calculated in relation to time (yr) since release. Five sites had > 10 quadrats in both sampling years and were analyzed with an exponential decay function to model localized population growth and spread. Annual increases in mean local abundance and maximum distance of colonization were observed. These results were substantiated by localized growth (at 3 sites) and spread (at 2 sites) that occurred at the 5 sites analyzed with the exponential decay function. These findings suggest that in years following L. minutus introduction, assuming similar release strategies and environmental conditions, consistent increases in weevil infestation rates and spread from the release site may be expected in about half the sites at which populations establish. USDA-APHIS-PPQUnited States Department of Agriculture (USDA) We thank Edward Gbur Jr. and Fred Stephen for their input on experimental design, and Elizabeth Long for her feedback on the manuscript. This work was supported in part through a Cooperative Agreement with USDA-APHIS-PPQ.

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.

Many systems are prone to both exotic plant invasion and frequent natural disturbances. Native species richness can buffer the effects of invasion or disturbance when imposed in isolation, but it is largely unknown whether richness provides substantial resistance against invader impact in the face of disturbance. We experimentally examined how disturbance (drought/burning) influenced the impact of three exotic invaders (Centaurea stoebe, Linaria dalmatica, or Potentilla recta) on native abundance across a gradient of species richness, using previously constructed grassland assemblages. We found that invaders had higher cover in experimentally disturbed plots than in undisturbed plots across all levels of native species richness. Although exotic species varied in cover, all three invaders had significant impacts on native cover in disturbed plots. Regardless of disturbance, however, invader cover diminished with increasing richness. Invader impacts on native cover also diminished at higher richness levels, but only in undisturbed plots. In disturbed plots, invaders strongly impacted native cover across all richness levels, as disturbance favoured invaders over native species. By examining these ecological processes concurrently, we found that disturbance exacerbated invader impacts on native abundance. Although diversity provided a buffering effect against invader impact without disturbance, the combination of invasion and disturbance markedly depressed native abundance, even in high richness assemblages.

Uroleucon cichorii (Koch, 1855) Aphis intybii (Koch, 1855) and Aphis fabae cirsiiacanthoidis Scopoli, 1763 colonies occurred on Cichorium intybus L., Cirsium arvense (L.) Scop. was colonised by Aphis fabae cirsiiacanthoidis. Artemisia vulgaris L. accommodated Macrosiphoniella artemisiae (Boyer de Fonscolombe, 1841). The annual Crepis setosa Haller was the host plant of Uroleucon cirsii (Linnaeus, 1758) and Uroleucon cichorii. The perennial Picris hieracioides L. also accommodated Uroleucon cirsii. Centaurea arenaria M. Bieb. ex Willd. and Centaurea stoebe L. subsp. micranthos (Gugler) Hayek accommodated Uroleucon jaceae (Linnaeus, 1758). Carduus acanthoides L. was colonized by Uroleucon aeneum (Hille Ris Lambers, 1939). Artemisia absinthium L. was the host of Macrosiphoniella absinthii (Linnaeus, 1758). Achillea millefolium L. was colonized by Macrosiphoniella millefolii (De Geer, 1773).

Spotted knapweed is a prohibited noxious weed that is primarily controlled with auxinic herbicides. A population collected from a managed rangeland in East Kootenay, BC, was highly resistant to bot...

Increasing frequency and magnitude of climatic extremes, such as heat waves are expected to enhance abiotic stresses on ecological communities. It has been proposed that ecological communities in disturbed habitats may be most sensitive to climatic extremes, as disturbance may reduce density and diversity of higher trophic level organisms like predators. However, there is little experimental evidence that climatic extremes indeed have stronger impact on functioning of such trophically downgraded ecosystems. Here, we experimentally examine how removal of predators from soil communities affects plant performance under periodic heat waves. We used a native plant species, and a congeneric native that is currently expanding its range because of climate change. We used soil nematode communities as the model system, as these are most abundant soil animals and their communities are trophically diverse. Predatory nematodes were manually removed from intact soil nematode communities (mainly the adults as some juveniles are impossible to manually remove) to create a trophically downgraded soil. Intact nematode communities and communities with reduced predatory nematodes were added separately to soils that were planted with either the native Centaurea jacea or the range-expanding congener Centaurea stoebe. Half the experimental units were exposed periodically to experimental heat waves of 10 °C above the control temperature. Our results show that the C: N and C: N: P ratio of plant shoots in predator-reduced soils became lower when exposed to periodic heat waves, however, only in the native plant C. jacea. The decrease in C: N ratio corresponded with increase of an herbivorous nematode in trophically intact soils of C. jacea independent of warming, whereas this relationships disappeared in warmed and predator-reduced soils. Our results accordingly highlight that periodic heat waves may affect stoichiometry of certain plant species by altering trophic interactions in predator-reduced soils.

Biological invasions are of particular concern in grasslands, as these systems are highly susceptible to changes in ecosystem energy flows following invasions by exotic plants. Spotted knapweed (Centaurea stoebe L.), a Eurasian, perennial forb, is considered one of the most ecologically harmful invasive plants in North American grasslands and may lead to changes in trophic dynamics, particularly within arthropod communities, which depend on plants for food and habitat. Using DNA metabarcoding to assess community dynamics of arthropods collected from pitfall traps and sweep nets, we explored the effects of C. stoebe density on the alpha, beta and functional diversity of arthropods in a semi-arid grassland in British Columbia, Canada. We used trait-based approaches to investigate the functional responses of terrestrial arthropod communities to better understand the effects of C. stoebe on trophic dynamics. Our study found seasonal differences in the beta-diversity of arthropods, but no differences in arthropod alpha-diversity in knapweed populations. However, our study found a significant reduction in detritivore relative abundance coupled with increases in predator relative abundance, indicating that knapweed density altered the detrital food web. Conversely, herbivores were unaffected by knapweed density, suggesting evidence for greater stability of the grazing food web. Further, predator:prey ratios were highest under high knapweed density. These ratios suggest that top-down effects are likely stronger than the bottom-up effects of C. stoebe invasion. DNA metabarcoding provides the tools to develop detailed surveys of species diversity across a range of environments and trophic levels, which could be a useful guide for planning restoration.

Volatile organic compounds (VOCs) emitted by plant leaves can influence the physiology of neighboring plants. In contrast to interactions above ground, little is known about the role of VOCs in belowground plant-plant interactions. Here, we characterize constitutive root volatile emissions of the spotted knapweed (Centaurea stoebe) and explore the impact of these volatiles on the germination and growth of different sympatric plant species. We show that C. stoebe roots emit high amounts of sesquiterpenes, with estimated release rates of (E)-β-caryophyllene above 3 μg g−1 dw h−1. Sesquiterpene emissions show little variation between different C. stoebe populations, but vary substantially between different Centaurea species. Through root transcriptome sequencing, we identify six root-expressed sesquiterpene synthases (TPSs). Two root-specific TPSs, CsTPS4 and CsTPS5, are sufficient to produce the full blend of emitted root sesquiterpenes. Volatile exposure experiments demonstrate that C. stoebe root volatiles have neutral to positive effects on the germination and growth of different sympatric neighbors. Thus, constitutive root sesquiterpenes produced by two C. stoebe TPSs are associated with facilitation of sympatric neighboring plants. The release of root VOCs may thus influence C. stoebe abundance and plant community structure in nature.

Volatile organic compounds (VOCs) emitted by plant roots can influence the germination and growth of neighboring plants. However, little is known about the effects of root VOCs on plant-herbivore interactions. The spotted knapeed (Centaurea stoebe) constitutively releases high amounts of sesquiterpenes into the rhizosphere. Here, we examine the impact of C. stoebe root VOCs on primary and secondary metabolites of sympatric Taraxacum officinale plants and the resulting plant-mediated effects on a generalist root herbivore, the white grub Melolontha melolontha. We show that exposure of T. officinale to C. stoebe root VOCs does not affect the accumulation of defensive secondary metabolites, but modulates carbohydrate and total protein levels in T. officinale roots. Furthermore, VOC exposure increases M. melolontha growth on T. officinale plants. Exposure of T. officinale to a major C. stoebe root VOC, the sesquiterpene (E)-β-caryophyllene, partially mimics the effect of the full root VOC blend on M. melolontha growth. Thus, releasing root VOCs can modify plant-herbivore interactions of neighboring plants. The release of VOCs to increase the susceptibility of other plants may be a form of plant offense.

The plant microbiome may be bottlenecked at the level of endophytes of individual seeds. Strong defense of developing seeds is predicted by optimal defense theory, and we have experimentally demonstrated exclusionary interactions among endophytic microbes infecting individual seeds of Centaurea stoebe. Having found a single, PDA-culturable microbe per seed or none in an exploratory study with Centaurea stoebe, we completed a more extensive survey of an additional 98 plant species representing 39 families. We again found that individual, surface-sterilized seeds of all species hosted only one PDA-culturable bacterial or fungal endophyte per seed, or none. PDA-unculturables were not determined but we expect them to also be bottlenecked in individual seeds, as they too should be governed by exclusionary interactions. If the bottleneck were confirmed with high-throughput sequencing of individual seeds then it would make sense to further investigate the Primary Symbiont Hypothesis (PSH). This includes the prediction that primary symbionts (i.e., the winners of the exclusionary battles among seed endophytes) have strong effects on seedlings depending on symbiont identity.

Premise of the study Genome duplication is associated with multiple changes at different levels, including interactions with pollinators and herbivores. Yet little is known whether polyploidy may also shape belowground interactions. Methods To elucidate potential ploidy-specific interactions with arbuscular mycorrhizal fungi (AMF), we compared mycorrhizal colonization and assembly of AMF communities in roots of diploid and tetraploid Centaurea stoebe s.l. (Asteraceae) co-occurring in a Central European population. In a follow-up greenhouse experiment, we tested inter-cytotype differences in mycorrhizal growth response by combining ploidy, substrate, and inoculation with native AMF in a full-factorial design. Key results All sampled plants were highly colonized by AMF, with the Glomeraceae predominating. AMF-community composition revealed by 454-pyrosequencing reflected the spatial distribution of the hosts, but not their ploidy level or soil characteristics. In the greenhouse experiment, the tetraploids produced more shoot biomass than the diploids did when grown in a more fertile substrate, while no inter-cytotype differences were found in a less fertile substrate. AMF inoculation significantly reduced plant growth and improved P uptake, but its effects did not differ between the cytotypes. Conclusions The results do not support our hypotheses that the cytotype structure in a mixed-ploidy population of C. stoebe is mirrored in AMF-community composition and that ploidy-specific fungal communities contribute to cytotype co-existence. Causes and implications of the observed negative growth response to AMF are discussed.

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.

The 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.

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.

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.

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 Szelenyi & Erdos, Eupelmus vesicularis Retzius, Scambus brevicornis Gravenhorst, Pteromalus cardui Erdos, 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.

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.

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.

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.

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.

Endophytic plant symbionts can have powerful effects on the way their hosts interact with pathogens, competitors, and consumers. The presence of endophytes in plants can alter food webs, community composition and ecosystem processes, suggesting that endophyte-plant symbioses may represent unique forms of extended phenotypes. We tested the impact of the fungal endophyte Alternaria alternata (phylotype CID 120) on the allelopathic effect of the invasive forb Centaurea stoebe when in competition with the North American native bunchgrass Koeleria macrantha in a greenhouse competition experiment. The allelopathic effect of C. stoebe on K. macrantha when infected with the fungal endophyte was more than twice that of endophyte-free C. stoebe. However, this allelopathic effect was a small part of the very large competitive effect of C. stoebe on K. macrantha in all treatments, likely because of the priority effects in our experimental design. To our knowledge, these results are the first experimental evidence for a symbiotic relationship between plants and fungal endophytes affecting allelopathic interactions between competing plants, and thus provide insight into the mechanisms by which fungal endophytes may increase the competitive ability of their hosts.

Controlled experiments show that arbuscular mycorrhizal fungi (AMF) can increase competitiveness of exotic plants, potentially increasing invasion success. We surveyed AMF abundance and community composition in Centaurea stoebe and Potentilla recta invasions in the western USA to assess whether patterns were consistent with mycorrhizal-mediated invasions. We asked whether (1) AMF abundance and community composition differ between native and exotic forbs, (2) associations between native plants and AMF shift with invading exotic plants, and (3) AMF abundance and/or community composition differ in areas where exotic plants are highly invasive and in areas where they are not. We collected soil and roots from invaded and native forb communities along invasion gradients and in regions with different invasion densities. We used AMF root colonization as a measure of AMF abundance and characterized AMF communities in roots using 454-sequencing of the LSU-rDNA region. All plants were highly colonized (>60%), but exotic forbs tended to be more colonized than natives (P < 0.001). We identified 30 AMF operational taxonomic units (OTUs) across sites, and community composition was best predicted by abiotic factors (soil texture, pH). Two OTUs in the genera Glomus and Rhizophagus dominated in most communities, and their dominance increased with invasion density (r = 0.57, P = 0.010), while overall OTU richness decreased with invasion density (r = −0.61, P = 0.006). Samples along P. recta invasion gradients revealed small and reciprocal shifts in AMF communities with >45% fungal OTUs shared between neighboring native and P. recta plants. Overall, we observed significant, but modest, differences in AMF colonization and communities between co-occurring exotic and native forbs and among exotic forbs across regions that differ in invasion pressure. While experimental manipulations are required to assess functional consequences, the observed patterns are not consistent with those expected from strong mycorrhizal-mediated invasions.

Exotic species can provide abundant food resources for native consumers, but predicting which native species will respond positively remains a challenge. We studied the foraging behavior of black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees in western Montana to compare the degree to which these congeric and syntopic consumers exploited larvae of Urophora, an exotic biological control insect living within the seedheads of the invasive forb, spotted knapweed (Centaurea stoebe). Chickadees typically forage within tree or shrub cover, whereas knapweed and hence Urophora larvae thrive in open grassland away from cover. We found that black-capped chickadees were much more likely than mountain chickadees to forage for Urophora. Black-capped chickadees strategically minimized time spent in open habitats by flying out from cover to retrieve knapweed seedheads and immediately returning to cover to extract the larvae. Black-capped chickadees also employed an atypical hovering technique nearly twice as often as their congeners did, particularly when foraging away from cover. Via this hovering technique, birds were able to gather knapweed seedheads from erect plants rather than searching for seedheads on the ground. These shifts in foraging behavior allowed black-capped chickadees to exploit Urophora larvae to a much greater degree than their congeners while minimizing exposure to a high-risk habitat, an outcome with potentially important community-wide consequences. Behavioral flexibility has been used to predict the success of invading species. We suggest that behavioral flexibility may also be used to predict how native species will respond to invasions, particularly the availability of exotic food resources.

Extensive areas in the upper Midwest have been invaded by spotted knapweed, and effective management strategies are required to reestablish native plant communities. We examined effects of mowing, mowing plus clopyralid, or mowing plus glyphosate in factorial combination with hand pulling and burning on knapweed abundances on a knapweed-infested site in western Michigan. We applied mowing and herbicide treatments in summer 2008, and seeded all plots with native grasses and forbs in spring 2009. We conducted the knapweed pulling treatment from 2009 to 2012 in July. The prescribed burn was conducted in April 2012. By 2012, hand pulling reduced adult knapweed densities to 0.57 ± 0.12 m−2(0.053 ± 0.011 ft−2) (mean ± SE), which was 5.8% of nonpulled treatments, juvenile densities to 0.29 ± 0.07 m−2(2.1% of nonpulled treatments), and seedling densities to 0.07 ± 0.06 m−2(2.6% of nonpulled treatments). After 3 yr, hand pulling reduced seed bank densities to 68 ± 26 m−2as compared to 524 ± 254 m−2in nonpulled treatments and 369 ± 66 m−2in adjacent untreated areas of the study site. Without hand pulling, effects of mowing or mowing plus glyphosate were short-lived and allowed knapweed to rapidly resurge. In comparison, although a single mowing plus clopyralid treatment maintained significantly reduced densities of knapweed for 4 yr, by 2012 knapweed biomass in the nonpulled clopyralid treatment was approximately 60% of that in the other nonpulled treatments. Burning had minimal impacts on knapweed densities regardless of treatment combination, probably as a result of low fire intensity. Results demonstrated that persistent hand pulling used as a follow-up to single mowing or mowing plus herbicide treatments can be an effective practice for treating isolated spotted knapweed infestations or for removing small numbers of knapweed that survive herbicide applications.

Predicting site vulnerability to nonnative plant establishment remains a difficult goal. Seedling survival is an important component of population dynamics and can affect the success of control strategies. Field manipulations allow potential causal mechanisms of site vulnerability to be evaluated under realistic environmental conditions. We conducted field studies to determine the effects of plant competition and differing precipitation regimes on spotted knapweed seedling survival. We also examined the effect of herbivory on rosette survival and growth. Seeds were sown into plots with vegetation intact or removed at three sites. Seeds were also sown into plots where plant competition and precipitation were manipulated in a factorial design at a single site. Field studies demonstrated that site accounted for much of the variation in emergence rate, while herbivory and plant competition affected seedling survival rates. We observed a wide range in emergence rates, with site averages ranging from 13.1 to 42.5%. Survival the following year ranged from 0.5 to 9.4% of sown seeds. Rosette survival was significantly higher when herbivores were excluded from plots. Below average precipitation reduced seedling survival; however, even with supplemental water, dry-down of exposed sites resulted in low seedling survival. Of the 8,000 seeds added to plots in one study, by autumn, only eight plants resulted, seven of which survived in watered plots with intact vegetation. Collectively, these results show that seedling survival is a critical phase in spotted knapweed population dynamics and can vary among habitats on the basis of plant competition and precipitation. Furthermore, herbivory affects all stages of the lifecycle from the seedling onward. The observed differences help explain the reported variability in seedling survival in the literature and inform efforts to control spotted knapweed using plant competition and biological controls.

Summary A prominent hypothesis for plant invasions is escape from the inhibitory effects of soil biota. Although the strength of these inhibitory effects, measured as soil feedbacks, has been assessed between natives and exotics in non-native ranges, few studies have compared the strength of plant–soil feedbacks for exotic species in soils from non-native versus native ranges. We examined whether 6 perennial European forb species that are widespread invaders in North American grasslands (Centaurea stoebe, Euphorbia esula, Hypericum perforatum, Linaria vulgaris, Potentilla recta and Leucanthemum vulgare) experienced different suppressive effects of soil biota collected from 21 sites across both ranges. Four of the six species tested exhibited substantially reduced shoot biomass in ‘live’ versus sterile soil from Europe. In contrast, North American soils produced no significant feedbacks on any of the invasive species tested indicating a broad scale escape from the inhibitory effects of soil biota. Negative feedbacks generated by European soil varied idiosyncratically among sites and species. Since this variation did not correspond with the presence of the target species at field sites, it suggests that negative feedbacks can be generated from soil biota that are widely distributed in native ranges in the absence of density-dependent effects. Synthesis. Our results show that for some invasives, native soils have strong suppressive potential, whereas this is not the case in soils from across the introduced range. Differences in regional-scale evolutionary history among plants and soil biota could ultimately help explain why some exotics are able to occur at higher abundance in the introduced versus native range.